[PATCH v2 0/9] seq_buf: Add seq_buf_strlen()

[PATCH v2 0/9] seq_buf: Add seq_buf_strlen()

[Kees Cook]

Hi,

While working on seq_buf conversions, we found there was a need for
seq_buf_strlen() (since calling strlen(seq_buf_str()) would be a waste of
time: seq_buf already knows the length), and seq_buf_init_append() (to
start a seq_buf from an existing string buffer). And while implementing
that, we found a bunch of other related things that needed fixing. This
is that series, with tests for each fix.

 v1: https://lore.kernel.org/all/20260917002312.i.923-kees@kernel.org/
 v2:
 - seq_buf_do_printk() simplification and more tests
 - add seq_buf_init_append() and tests[1]
 - finish documenting seq_buf[1] and link it to kernel api docs
 - test improvements throughout

-Kees

[1] https://lore.kernel.org/all/20260917011359.1585961-1-morbo@google.com/


Bill Wendling (1):
  seq_buf: Add seq_buf_init_append()

Kees Cook (8):
  seq_buf: Do not print an empty line from an overflowed
    seq_buf_do_printk()
  seq_buf: Do not pop from an overflowed seq_buf
  seq_buf: Copy what fits when seq_buf_puts() and seq_buf_putmem()
    overflow
  seq_buf: Clear what a writer did not claim when a seq_buf overflows
  seq_buf: Add seq_buf_strlen()
  powerpc/papr_scm: Return the string length from the sysfs show
    functions
  nvdimm: ndtest: Return the string length from flags_show()
  docs: core-api: Document the seq_buf API

 Documentation/core-api/kernel-api.rst     |   9 +
 include/linux/seq_buf.h                   | 103 ++++-
 include/linux/trace_seq.h                 |   5 +-
 arch/powerpc/platforms/pseries/papr_scm.c |   4 +-
 lib/seq_buf.c                             |  35 +-
 lib/tests/seq_buf_kunit.c                 | 471 +++++++++++++++++++++-
 tools/testing/nvdimm/test/ndtest.c        |   2 +-
 7 files changed, 607 insertions(+), 22 deletions(-)

-- 
2.34.1

[PATCH v2 1/9] seq_buf: Do not print an empty line from an overflowed seq_buf_do_printk()

[Kees Cook]

seq_buf_do_printk() prints a buffer line by line, then prints whatever
follows the last newline. When a string has overflowed at exactly a
newline, an empty line is printed since the pointer hasn't reached the
overflow mark of the seq_buf. Switch to just check if the string is
already empty and only print if not.

The only caller is the memory cgroup OOM report, so this could only ever
add a blank line to a report whose statistics already did not fit.

Add a test that registers a console to count the records that
seq_buf_do_printk() emits. It counts the records carrying the test's
marker, and the records holding nothing but a line feed that arrive
after one, so that unrelated kernel messages do not disturb it. Both
states that reach the flaw are covered: exactly full, and overflowed.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Fixes: 96928d9032a7c ("seq_buf: Add seq_buf_do_printk() helper")
Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Gow <david@davidgow.net>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 lib/seq_buf.c             |   2 +-
 lib/tests/seq_buf_kunit.c | 133 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+), 1 deletion(-)

diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index a92093f346da..35a5964370b4 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -128,7 +128,7 @@ void seq_buf_do_printk(struct seq_buf *s, const char *lvl)
 	}
 
 	/* No trailing LF */
-	if (start < s->buffer + s->len)
+	if (*start)
 		printk("%s%s\n", lvl, start);
 }
 EXPORT_SYMBOL_GPL(seq_buf_do_printk);
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index eb466386bbef..9ceccdc3029f 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -6,7 +6,9 @@
  */
 
 #include <kunit/test.h>
+#include <linux/console.h>
 #include <linux/seq_buf.h>
+#include <linux/string.h>
 
 static void seq_buf_init_test(struct kunit *test)
 {
@@ -216,6 +218,136 @@ static void seq_buf_putmem_hex_overflow_test(struct kunit *test)
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected);
 }
 
+
+/*
+ * Counters for the console that seq_buf_do_printk_test() registers while it
+ * runs. Only records carrying the marker are counted, so unrelated kernel
+ * messages do not disturb them.
+ *
+ * An empty record carries nothing to recognize it by, so count one only
+ * where the flaw puts it: directly after a record of ours, with nothing in
+ * between. That still misreads a bare line feed printed by another CPU in
+ * exactly that gap, but no longer counts one printed at any point while the
+ * console happens to be registered.
+ */
+#define SEQ_BUF_PRINTK_MARKER	"sbdpkx"
+
+static unsigned int seq_buf_printk_marked;
+static unsigned int seq_buf_printk_empty;
+static bool seq_buf_printk_last_was_ours;
+
+static void seq_buf_printk_capture(struct console *con, const char *s,
+				   unsigned int count)
+{
+	const char *text = s;
+	const char *prefix;
+
+	/*
+	 * Skip what printk() puts in front of the message: a timestamp,
+	 * and the caller id as well under CONFIG_PRINTK_CALLER, so strip
+	 * every bracketed group rather than just the first.
+	 */
+	while (count && text[0] == '[') {
+		prefix = memchr(text, ']', count);
+		if (!prefix)
+			break;
+		count -= prefix + 1 - text;
+		text = prefix + 1;
+		if (count && text[0] == ' ') {
+			text++;
+			count--;
+		}
+	}
+
+	if (strnstr(text, SEQ_BUF_PRINTK_MARKER, count)) {
+		seq_buf_printk_marked++;
+		seq_buf_printk_last_was_ours = true;
+		return;
+	}
+
+	if (seq_buf_printk_last_was_ours &&
+	    (count == 0 || (count == 1 && text[0] == '\n')))
+		seq_buf_printk_empty++;
+
+	seq_buf_printk_last_was_ours = false;
+}
+
+static void seq_buf_printk_run(struct console *capture, struct seq_buf *s)
+{
+	seq_buf_printk_marked = 0;
+	seq_buf_printk_empty = 0;
+	seq_buf_printk_last_was_ours = false;
+
+	/*
+	 * register_console() will not take an unmatched console without
+	 * CON_ENABLED, and unregister_console() clears it, so set it on
+	 * every run to keep the test repeatable.
+	 */
+	capture->flags = CON_ENABLED;
+	register_console(capture);
+	seq_buf_do_printk(s, KERN_INFO);
+	unregister_console(capture);
+}
+
+static void seq_buf_do_printk_test(struct kunit *test)
+{
+	/*
+	 * A registered console is a global object: printk() reaches it
+	 * through the console list from any CPU, and the console code writes
+	 * back into it, so keep it out of this function's stack frame the
+	 * way every other console in the tree does.
+	 */
+	static struct console capture = {
+		.name = "sbufcap",
+		.write = seq_buf_printk_capture,
+		.index = -1,
+	};
+	DECLARE_SEQ_BUF(s, 8);
+	DECLARE_SEQ_BUF(t, 16);
+	DECLARE_SEQ_BUF(u, 8);
+
+	/*
+	 * Fill the buffer exactly, so that the NUL takes the place of the
+	 * last byte and the string ends with the line feed before it.
+	 */
+	seq_buf_puts(&s, SEQ_BUF_PRINTK_MARKER);
+	seq_buf_putc(&s, '\n');
+	seq_buf_putc(&s, '!');
+	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_ASSERT_EQ(test, seq_buf_used(&s), 8);
+	KUNIT_ASSERT_EQ(test, strlen(seq_buf_str(&s)), 7);
+
+	seq_buf_printk_run(&capture, &s);
+
+	/* The one line that was written, and nothing after it. */
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_marked, 1);
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_empty, 0);
+
+	/* Check that lines without a trailing newline are shown. */
+	seq_buf_puts(&t, SEQ_BUF_PRINTK_MARKER "\n" SEQ_BUF_PRINTK_MARKER);
+	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&t));
+
+	seq_buf_printk_run(&capture, &t);
+
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_marked, 2);
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_empty, 0);
+
+	/*
+	 * The buffer above was exactly full, where "len" equals the size. A
+	 * buffer that actually overflowed reaches the same bug by the other
+	 * route the old test had, with "len" one past the size.
+	 */
+	seq_buf_puts(&u, SEQ_BUF_PRINTK_MARKER "\n");
+	KUNIT_EXPECT_EQ(test, seq_buf_puts(&u, "yy"), -1);
+	KUNIT_ASSERT_TRUE(test, seq_buf_has_overflowed(&u));
+	KUNIT_ASSERT_EQ(test, u.len, u.size + 1);
+
+	seq_buf_printk_run(&capture, &u);
+
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_marked, 1);
+	KUNIT_EXPECT_EQ(test, seq_buf_printk_empty, 0);
+}
+
 static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_init_test),
 	KUNIT_CASE(seq_buf_declare_test),
@@ -228,6 +360,7 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_get_buf_commit_test),
 	KUNIT_CASE(seq_buf_putmem_hex_test),
 	KUNIT_CASE(seq_buf_putmem_hex_overflow_test),
+	KUNIT_CASE(seq_buf_do_printk_test),
 	{}
 };
 
-- 
2.34.1

[PATCH v2 2/9] seq_buf: Do not pop from an overflowed seq_buf

[Kees Cook]

When a seq_buf has overflowed, its len is size + 1, so seq_buf_pop()
decrements len to size and reads buffer[size], one byte past the end of
the buffer. It also leaves len equal to size, which no longer counts as
overflowed, so a truncated seq_buf then looks like a complete, full one.

An overflowed seq_buf logically has no last character to pop: the
length of what was written has been lost, and the last byte of the
buffer may be the NUL written by vsnprintf() or bytes that were never
committed. Return -1 for an overflowed seq_buf, as for an empty one,
and leave it overflowed, as the rest of the seq_buf API does until
seq_buf_clear() or seq_buf_init().

The current callers do not reach this, e.g. trace_syscalls only calls
trace_seq_pop() when the trace_seq it pops from has not overflowed, and
kernel/bpf/diagnostics.c sets the length from strnlen() before popping.

Add tests for the pop corner cases.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Fixes: 32e0f607ac6a2 ("tracing: Add trace_seq_pop() and seq_buf_pop()")
Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Günther Noack" <gnoack@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: "Mickaël Salaün" <mic@digikod.net>
Cc: <bpf@vger.kernel.org>
Cc: <linux-security-module@vger.kernel.org>
Cc: <linux-trace-kernel@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Gow <david@davidgow.net>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/seq_buf.h   |  4 ++--
 include/linux/trace_seq.h |  5 ++++-
 lib/tests/seq_buf_kunit.c | 42 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 9f2839e73f8a..f5a350347bc5 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -155,11 +155,11 @@ static inline void seq_buf_commit(struct seq_buf *s, int num)
  *
  * Removes the last written character to the seq_buf @s.
  *
- * Returns the last character or -1 if it is empty.
+ * Returns the last character, or -1 if @s is empty or has overflowed.
  */
 static inline int seq_buf_pop(struct seq_buf *s)
 {
-	if (!s->len)
+	if (!s->len || seq_buf_has_overflowed(s))
 		return -1;
 
 	s->len--;
diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h
index 697d619aafdc..7174ebf3f015 100644
--- a/include/linux/trace_seq.h
+++ b/include/linux/trace_seq.h
@@ -86,7 +86,10 @@ static inline bool trace_seq_has_overflowed(struct trace_seq *s)
  *
  * Removes the last written character to the trace_seq @s.
  *
- * Returns the last character or -1 if it is empty.
+ * Returns the last character, or -1 if the underlying seq_buf is empty or
+ * has overflowed. Note that only that buffer is consulted: a @s marked
+ * full by a write that did not fit, which trace_seq_has_overflowed()
+ * reports as overflowed, still pops the last character written.
  */
 static inline int trace_seq_pop(struct trace_seq *s)
 {
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index 9ceccdc3029f..d5a0c618b880 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -115,6 +115,47 @@ static void seq_buf_putc_test(struct kunit *test)
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "");
 }
 
+static void seq_buf_pop_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 8);
+	struct seq_buf t;
+	char *buf;
+
+	/* Nothing to pop. */
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), -1);
+	KUNIT_EXPECT_EQ(test, s.len, 0);
+
+	seq_buf_puts(&s, "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'o');
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell");
+
+	/* A 0xff byte must not be mistaken for an empty buffer. */
+	seq_buf_putc(&s, 0xff);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 0xff);
+
+	/* A full buffer pops its last byte. */
+	seq_buf_puts(&s, "abc");
+	seq_buf_putc(&s, 'd');
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 8);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'd');
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 7);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hellabc");
+
+	/*
+	 * An overflowed buffer has nothing to pop, and stays overflowed. Use
+	 * a buffer allocated at its exact size, so that KASAN reports any
+	 * read past its end.
+	 */
+	buf = kunit_kmalloc(test, 16, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, buf);
+	seq_buf_init(&t, buf, 16);
+	KUNIT_EXPECT_EQ(test, seq_buf_printf(&t, "%s", "longer than sixteen"), -1);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&t), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
+}
+
 static void seq_buf_printf_test(struct kunit *test)
 {
 	DECLARE_SEQ_BUF(s, 32);
@@ -355,6 +396,7 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_puts_test),
 	KUNIT_CASE(seq_buf_puts_overflow_test),
 	KUNIT_CASE(seq_buf_putc_test),
+	KUNIT_CASE(seq_buf_pop_test),
 	KUNIT_CASE(seq_buf_printf_test),
 	KUNIT_CASE(seq_buf_printf_overflow_test),
 	KUNIT_CASE(seq_buf_get_buf_commit_test),
-- 
2.34.1

[PATCH v2 3/9] seq_buf: Copy what fits when seq_buf_puts() and seq_buf_putmem() overflow

[Kees Cook]

When seq_buf_puts() or seq_buf_putmem() is given more than fits, it
copies nothing and only marks the seq_buf as overflowed. If seq_buf_str()
is used, it will terminate the buffer in its last byte, so every byte
between the end of the data and the end of the buffer becomes part of
the string, though the seq_buf never wrote them.

seq_buf_printf() does not have this problem, because vsnprintf() writes
as much of the output as fits, followed by a NUL. Repeat this behavior
in seq_buf_puts(), using strscpy(), and in seq_buf_putmem(), which also
covers seq_buf_putmem_hex(). seq_buf_putc() needs no change, as it can
only overflow when the buffer is already full.

Each writer now records the buffer as full once it has copied what fits,
so that what it wrote can be told apart from bytes nothing touched.

Update seq_buf_putmem_hex_overflow_test, which expected a hex group that
did not fit whole to be left out entirely, and add tests that overflow
seq_buf_puts(), seq_buf_putmem() and seq_buf_putmem_hex() with stale
bytes in the buffer.

The three partial-overflow tests check the buffer itself rather than
seq_buf_str(). Neither writer leaves the last byte of the buffer alone:
seq_buf_putmem() copies raw bytes and writes no NUL, and seq_buf_puts()
relies on strscpy() to write one. That byte is exactly where
seq_buf_str() writes its terminator, so asserting only on the string
would pass whether the copy stopped a byte early or dropped the NUL
entirely.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Gow <david@davidgow.net>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/seq_buf.h   |  6 +++
 lib/seq_buf.c             | 16 +++++++-
 lib/tests/seq_buf_kunit.c | 84 ++++++++++++++++++++++++++++++++++++++-
 3 files changed, 102 insertions(+), 4 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index f5a350347bc5..77e76e283370 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -55,6 +55,12 @@ seq_buf_has_overflowed(struct seq_buf *s)
 	return s->len > s->size;
 }
 
+/*
+ * Mark @s as overflowed, which discards the length of what it holds. The
+ * bytes up to its last one are the string from then on, as that is where
+ * seq_buf_str() terminates it, so a caller that could not fill the buffer
+ * has to NUL them itself before calling this.
+ */
 static inline void
 seq_buf_set_overflow(struct seq_buf *s)
 {
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 35a5964370b4..00abdec8760e 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -175,7 +175,9 @@ int seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary)
  * @s: seq_buf descriptor
  * @str: simple string to record
  *
- * Copy a simple string into the sequence buffer.
+ * Copy a simple string into the sequence buffer. If @str does not fit,
+ * as much of it as fits is copied, followed by a null byte, as
+ * seq_buf_printf() does.
  *
  * Returns: zero on success, -1 on overflow.
  */
@@ -194,6 +196,11 @@ int seq_buf_puts(struct seq_buf *s, const char *str)
 		s->len += len - 1;
 		return 0;
 	}
+	/* Copy what fits, so the buffer never holds stale bytes */
+	if (s->len < s->size) {
+		strscpy(s->buffer + s->len, str, s->size - s->len);
+		s->len = s->size;
+	}
 	seq_buf_set_overflow(s);
 	return -1;
 }
@@ -229,7 +236,7 @@ EXPORT_SYMBOL_GPL(seq_buf_putc);
  *
  * There may be cases where raw memory needs to be written into the
  * buffer and a strcpy() would not work. Using this function allows
- * for such cases.
+ * for such cases. If @mem does not fit, as much of it as fits is copied.
  *
  * Returns: zero on success, -1 on overflow.
  */
@@ -242,6 +249,11 @@ int seq_buf_putmem(struct seq_buf *s, const void *mem, unsigned int len)
 		s->len += len;
 		return 0;
 	}
+	/* Copy what fits, so the buffer never holds stale bytes */
+	if (s->len < s->size) {
+		memcpy(s->buffer + s->len, mem, s->size - s->len);
+		s->len = s->size;
+	}
 	seq_buf_set_overflow(s);
 	return -1;
 }
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index d5a0c618b880..e0057eaeeb36 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -248,9 +248,9 @@ static void seq_buf_putmem_hex_overflow_test(struct kunit *test)
 	DECLARE_SEQ_BUF(s, 20);
 	const u8 data[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 #ifdef __BIG_ENDIAN
-	const char *expected = "0001020304050607 ";
+	const char *expected = "0001020304050607 08";
 #else
-	const char *expected = "0706050403020100 ";
+	const char *expected = "0706050403020100 09";
 #endif
 
 	KUNIT_EXPECT_EQ(test, seq_buf_putmem_hex(&s, data, sizeof(data)), -1);
@@ -259,6 +259,83 @@ static void seq_buf_putmem_hex_overflow_test(struct kunit *test)
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected);
 }
 
+static void seq_buf_puts_partial_overflow_test(struct kunit *test)
+{
+	static const char expected[] = "abcdefg";
+	DECLARE_SEQ_BUF(s, 16);
+	struct seq_buf t;
+	char buf[8];
+
+	/* As much of the string as fits is written, like seq_buf_printf(). */
+	seq_buf_puts(&s, "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_puts(&s, " world, again"), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 16);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world, ag");
+
+	/* Stale bytes after the data must not show up in the string. */
+	memset(buf, 'X', sizeof(buf));
+	seq_buf_init(&t, buf, sizeof(buf));
+	seq_buf_putc(&t, 'a');
+	KUNIT_EXPECT_EQ(test, seq_buf_puts(&t, "bcdefghij"), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
+
+	/*
+	 * Check the buffer before seq_buf_str() does: it would write the
+	 * terminator over the last byte itself, hiding whether the copy
+	 * placed one there. The literal's own NUL is the eighth byte.
+	 */
+	KUNIT_EXPECT_MEMEQ(test, buf, expected, sizeof(buf));
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "abcdefg");
+}
+
+static void seq_buf_putmem_partial_overflow_test(struct kunit *test)
+{
+	const u8 data[] = { 1, 2, 3, 4, 5, 6, 7 };
+	const char expected[] = { 'a', 'b', 1, 2, 3, 4, 5, 6 };
+	struct seq_buf s;
+	char buf[8];
+
+	memset(buf, 'X', sizeof(buf));
+	seq_buf_init(&s, buf, sizeof(buf));
+	seq_buf_putmem(&s, "ab", 2);
+
+	/* One byte too many, so the last byte of @data is dropped. */
+	KUNIT_EXPECT_EQ(test, seq_buf_putmem(&s, data, sizeof(data)), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+
+	/*
+	 * Check the buffer rather than seq_buf_str(): seq_buf_putmem() writes
+	 * no NUL of its own, so a short copy leaves a stale byte at the end,
+	 * exactly where seq_buf_str() would then write the terminator and
+	 * hide it.
+	 */
+	KUNIT_EXPECT_MEMEQ(test, buf, expected, sizeof(buf));
+}
+
+static void seq_buf_putmem_hex_partial_overflow_test(struct kunit *test)
+{
+	const u8 data[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
+#ifdef __BIG_ENDIAN
+	const char *expected = "0001020304050607 08";
+	static const char expected_raw[] = "0001020304050607 080";
+#else
+	const char *expected = "0706050403020100 09";
+	static const char expected_raw[] = "0706050403020100 090";
+#endif
+	struct seq_buf s;
+	char buf[20];
+
+	/* Stale bytes after the data must not show up in the string. */
+	memset(buf, 'X', sizeof(buf));
+	seq_buf_init(&s, buf, sizeof(buf));
+	KUNIT_EXPECT_EQ(test, seq_buf_putmem_hex(&s, data, sizeof(data)), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+
+	/* Before seq_buf_str() writes the terminator over the last byte. */
+	KUNIT_EXPECT_MEMEQ(test, buf, expected_raw, sizeof(buf));
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected);
+}
 
 /*
  * Counters for the console that seq_buf_do_printk_test() registers while it
@@ -402,6 +479,9 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_get_buf_commit_test),
 	KUNIT_CASE(seq_buf_putmem_hex_test),
 	KUNIT_CASE(seq_buf_putmem_hex_overflow_test),
+	KUNIT_CASE(seq_buf_puts_partial_overflow_test),
+	KUNIT_CASE(seq_buf_putmem_partial_overflow_test),
+	KUNIT_CASE(seq_buf_putmem_hex_partial_overflow_test),
 	KUNIT_CASE(seq_buf_do_printk_test),
 	{}
 };
-- 
2.34.1

[PATCH v2 4/9] seq_buf: Clear what a writer did not claim when a seq_buf overflows

[Kees Cook]

Using seq_buf_set_overflow() would leave the bytes between "len"
and "size" untouched, so if seq_buf_str() is used on an overflowed
seq_buf, those bytes may be exposed. For any paths that don't claim
partially written bytes, by setting "len = size" before calling
seq_buf_set_overflow(), wipe the unclaimed bytes. The seq_buf_puts()
and related APIs already claim those bytes now, so only the unclaimed
cases remain. A specific example of this was seq_buf_path() which uses
d_path() and would write to the tail before discovering it was out
of space, and would correctly mark a seq_buf as overflowed, but the
path fragment would be left over.

Clear from len to the end of the buffer in seq_buf_set_overflow(), which
every overflow goes through, including seq_buf_commit() with a negative
count.

Add a test that fills a seq_buf, leaves it too little room for a path, and
checks that nothing of the path is left in the buffer. The tests run before
anything writable is mounted, so it takes its file from shmem.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Gow <david@davidgow.net>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/seq_buf.h   |  8 +++++--
 lib/seq_buf.c             |  4 ++++
 lib/tests/seq_buf_kunit.c | 45 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 77e76e283370..0c0a0db04b09 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -5,6 +5,7 @@
 #include <linux/bug.h>
 #include <linux/minmax.h>
 #include <linux/seq_file.h>
+#include <linux/string.h>
 #include <linux/types.h>
 
 /*
@@ -58,12 +59,15 @@ seq_buf_has_overflowed(struct seq_buf *s)
 /*
  * Mark @s as overflowed, which discards the length of what it holds. The
  * bytes up to its last one are the string from then on, as that is where
- * seq_buf_str() terminates it, so a caller that could not fill the buffer
- * has to NUL them itself before calling this.
+ * seq_buf_str() terminates it, so clear whatever was not written: a writer
+ * sets len to how much it filled, and anything past that was never adopted.
  */
 static inline void
 seq_buf_set_overflow(struct seq_buf *s)
 {
+	if (s->len < s->size)
+		memset(s->buffer + s->len, 0, s->size - s->len);
+
 	s->len = s->size + 1;
 }
 
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 00abdec8760e..7e3bf837da01 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -76,6 +76,8 @@ int seq_buf_vprintf(struct seq_buf *s, const char *fmt, va_list args)
 			s->len += len;
 			return 0;
 		}
+		/* vsnprintf() wrote as much as fits, so none of it is stale */
+		s->len = s->size;
 	}
 	seq_buf_set_overflow(s);
 	return -1;
@@ -164,6 +166,8 @@ int seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary)
 			s->len += ret;
 			return 0;
 		}
+		/* bstr_printf() wrote as much as fits, so none of it is stale */
+		s->len = s->size;
 	}
 	seq_buf_set_overflow(s);
 	return -1;
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index e0057eaeeb36..852eb645e253 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -6,6 +6,9 @@
  */
 
 #include <kunit/test.h>
+#include <linux/fs.h>
+#include <linux/seq_buf.h>
+#include <linux/shmem_fs.h>
 #include <linux/console.h>
 #include <linux/seq_buf.h>
 #include <linux/string.h>
@@ -466,6 +469,47 @@ static void seq_buf_do_printk_test(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, seq_buf_printk_empty, 0);
 }
 
+/* Long enough that it cannot fit in the room the test leaves for it. */
+#define SEQ_BUF_TEST_PATH	"/seq_buf_kunit_path_name"
+
+static void seq_buf_path_overflow_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 32);
+	const char *expected = "keep:xxxxxxxxxxxxxxxxxxxxxxx";
+	struct file *file;
+	size_t len;
+	int i;
+
+	/*
+	 * The tests run before anything writable is mounted, so take the file
+	 * whose path gets printed from shmem, which needs no mount of its own.
+	 */
+	file = shmem_file_setup(SEQ_BUF_TEST_PATH, 0, EMPTY_VMA_FLAGS);
+	if (IS_ERR(file))
+		kunit_skip(test, "cannot create a file to print the path of");
+
+	/* Leave less room than the path needs, so d_path() cannot fit it. */
+	seq_buf_puts(&s, "keep:");
+	len = seq_buf_used(&s);
+	for (i = len; i < 28; i++)
+		seq_buf_putc(&s, 'x');
+
+	KUNIT_EXPECT_EQ(test, seq_buf_path(&s, &file->f_path, "\n"), -1);
+	fput(file);
+
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+
+	/*
+	 * d_path() keeps as much of the path as fits when it does not fit
+	 * whole, and seq_buf_str() would hand out that fragment, as it ends
+	 * the string at the last byte of an overflowed buffer.
+	 */
+	for (i = 28; i < 32; i++)
+		KUNIT_EXPECT_EQ_MSG(test, s.buffer[i], '\0',
+				    "byte %d past the data is not cleared", i);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected);
+}
+
 static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_init_test),
 	KUNIT_CASE(seq_buf_declare_test),
@@ -482,6 +526,7 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_puts_partial_overflow_test),
 	KUNIT_CASE(seq_buf_putmem_partial_overflow_test),
 	KUNIT_CASE(seq_buf_putmem_hex_partial_overflow_test),
+	KUNIT_CASE(seq_buf_path_overflow_test),
 	KUNIT_CASE(seq_buf_do_printk_test),
 	{}
 };
-- 
2.34.1

[PATCH v2 5/9] seq_buf: Add seq_buf_strlen()

[Kees Cook]

Several strlcat() call sites being converted to seq_buf need behavior
seq_buf doesn't currently provide. The return from seq_buf_used() is
not the length of the string in a seq_buf. Once the buffer is full or
has overflowed it returns the buffer size, which counts the byte that
seq_buf_str() replaces with the NUL, so a caller that needs the string
and its length has to call seq_buf_str() and then walk the string with
strlen().

Move the termination out of seq_buf_str() into a helper that returns
where it put the NUL, and add seq_buf_strlen(), which terminates the
buffer in the same way and returns that offset.

Add tests comparing seq_buf_strlen() against strlen() of seq_buf_str()
for empty, appended, truncated, exactly full, and overflowed buffers,
and checking that seq_buf_strlen() alone terminates a full buffer.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Gow <david@davidgow.net>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/seq_buf.h   |  57 +++++++++++++++++--
 lib/tests/seq_buf_kunit.c | 114 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 166 insertions(+), 5 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 0c0a0db04b09..7f025c7a68be 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -89,6 +89,27 @@ static inline unsigned int seq_buf_used(struct seq_buf *s)
 	return min(s->len, s->size);
 }
 
+/*
+ * NUL-terminate the buffer in @s: directly after the data when there is
+ * room for it, otherwise in the last byte of the buffer. @s->size must not
+ * be zero.
+ *
+ * Returns: the offset of the NUL.
+ */
+static inline size_t __seq_buf_terminate(struct seq_buf *s)
+{
+	size_t end;
+
+	if (seq_buf_buffer_left(s))
+		end = s->len;
+	else
+		end = s->size - 1;
+
+	s->buffer[end] = 0;
+
+	return end;
+}
+
 /**
  * seq_buf_str - get NUL-terminated C string from seq_buf
  * @s: the seq_buf handle
@@ -98,7 +119,9 @@ static inline unsigned int seq_buf_used(struct seq_buf *s)
  *
  * Note, if this is called when the buffer has overflowed, then
  * the last byte of the buffer is zeroed, and the len will still
- * point passed it.
+ * point passed it. The same happens when the buffer is exactly
+ * full: the NUL takes the place of the last byte written, which is
+ * lost, though seq_buf_used() still counts it.
  *
  * After this function is called, s->buffer is safe to use
  * in string operations.
@@ -110,14 +133,38 @@ static inline const char *seq_buf_str(struct seq_buf *s)
 	if (WARN_ON(s->size == 0))
 		return "";
 
-	if (seq_buf_buffer_left(s))
-		s->buffer[s->len] = 0;
-	else
-		s->buffer[s->size - 1] = 0;
+	__seq_buf_terminate(s);
 
 	return s->buffer;
 }
 
+/**
+ * seq_buf_strlen - get the length of the NUL-terminated C string in seq_buf
+ * @s: the seq_buf handle
+ *
+ * This makes sure that the buffer in @s is NUL-terminated, exactly as
+ * seq_buf_str() does, and returns the length of the resulting string
+ * without walking it. Unlike seq_buf_used(), this does not count the byte
+ * given up to the NUL when the buffer is full or has overflowed. When the
+ * buffer is exactly full, that byte is the last one written, and calling
+ * either function loses it.
+ *
+ * After this function is called, s->buffer is safe to use
+ * in string operations.
+ *
+ * Returns: the offset of the NUL that terminates @s->buffer. That is the
+ * length of the string unless an earlier NUL is in the way, either one the
+ * data written to @s carried itself, or one seq_buf_set_overflow() left
+ * behind when it cleared what no writer had claimed.
+ */
+static inline size_t seq_buf_strlen(struct seq_buf *s)
+{
+	if (WARN_ON(s->size == 0))
+		return 0;
+
+	return __seq_buf_terminate(s);
+}
+
 /**
  * seq_buf_get_buf - get buffer to write arbitrary data to
  * @s: the seq_buf handle
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index 852eb645e253..0259c8506b89 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -26,8 +26,10 @@ static void seq_buf_init_test(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 32);
 	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 0);
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0);
 }
 
+
 static void seq_buf_declare_test(struct kunit *test)
 {
 	DECLARE_SEQ_BUF(s, 24);
@@ -510,6 +512,113 @@ static void seq_buf_path_overflow_test(struct kunit *test)
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected);
 }
 
+static void seq_buf_strlen_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 16);
+
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "");
+
+	seq_buf_puts(&s, "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+
+	seq_buf_printf(&s, " %s", "world");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+}
+
+static void seq_buf_strlen_printf_overflow_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 16);
+	DECLARE_SEQ_BUF(t, 8);
+
+	seq_buf_printf(&s, "%s", "1234567890abcdefghij");
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 16);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 15);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "1234567890abcde");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+
+	/* Output one byte too long for the NUL. */
+	seq_buf_printf(&t, "%s", "12345678");
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&t), 8);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 7);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "1234567");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), strlen(seq_buf_str(&t)));
+}
+
+static void seq_buf_strlen_full_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 4);
+	DECLARE_SEQ_BUF(t, 8);
+	char *buf;
+	size_t len;
+
+	/* Filled exactly, with no room left for a NUL, but not overflowed. */
+	seq_buf_putc(&s, 'a');
+	seq_buf_putc(&s, 'b');
+	seq_buf_putc(&s, 'c');
+	seq_buf_putc(&s, 'd');
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 3);
+	/* seq_buf_strlen() terminates the buffer by itself. */
+	KUNIT_EXPECT_EQ(test, s.buffer[3], '\0');
+	KUNIT_EXPECT_EQ(test, strnlen(s.buffer, s.size), 3);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+
+	/* A printf into a full buffer writes nothing. */
+	KUNIT_EXPECT_EQ(test, seq_buf_printf(&s, "%s", "x"), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 3);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+
+	len = seq_buf_get_buf(&t, &buf);
+	KUNIT_ASSERT_EQ(test, len, 8);
+	memset(buf, 'z', len);
+	seq_buf_commit(&t, len);
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&t));
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 7);
+	KUNIT_EXPECT_EQ(test, t.buffer[7], '\0');
+	KUNIT_EXPECT_EQ(test, strnlen(t.buffer, t.size), 7);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "zzzzzzz");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), strlen(seq_buf_str(&t)));
+}
+
+static void seq_buf_strlen_puts_overflow_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 16);
+
+	/* A puts that does not fit copies as much as fits. */
+	seq_buf_puts(&s, "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_puts(&s, " this does not fit"), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 15);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello this does");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s)));
+}
+
+
+static void seq_buf_strlen_embedded_nul_test(struct kunit *test)
+{
+	static const char data[] = "ab\0cd";
+	DECLARE_SEQ_BUF(s, 16);
+
+	/*
+	 * seq_buf_strlen() reports where it put the terminator, not where
+	 * the first NUL is, so data carrying a NUL of its own makes the two
+	 * disagree. That is expected, and is what the documented caveat is
+	 * about.
+	 */
+	seq_buf_putmem(&s, data, sizeof(data) - 1);
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5);
+	KUNIT_EXPECT_EQ(test, strlen(seq_buf_str(&s)), 2);
+}
+
 static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_init_test),
 	KUNIT_CASE(seq_buf_declare_test),
@@ -527,6 +636,11 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_putmem_partial_overflow_test),
 	KUNIT_CASE(seq_buf_putmem_hex_partial_overflow_test),
 	KUNIT_CASE(seq_buf_path_overflow_test),
+	KUNIT_CASE(seq_buf_strlen_test),
+	KUNIT_CASE(seq_buf_strlen_printf_overflow_test),
+	KUNIT_CASE(seq_buf_strlen_full_test),
+	KUNIT_CASE(seq_buf_strlen_puts_overflow_test),
+	KUNIT_CASE(seq_buf_strlen_embedded_nul_test),
 	KUNIT_CASE(seq_buf_do_printk_test),
 	{}
 };
-- 
2.34.1

[PATCH v2 6/9] seq_buf: Add seq_buf_init_append()

[Kees Cook]

From: Bill Wendling <morbo@google.com>

Several strlcat() call sites being converted to seq_buf need behavior
seq_buf doesn't currently provide. The normal seq_buf_init() always
sets the new buffer size to 0 via seq_buf_clear(). Code migrating from
strlcat(buf, ...), which appends to whatever buf already contains,
can't use seq_buf_init() without discarding that existing content. Add
seq_buf_init_append(), which preserves the existing contents and positions
the seq_buf to append after it. Add KUnit tests for behavior coverage.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0.

Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
Co-developed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Gow <david@davidgow.net>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuvam Pandey <shuvampandey1@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/seq_buf.h   | 20 +++++++++++++++
 lib/tests/seq_buf_kunit.c | 53 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 7f025c7a68be..89d847d40626 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -46,6 +46,26 @@ seq_buf_init(struct seq_buf *s, char *buf, unsigned int size)
 	seq_buf_clear(s);
 }
 
+/**
+ * seq_buf_init_append - initialize a seq_buf over a buffer that may
+ *			 already hold NUL-terminated content
+ * @s: the seq_buf handle
+ * @buf: pointer to the (possibly non-empty) buffer
+ * @size: total size of @buf
+ *
+ * Unlike seq_buf_init(), which always starts @buf at len=0, this
+ * preserves whatever NUL-terminated content @buf already holds and
+ * positions @s to append after it. Useful for converting code that used
+ * to append to an existing buffer with strlcat()/scnprintf() and friends.
+ */
+static inline void
+seq_buf_init_append(struct seq_buf *s, char *buf, unsigned int size)
+{
+	s->buffer = buf;
+	s->size = size;
+	s->len = strnlen(buf, size);
+}
+
 /*
  * seq_buf have a buffer that might overflow. When this happens
  * len is set to be greater than size.
diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
index 0259c8506b89..f3058771d96c 100644
--- a/lib/tests/seq_buf_kunit.c
+++ b/lib/tests/seq_buf_kunit.c
@@ -29,6 +29,58 @@ static void seq_buf_init_test(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0);
 }
 
+static void seq_buf_init_append_test(struct kunit *test)
+{
+	char buf[32] = "hello";
+	struct seq_buf s;
+
+	/* Initial string contents match. */
+	seq_buf_init_append(&s, buf, sizeof(buf));
+	KUNIT_EXPECT_EQ(test, s.size, 32);
+	KUNIT_EXPECT_EQ(test, s.len, 5);
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 32 - 5);
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 5);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5);
+
+	/* Appending with space works. */
+	seq_buf_puts(&s, " world");
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11);
+
+	/* No truncation when space for NUL is present. */
+	seq_buf_init_append(&s, buf, 12);
+	KUNIT_EXPECT_EQ(test, s.size, 12);
+	KUNIT_EXPECT_EQ(test, s.len, 11);
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11);
+
+	/* Check for required truncation when full. */
+	seq_buf_init_append(&s, buf, 11);
+	KUNIT_EXPECT_EQ(test, s.size, 11);
+	KUNIT_EXPECT_EQ(test, s.len, 11);
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 0);
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello worl");
+	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 10);
+
+	/*
+	 * The size bounds the scan: the string reaches past it, so an
+	 * unbounded strlen() would report 10 here and leave @s overflowed.
+	 */
+	seq_buf_init_append(&s, buf, 5);
+	KUNIT_EXPECT_EQ(test, s.size, 5);
+	KUNIT_EXPECT_EQ(test, s.len, 5);
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 0);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell");
+}
 
 static void seq_buf_declare_test(struct kunit *test)
 {
@@ -621,6 +673,7 @@ static void seq_buf_strlen_embedded_nul_test(struct kunit *test)
 
 static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_init_test),
+	KUNIT_CASE(seq_buf_init_append_test),
 	KUNIT_CASE(seq_buf_declare_test),
 	KUNIT_CASE(seq_buf_clear_test),
 	KUNIT_CASE(seq_buf_puts_test),
-- 
2.34.1

[PATCH v2 7/9] powerpc/papr_scm: Return the string length from the sysfs show functions

[Kees Cook]

perf_stats_show() and flags_show() build their output with a seq_buf
and return seq_buf_used(), which may include the trailing NUL byte
when the seq_buf has overflowed. Use seq_buf_strlen() instead.

Build tested ARCH=powerpc ppc64_defconfig with GCC powerpc64-linux-gnu
16.1.0:
arch/powerpc/platforms/pseries/papr_scm.o

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Cc: "Uwe Kleine-König" <u.kleine-koenig@baylibre.com>
Cc: <linuxppc-dev@lists.ozlabs.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Cc: Thorsten Blum <blum@kernel.org>
---
 arch/powerpc/platforms/pseries/papr_scm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
index 75da96c08cdd..b03cfd8528f5 100644
--- a/arch/powerpc/platforms/pseries/papr_scm.c
+++ b/arch/powerpc/platforms/pseries/papr_scm.c
@@ -1108,7 +1108,7 @@ static ssize_t perf_stats_show(struct device *dev,
 
 free_stats:
 	kfree(stats);
-	return rc ? rc : (ssize_t)seq_buf_used(&s);
+	return rc ?: (ssize_t)seq_buf_strlen(&s);
 }
 static DEVICE_ATTR_ADMIN_RO(perf_stats);
 
@@ -1150,7 +1150,7 @@ static ssize_t flags_show(struct device *dev,
 	if (seq_buf_used(&s))
 		seq_buf_printf(&s, "\n");
 
-	return seq_buf_used(&s);
+	return seq_buf_strlen(&s);
 }
 DEVICE_ATTR_RO(flags);
 
-- 
2.34.1

[PATCH v2 8/9] nvdimm: ndtest: Return the string length from flags_show()

[Kees Cook]

flags_show() build their output with a seq_buf and return seq_buf_used(),
which may include the trailing NUL byte when the seq_buf has
overflowed. Use seq_buf_strlen() instead.

The flag names are far shorter than the PAGE_SIZE buffer sysfs
provides, so this cannot overflow today.

Build tested ARCH=x86_64 with GCC 16.2.0, built out of tree with
make M=tools/testing/nvdimm:
tools/testing/nvdimm/test/ndtest.o

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: <nvdimm@lists.linux.dev>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guangshuo Li <lgs201920130244@gmail.com>
Cc: Ira Weiny <iweiny@kernel.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Vishal Verma <vishal.l.verma@intel.com>
---
 tools/testing/nvdimm/test/ndtest.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/nvdimm/test/ndtest.c b/tools/testing/nvdimm/test/ndtest.c
index 2051ad5d4882..f097f2992966 100644
--- a/tools/testing/nvdimm/test/ndtest.c
+++ b/tools/testing/nvdimm/test/ndtest.c
@@ -693,7 +693,7 @@ static ssize_t flags_show(struct device *dev,
 	if (seq_buf_used(&s))
 		seq_buf_printf(&s, "\n");
 
-	return seq_buf_used(&s);
+	return seq_buf_strlen(&s);
 }
 static DEVICE_ATTR_RO(flags);
 
-- 
2.34.1

[PATCH v2 9/9] docs: core-api: Document the seq_buf API

[Kees Cook]

The kernel-doc in include/linux/seq_buf.h and lib/seq_buf.c documents
the seq_buf interface, but no .rst file pulls either of them in, so none
of it reaches the generated documentation.

Add the missing kernel-doc for seq_buf_clear() and seq_buf_init(), and a
Sequence Buffers section to the kernel API documentation. The static
internal helper seq_buf_can_fit() is left out. Additionally fix
seq_buf_hex_dump() indentation to avoid the reported Sphinx error:

  ERROR: Unexpected indentation.
  WARNING: Block quote ends without a blank line; unexpected unindent.

Verified with "make SPHINXDIRS=core-api htmldocs", which rendered
happily into core-api/kernel-api.html.

Assisted-by: LLM
Co-developed-by: Bill Wendling <morbo@google.com>
Signed-off-by: Bill Wendling <morbo@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-doc@vger.kernel.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 Documentation/core-api/kernel-api.rst |  9 +++++++++
 include/linux/seq_buf.h               | 12 ++++++++++++
 lib/seq_buf.c                         | 13 +++++++------
 3 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/Documentation/core-api/kernel-api.rst b/Documentation/core-api/kernel-api.rst
index 4c4a57c1c094..f5a0aedbbb48 100644
--- a/Documentation/core-api/kernel-api.rst
+++ b/Documentation/core-api/kernel-api.rst
@@ -96,6 +96,15 @@ Error Pointers
 .. kernel-doc:: include/linux/err.h
    :internal:
 
+Sequence Buffers
+----------------
+
+.. kernel-doc:: include/linux/seq_buf.h
+   :internal:
+
+.. kernel-doc:: lib/seq_buf.c
+   :no-identifiers: seq_buf_can_fit
+
 Sorting
 -------
 
diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 89d847d40626..416a8b67b9ad 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -31,6 +31,10 @@ struct seq_buf {
 		.size = SIZE,				\
 	}
 
+/**
+ * seq_buf_clear - reset the seq_buf to be read / appended from the beginning
+ * @s: the seq_buf handle
+ */
 static inline void seq_buf_clear(struct seq_buf *s)
 {
 	s->len = 0;
@@ -38,6 +42,14 @@ static inline void seq_buf_clear(struct seq_buf *s)
 		s->buffer[0] = '\0';
 }
 
+/**
+ * seq_buf_init - initialize a seq_buf
+ * @s: the seq_buf handle
+ * @buf: pointer to the buffer
+ * @size: total size of @buf
+ *
+ * The contents of the buffer are ignored.
+ */
 static inline void
 seq_buf_init(struct seq_buf *s, char *buf, unsigned int size)
 {
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 7e3bf837da01..1c86eae9e188 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -407,12 +407,13 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
  *
  * Function is an analogue of print_hex_dump() and thus has similar interface.
  *
- * linebuf size is maximal length for one line.
- * 32 * 3 - maximum bytes per line, each printed into 2 chars + 1 for
- *	separating space
- * 2 - spaces separating hex dump and ASCII representation
- * 32 - ASCII representation
- * 1 - terminating '\0'
+ * linebuf size is maximal length for one line::
+ *
+ *	32 * 3 - maximum bytes per line, each printed into 2 chars + 1 for
+ *		 separating space
+ *	2 - spaces separating hex dump and ASCII representation
+ *	32 - ASCII representation
+ *	1 - terminating '\0'
  *
  * Returns: zero on success, -1 on overflow.
  */
-- 
2.34.1

Re: [PATCH v2 9/9] docs: core-api: Document the seq_buf API

[Randy Dunlap]

On 9/18/26 5:27 PM, Kees Cook wrote:
> The kernel-doc in include/linux/seq_buf.h and lib/seq_buf.c documents
> the seq_buf interface, but no .rst file pulls either of them in, so none
> of it reaches the generated documentation.
> 
> Add the missing kernel-doc for seq_buf_clear() and seq_buf_init(), and a
> Sequence Buffers section to the kernel API documentation. The static
> internal helper seq_buf_can_fit() is left out. Additionally fix
> seq_buf_hex_dump() indentation to avoid the reported Sphinx error:
> 
>   ERROR: Unexpected indentation.
>   WARNING: Block quote ends without a blank line; unexpected unindent.
> 
> Verified with "make SPHINXDIRS=core-api htmldocs", which rendered
> happily into core-api/kernel-api.html.
> 
> Assisted-by: LLM
> Co-developed-by: Bill Wendling <morbo@google.com>
> Signed-off-by: Bill Wendling <morbo@google.com>
> Signed-off-by: Kees Cook <kees@kernel.org>

Looks good. Thanks.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>

> ---
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: <linux-doc@vger.kernel.org>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Shuah Khan <skhan@linuxfoundation.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> ---
>  Documentation/core-api/kernel-api.rst |  9 +++++++++
>  include/linux/seq_buf.h               | 12 ++++++++++++
>  lib/seq_buf.c                         | 13 +++++++------
>  3 files changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/core-api/kernel-api.rst b/Documentation/core-api/kernel-api.rst
> index 4c4a57c1c094..f5a0aedbbb48 100644
> --- a/Documentation/core-api/kernel-api.rst
> +++ b/Documentation/core-api/kernel-api.rst
> @@ -96,6 +96,15 @@ Error Pointers
>  .. kernel-doc:: include/linux/err.h
>     :internal:
>  
> +Sequence Buffers
> +----------------
> +
> +.. kernel-doc:: include/linux/seq_buf.h
> +   :internal:
> +
> +.. kernel-doc:: lib/seq_buf.c
> +   :no-identifiers: seq_buf_can_fit
> +
>  Sorting
>  -------
>  
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index 89d847d40626..416a8b67b9ad 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -31,6 +31,10 @@ struct seq_buf {
>  		.size = SIZE,				\
>  	}
>  
> +/**
> + * seq_buf_clear - reset the seq_buf to be read / appended from the beginning
> + * @s: the seq_buf handle
> + */
>  static inline void seq_buf_clear(struct seq_buf *s)
>  {
>  	s->len = 0;
> @@ -38,6 +42,14 @@ static inline void seq_buf_clear(struct seq_buf *s)
>  		s->buffer[0] = '\0';
>  }
>  
> +/**
> + * seq_buf_init - initialize a seq_buf
> + * @s: the seq_buf handle
> + * @buf: pointer to the buffer
> + * @size: total size of @buf
> + *
> + * The contents of the buffer are ignored.
> + */
>  static inline void
>  seq_buf_init(struct seq_buf *s, char *buf, unsigned int size)
>  {
> diff --git a/lib/seq_buf.c b/lib/seq_buf.c
> index 7e3bf837da01..1c86eae9e188 100644
> --- a/lib/seq_buf.c
> +++ b/lib/seq_buf.c
> @@ -407,12 +407,13 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
>   *
>   * Function is an analogue of print_hex_dump() and thus has similar interface.
>   *
> - * linebuf size is maximal length for one line.
> - * 32 * 3 - maximum bytes per line, each printed into 2 chars + 1 for
> - *	separating space
> - * 2 - spaces separating hex dump and ASCII representation
> - * 32 - ASCII representation
> - * 1 - terminating '\0'
> + * linebuf size is maximal length for one line::
> + *
> + *	32 * 3 - maximum bytes per line, each printed into 2 chars + 1 for
> + *		 separating space
> + *	2 - spaces separating hex dump and ASCII representation
> + *	32 - ASCII representation
> + *	1 - terminating '\0'
>   *
>   * Returns: zero on success, -1 on overflow.
>   */

-- 
~Randy

Re: [PATCH v2 5/9] seq_buf: Add seq_buf_strlen()

[Greg KH]

On Fri, Sep 18, 2026 at 05:27:03PM -0700, Kees Cook wrote:
> +/**
> + * seq_buf_strlen - get the length of the NUL-terminated C string in seq_buf
> + * @s: the seq_buf handle
> + *
> + * This makes sure that the buffer in @s is NUL-terminated, exactly as
> + * seq_buf_str() does, and returns the length of the resulting string
> + * without walking it. Unlike seq_buf_used(), this does not count the byte
> + * given up to the NUL when the buffer is full or has overflowed. When the
> + * buffer is exactly full, that byte is the last one written, and calling
> + * either function loses it.
> + *
> + * After this function is called, s->buffer is safe to use
> + * in string operations.
> + *
> + * Returns: the offset of the NUL that terminates @s->buffer. That is the
> + * length of the string unless an earlier NUL is in the way, either one the
> + * data written to @s carried itself, or one seq_buf_set_overflow() left
> + * behind when it cleared what no writer had claimed.
> + */
> +static inline size_t seq_buf_strlen(struct seq_buf *s)
> +{
> +	if (WARN_ON(s->size == 0))
> +		return 0;

Why WARN_ON()?  Are you wanting to just mint new CVEs with this code
path, do we not give out enough already?  :)

I can see returning 0, if it's empty, but isn't that a valid check for
people to wish to know at times?  Why crash the box?  (remember about
panic-on-warn being enabled in a few billion Linux instances...)

thanks,

greg k-h