[PATCH 1/2] proc: add tgid_iter.pid_ns member

[PATCH 1/2] proc: add tgid_iter.pid_ns member

[Alexey Dobriyan]

next_tgid() accept pid namespace as an argument, but it is never changes
during readdir (which would be unthinkable thing to do anyway).

Move it inside iterator type and hide from using directly.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 fs/proc/base.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 4eec684baca9..7c1089226a47 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3540,8 +3540,10 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
 struct tgid_iter {
 	unsigned int tgid;
 	struct task_struct *task;
+	struct pid_namespace *pid_ns;
 };
-static struct tgid_iter next_tgid(struct pid_namespace *ns, struct tgid_iter iter)
+
+static struct tgid_iter next_tgid(struct tgid_iter iter)
 {
 	struct pid *pid;
 
@@ -3550,9 +3552,9 @@ static struct tgid_iter next_tgid(struct pid_namespace *ns, struct tgid_iter ite
 	rcu_read_lock();
 retry:
 	iter.task = NULL;
-	pid = find_ge_pid(iter.tgid, ns);
+	pid = find_ge_pid(iter.tgid, iter.pid_ns);
 	if (pid) {
-		iter.tgid = pid_nr_ns(pid, ns);
+		iter.tgid = pid_nr_ns(pid, iter.pid_ns);
 		iter.task = pid_task(pid, PIDTYPE_TGID);
 		if (!iter.task) {
 			iter.tgid += 1;
@@ -3571,7 +3573,7 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct tgid_iter iter;
 	struct proc_fs_info *fs_info = proc_sb_info(file_inode(file)->i_sb);
-	struct pid_namespace *ns = proc_pid_ns(file_inode(file)->i_sb);
+	struct pid_namespace *pid_ns = proc_pid_ns(file_inode(file)->i_sb);
 	loff_t pos = ctx->pos;
 
 	if (pos >= PID_MAX_LIMIT + TGID_OFFSET)
@@ -3589,9 +3591,10 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
 	}
 	iter.tgid = pos - TGID_OFFSET;
 	iter.task = NULL;
-	for (iter = next_tgid(ns, iter);
+	iter.pid_ns = pid_ns;
+	for (iter = next_tgid(iter);
 	     iter.task;
-	     iter.tgid += 1, iter = next_tgid(ns, iter)) {
+	     iter.tgid += 1, iter = next_tgid(iter)) {
 		char name[10 + 1];
 		unsigned int len;
 
-- 
2.51.2

[PATCH 1/1] proc: test lseek on /proc/net/dev

[Alexey Dobriyan]

---
 tools/testing/selftests/proc/.gitignore       |  1 +
 tools/testing/selftests/proc/Makefile         |  1 +
 .../selftests/proc/proc-net-dev-lseek.c       | 68 +++++++++++++++++++
 3 files changed, 70 insertions(+)
 create mode 100644 tools/testing/selftests/proc/proc-net-dev-lseek.c

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 19bb333e2485..243f4537a670 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -7,6 +7,7 @@
 /proc-loadavg-001
 /proc-maps-race
 /proc-multiple-procfs
+/proc-net-dev-lseek
 /proc-empty-vm
 /proc-pid-vm
 /proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index 50aba102201a..2a9547630115 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -10,6 +10,7 @@ TEST_GEN_PROGS += fd-003-kthread
 TEST_GEN_PROGS += proc-2-is-kthread
 TEST_GEN_PROGS += proc-loadavg-001
 TEST_GEN_PROGS += proc-maps-race
+TEST_GEN_PROGS += proc-net-dev-lseek
 TEST_GEN_PROGS += proc-empty-vm
 TEST_GEN_PROGS += proc-pid-vm
 TEST_GEN_PROGS += proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/proc-net-dev-lseek.c b/tools/testing/selftests/proc/proc-net-dev-lseek.c
new file mode 100644
index 000000000000..742a3e804451
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-net-dev-lseek.c
@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2025 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#undef _GNU_SOURCE
+#define _GNU_SOURCE
+#undef NDEBUG
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <unistd.h>
+#include <sched.h>
+/*
+ * Test that lseek("/proc/net/dev/", 0, SEEK_SET)
+ * a) works,
+ * b) does what you think it does.
+ */
+int main(void)
+{
+	/* /proc/net/dev output is deterministic in fresh netns only. */
+	if (unshare(CLONE_NEWNET) == -1) {
+		if (errno == ENOSYS || errno == EPERM) {
+			return 4;
+		}
+		return 1;
+	}
+
+	const int fd = open("/proc/net/dev", O_RDONLY);
+	assert(fd >= 0);
+
+	char buf1[4096];
+	const ssize_t rv1 = read(fd, buf1, sizeof(buf1));
+	/*
+	 * Not "<=", this file can't be empty:
+	 * there is header, "lo" interface with some zeroes.
+	 */
+	assert(0 < rv1);
+	assert(rv1 <= sizeof(buf1));
+
+	/* Believe it or not, this line broke one day. */
+	assert(lseek(fd, 0, SEEK_SET) == 0);
+
+	char buf2[4096];
+	const ssize_t rv2 = read(fd, buf2, sizeof(buf2));
+	/* Not "<=", see above. */
+	assert(0 < rv2);
+	assert(rv2 <= sizeof(buf2));
+
+	/* Test that lseek rewinds to the beginning of the file. */
+	assert(rv1 == rv2);
+	assert(memcmp(buf1, buf2, rv1) == 0);
+
+	/* Contents of the file is not validated: this test is about lseek(). */
+
+	return 0;
+}
-- 
2.49.1

[PATCH 2/2] proc: rewrite next_tgid()

[Alexey Dobriyan]

* deduplicate "iter.tgid += 1" line,
  it is done once inside next_tgid() itself and second time inside
  "for" loop

* deduplicate next_tgid() call itself with different loop style:

	auto it = make_xxx_iter();
	while (next_xxx(&it)) {
	}

  gcc seems to inline it twice in the original code:

  	$ ./scripts/bloat-o-meter ../vmlinux-000 ../obj/vmlinux
	add/remove: 0/1 grow/shrink: 1/0 up/down: 100/-245 (-145)
	Function                                     old     new   delta
	proc_pid_readdir                             531     631    +100
	next_tgid                                    245       -    -245

But if there is only one call, it doesn't matter if it is inlined or not!

* make tgid_iter.pid_ns const
  it never changes during readdir, returning instance + C99 initializer
  make it possible,

* rename "iter" to "it", this is what another language seems to be doing.

* limit declaration scope to prevent problems (in general).
---
 fs/proc/base.c | 69 ++++++++++++++++++++++++++++----------------------
 1 file changed, 39 insertions(+), 30 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 7c1089226a47..ddf5e16c795b 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3535,35 +3535,48 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
 
 /*
  * Find the first task with tgid >= tgid
- *
  */
 struct tgid_iter {
 	unsigned int tgid;
 	struct task_struct *task;
-	struct pid_namespace *pid_ns;
+	struct pid_namespace *const pid_ns;
 };
 
-static struct tgid_iter next_tgid(struct tgid_iter iter)
+static
+struct tgid_iter
+make_tgid_iter(unsigned int init_tgid, struct pid_namespace *pid_ns)
 {
-	struct pid *pid;
+	return (struct tgid_iter){
+		/* See preincrement below. */
+		.tgid = init_tgid - 1,
+		.pid_ns = pid_ns,
+	};
+}
+
+static bool next_tgid(struct tgid_iter *it)
+{
+	if (it->task) {
+		put_task_struct(it->task);
+		it->task = NULL;
+	}
 
-	if (iter.task)
-		put_task_struct(iter.task);
 	rcu_read_lock();
-retry:
-	iter.task = NULL;
-	pid = find_ge_pid(iter.tgid, iter.pid_ns);
-	if (pid) {
-		iter.tgid = pid_nr_ns(pid, iter.pid_ns);
-		iter.task = pid_task(pid, PIDTYPE_TGID);
-		if (!iter.task) {
-			iter.tgid += 1;
-			goto retry;
+	while (1) {
+		it->tgid += 1;
+		struct pid *pid = find_ge_pid(it->tgid, it->pid_ns);
+		if (pid) {
+			it->tgid = pid_nr_ns(pid, it->pid_ns);
+			it->task = pid_task(pid, PIDTYPE_TGID);
+			if (it->task) {
+				get_task_struct(it->task);
+				rcu_read_unlock();
+				return true;
+			}
+		} else {
+			rcu_read_unlock();
+			return false;
 		}
-		get_task_struct(iter.task);
 	}
-	rcu_read_unlock();
-	return iter;
 }
 
 #define TGID_OFFSET (FIRST_PROCESS_ENTRY + 2)
@@ -3571,7 +3584,6 @@ static struct tgid_iter next_tgid(struct tgid_iter iter)
 /* for the /proc/ directory itself, after non-process stuff has been done */
 int proc_pid_readdir(struct file *file, struct dir_context *ctx)
 {
-	struct tgid_iter iter;
 	struct proc_fs_info *fs_info = proc_sb_info(file_inode(file)->i_sb);
 	struct pid_namespace *pid_ns = proc_pid_ns(file_inode(file)->i_sb);
 	loff_t pos = ctx->pos;
@@ -3589,24 +3601,21 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
 			return 0;
 		ctx->pos = pos = pos + 1;
 	}
-	iter.tgid = pos - TGID_OFFSET;
-	iter.task = NULL;
-	iter.pid_ns = pid_ns;
-	for (iter = next_tgid(iter);
-	     iter.task;
-	     iter.tgid += 1, iter = next_tgid(iter)) {
+
+	auto it = make_tgid_iter(pos - TGID_OFFSET, pid_ns);
+	while (next_tgid(&it)) {
 		char name[10 + 1];
 		unsigned int len;
 
 		cond_resched();
-		if (!has_pid_permissions(fs_info, iter.task, HIDEPID_INVISIBLE))
+		if (!has_pid_permissions(fs_info, it.task, HIDEPID_INVISIBLE))
 			continue;
 
-		len = snprintf(name, sizeof(name), "%u", iter.tgid);
-		ctx->pos = iter.tgid + TGID_OFFSET;
+		len = snprintf(name, sizeof(name), "%u", it.tgid);
+		ctx->pos = it.tgid + TGID_OFFSET;
 		if (!proc_fill_cache(file, ctx, name, len,
-				     proc_pid_instantiate, iter.task, NULL)) {
-			put_task_struct(iter.task);
+				     proc_pid_instantiate, it.task, NULL)) {
+			put_task_struct(it.task);
 			return 0;
 		}
 	}
-- 
2.51.2