[PATCH] exec: improve clarity of comment regarding pid save-asides

[Nir Lichtman <nir@lichtman.org>]

Problem: Current comment regarding saving aside the old pid and old vpid
is very vague, especially when considering that it is unexpected that
execve can actually kill the current process.

Solution: Improve the description of the comment explaining more
in-depth the reasoning behind.

Suggested-by: Kees Cook <kees@kernel.org>
Signed-off-by: Nir Lichtman <nir@lichtman.org>
---
 fs/exec.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 506cd411f4ac..343c435b00ee 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1792,7 +1792,14 @@ static int exec_binprm(struct linux_binprm *bprm)
 	pid_t old_pid, old_vpid;
 	int ret, depth;
 
-	/* Need to fetch pid before load_binary changes it */
+	/*
+	 * Need to save aside the current thread pid and vpid
+	 * since if the current thread is not a thread group leader
+	 * the logic in de_thread kills the current thread and all
+	 * other threads in the current thread group, except the leader.
+	 * The new program will execute in the leader, with the leader pid
+	 * ("man 2 clone" CLONE_THREAD flag for more info)
+	 */
 	old_pid = current->pid;
 	rcu_read_lock();
 	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
-- 
2.39.5