[PATCH RFC v2 02/11] stackdepot: add caller-owned stack trace fetching

[Caleb Kan <calebkan1106@gmail.com>]

From: Caleb Kan <ckan@cloudflare.com>

stack_depot_fetch() returns a pointer to contiguous storage owned by stack
depot. That cannot work for a backend whose frames are not contiguous, so
callers need an interface that copies the trace before they can support
both backends.

Add stack_depot_fetch_into() to copy a complete trace into caller-owned
storage. Leave the destination unchanged when it is too small, keep a zero
handle as a no-op, and document that callers must keep the handle valid
while copying it. Warn if a caller passes a NULL buffer or zero capacity
for a valid handle instead of treating the stack as missing.

Unpoison the copied entries before returning them because
lib/stackdepot.c is not instrumented by KMSAN. Add built-in KUnit tests for
exact and oversized destinations, zero handles, and undersized buffers.
Later patches add tests for stack depot internals.

Signed-off-by: Caleb Kan <ckan@cloudflare.com>
---
 include/linux/stackdepot.h   | 36 ++++++++++++++++++
 lib/Kconfig.debug            | 16 ++++++++
 lib/stackdepot.c             | 28 ++++++++++++++
 lib/tests/Makefile           |  1 +
 lib/tests/stackdepot_kunit.c | 89 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 170 insertions(+)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index 2cc21ffcdaf9..734529767c8a 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -199,6 +199,42 @@ struct stack_record *__stack_depot_get_stack_record(depot_stack_handle_t handle)
 unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 			       unsigned long **entries);
 
+/**
+ * stack_depot_fetch_into - Fetch a stack trace into caller-owned storage
+ *
+ * @handle:	Stack depot handle
+ * @entries:	Caller-owned buffer to copy the stack trace into
+ * @max_entries:	Number of frames that fit in @entries
+ *
+ * Copies the stored frames into caller-owned @entries. If fewer frames are
+ * stored than @max_entries, only the stored frames are written and their count
+ * is returned. If more frames are stored than @max_entries, the copy is skipped
+ * entirely and 0 is returned.
+ *
+ * Passing a NULL @entries buffer or zero @max_entries for a valid @handle is
+ * invalid. Callers must provide storage for @max_entries frames.
+ *
+ * Callers should size @entries to match the save-side stack depth cap (for
+ * example, %CONFIG_STACKDEPOT_MAX_FRAMES or the local stack_trace_save() limit)
+ * when losing diagnostics on an undersized buffer would be surprising.
+ *
+ * A non-zero invalid @handle, including a post-put handle, may WARN. Its return
+ * value and copied contents are undefined because the record may have been
+ * reused for another stack.
+ *
+ * Callers must ensure @handle remains valid for the duration of this call.
+ * Persistent handles saved without %STACK_DEPOT_FLAG_GET require no extra
+ * reference; handles saved with %STACK_DEPOT_FLAG_GET require a held reference.
+ * Callers must not call stack_depot_put() on persistent handles.
+ * Racing this helper with stack_depot_put() on the same handle is invalid.
+ *
+ * Return: Number of frames copied, 0 if @handle is 0, stack depot is disabled,
+ * or @max_entries is less than the number of stored frames.
+ */
+unsigned int stack_depot_fetch_into(depot_stack_handle_t handle,
+				    unsigned long *entries,
+				    unsigned int max_entries);
+
 /**
  * stack_depot_print - Print a stack trace from stack depot
  *
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 134b15a44625..fcd74edfd93a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2785,6 +2785,22 @@ config RESOURCE_KUNIT_TEST
 
 	  If unsure, say N.
 
+config STACKDEPOT_KUNIT_TEST
+	bool "KUnit test for stack depot" if !KUNIT_ALL_TESTS
+	depends on KUNIT=y && STACKDEPOT
+	depends on STACKDEPOT_MAX_FRAMES >= 3
+	default KUNIT_ALL_TESTS
+	help
+	  Enable this option to test stack depot API behavior at boot.
+	  This test is built in, so KUNIT must also be built in.
+
+	  KUnit tests run during boot and output the results to the debug log
+	  in TAP format (https://testanything.org/). Only useful for kernel
+	  developers running the KUnit test harness, and not intended for
+	  inclusion into a production build.
+
+	  If unsure, say N.
+
 config SYSCTL_KUNIT_TEST
 	tristate "KUnit test for sysctl" if !KUNIT_ALL_TESTS
 	depends on KUNIT
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 90c52f2e0d3f..4da7279d9f83 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -785,6 +785,34 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 }
 EXPORT_SYMBOL_GPL(stack_depot_fetch);
 
+unsigned int stack_depot_fetch_into(depot_stack_handle_t handle,
+				    unsigned long *entries,
+				    unsigned int max_entries)
+{
+	struct stack_record *stack;
+	unsigned int nr_entries;
+
+	if (!handle)
+		return 0;
+	if (stack_depot_disabled)
+		return 0;
+	WARN_ON_ONCE(!entries || !max_entries);
+
+	stack = depot_fetch_stack(handle);
+	if (!stack)
+		return 0;
+	nr_entries = stack->size;
+	if (WARN_ON_ONCE(!nr_entries))
+		return 0;
+	if (nr_entries > max_entries)
+		return 0;
+
+	memcpy(entries, stack->entries, nr_entries * sizeof(*entries));
+	kmsan_unpoison_memory(entries, nr_entries * sizeof(*entries));
+	return nr_entries;
+}
+EXPORT_SYMBOL_GPL(stack_depot_fetch_into);
+
 void stack_depot_put(depot_stack_handle_t handle)
 {
 	struct stack_record *stack;
diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 3cac3b63a752..1f72191f98bb 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -48,6 +48,7 @@ obj-$(CONFIG_SCANF_KUNIT_TEST) += scanf_kunit.o
 obj-$(CONFIG_SEQ_BUF_KUNIT_TEST) += seq_buf_kunit.o
 obj-$(CONFIG_SIPHASH_KUNIT_TEST) += siphash_kunit.o
 obj-$(CONFIG_SLUB_KUNIT_TEST) += slub_kunit.o
+obj-$(CONFIG_STACKDEPOT_KUNIT_TEST) += stackdepot_kunit.o
 obj-$(CONFIG_TEST_SORT) += test_sort.o
 CFLAGS_stackinit_kunit.o += $(call cc-disable-warning, switch-unreachable)
 obj-$(CONFIG_STACKINIT_KUNIT_TEST) += stackinit_kunit.o
diff --git a/lib/tests/stackdepot_kunit.c b/lib/tests/stackdepot_kunit.c
new file mode 100644
index 000000000000..b0c44c096976
--- /dev/null
+++ b/lib/tests/stackdepot_kunit.c
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <kunit/test.h>
+#include <linux/array_size.h>
+#include <linux/gfp.h>
+#include <linux/stackdepot.h>
+#include <linux/string.h>
+
+static void stackdepot_fetch_into_roundtrip(struct kunit *test)
+{
+	unsigned long entries[] = {
+		0x101000UL,
+		0x102000UL,
+		0x103000UL,
+	};
+	unsigned long exact[ARRAY_SIZE(entries)] = {};
+	unsigned long fetched[ARRAY_SIZE(entries) + 1] = {
+		[ARRAY_SIZE(entries)] = 0xa5a5a5a5UL,
+	};
+	unsigned long expected_tail = fetched[ARRAY_SIZE(entries)];
+	depot_stack_handle_t handle;
+	unsigned int nr_entries;
+
+	KUNIT_ASSERT_EQ(test, stack_depot_init(), 0);
+
+	handle = stack_depot_save(entries, ARRAY_SIZE(entries), GFP_KERNEL);
+	KUNIT_ASSERT_NE(test, handle, (depot_stack_handle_t)0);
+
+	nr_entries = stack_depot_fetch_into(handle, exact, ARRAY_SIZE(exact));
+	KUNIT_EXPECT_EQ(test, nr_entries, (unsigned int)ARRAY_SIZE(entries));
+	KUNIT_EXPECT_MEMEQ(test, exact, entries, sizeof(entries));
+
+	nr_entries = stack_depot_fetch_into(handle, fetched, ARRAY_SIZE(fetched));
+	KUNIT_EXPECT_EQ(test, nr_entries, (unsigned int)ARRAY_SIZE(entries));
+	KUNIT_EXPECT_MEMEQ(test, fetched, entries, sizeof(entries));
+	KUNIT_EXPECT_EQ(test, fetched[ARRAY_SIZE(entries)], expected_tail);
+}
+
+static void stackdepot_fetch_into_rejects_missing_or_short_stack(struct kunit *test)
+{
+	unsigned long entries[] = {
+		0x111000UL,
+		0x112000UL,
+		0x113000UL,
+	};
+	unsigned long fetched[ARRAY_SIZE(entries)] = {
+		0xa1a1a1a1UL,
+		0xb2b2b2b2UL,
+		0xc3c3c3c3UL,
+	};
+	unsigned long expected[ARRAY_SIZE(fetched)];
+	depot_stack_handle_t handle;
+	unsigned int nr_entries;
+
+	KUNIT_ASSERT_EQ(test, stack_depot_init(), 0);
+
+	handle = stack_depot_save(entries, ARRAY_SIZE(entries), GFP_KERNEL);
+	KUNIT_ASSERT_NE(test, handle, (depot_stack_handle_t)0);
+	memcpy(expected, fetched, sizeof(expected));
+
+	nr_entries = stack_depot_fetch_into(0, fetched, ARRAY_SIZE(fetched));
+	KUNIT_EXPECT_EQ(test, nr_entries, 0U);
+	KUNIT_EXPECT_MEMEQ(test, fetched, expected, sizeof(expected));
+
+	nr_entries = stack_depot_fetch_into(0, NULL, 0);
+	KUNIT_EXPECT_EQ(test, nr_entries, 0U);
+
+	nr_entries = stack_depot_fetch_into(handle, fetched,
+					    ARRAY_SIZE(fetched) - 1);
+	KUNIT_EXPECT_EQ(test, nr_entries, 0U);
+	KUNIT_EXPECT_MEMEQ(test, fetched, expected, sizeof(expected));
+}
+
+static struct kunit_case stackdepot_test_cases[] = {
+	KUNIT_CASE(stackdepot_fetch_into_roundtrip),
+	KUNIT_CASE(stackdepot_fetch_into_rejects_missing_or_short_stack),
+	{}
+};
+
+static struct kunit_suite stackdepot_test_suite = {
+	.name = "stackdepot",
+	.test_cases = stackdepot_test_cases,
+};
+
+kunit_test_suite(stackdepot_test_suite);
+
+MODULE_DESCRIPTION("KUnit tests for stack depot");
+MODULE_AUTHOR("Caleb Kan <ckan@cloudflare.com>");
+MODULE_LICENSE("GPL");

-- 
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