[v4,01/11] fs/exec: Drop task_lock() inside __get_task_comm()

Commit Message

Yafang Shao June 28, 2024, 9:05 a.m. UTC

Quoted from Linus [0]:

  Since user space can randomly change their names anyway, using locking
  was always wrong for readers (for writers it probably does make sense
  to have some lock - although practically speaking nobody cares there
  either, but at least for a writer some kind of race could have
  long-term mixed results

Suggested-by: Linus Torvalds <[e-mail]>
Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=[e-mail] [0]
Signed-off-by: Yafang Shao <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Eric Biederman <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Matus Jokay <[e-mail]>
---
 fs/exec.c             | 10 ++++++++--
 include/linux/sched.h |  4 ++--
 2 files changed, 10 insertions(+), 4 deletions(-)

Patch

diff --git a/fs/exec.c b/fs/exec.c
index 40073142288f..fa6b61c79df8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1238,12 +1238,18 @@  static int unshare_sighand(struct task_struct *me)
 	return 0;
 }
 
+/*
+ * User space can randomly change their names anyway, so locking for readers
+ * doesn't make sense. For writers, locking is probably necessary, as a race
+ * condition could lead to long-term mixed results.
+ * The strscpy_pad() in __set_task_comm() can ensure that the task comm is
+ * always NUL-terminated. Therefore the race condition between reader and writer
+ * is not an issue.
+ */
 char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
 {
-	task_lock(tsk);
 	/* Always NUL terminated and zero-padded */
 	strscpy_pad(buf, tsk->comm, buf_size);
-	task_unlock(tsk);
 	return buf;
 }
 EXPORT_SYMBOL_GPL(__get_task_comm);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 61591ac6eab6..95888d1da49e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1086,9 +1086,9 @@  struct task_struct {
 	/*
 	 * executable name, excluding path.
 	 *
-	 * - normally initialized setup_new_exec()
+	 * - normally initialized begin_new_exec()
 	 * - access it with [gs]et_task_comm()
-	 * - lock it with task_lock()
+	 * - lock it with task_lock() for writing
 	 */
 	char				comm[TASK_COMM_LEN];