[PATCH v3 1/2] exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case

[Tycho Andersen <tycho@tycho.pizza>]

From: Tycho Andersen <tandersen@netflix.com>

Zbigniew mentioned at Linux Plumber's that systemd is interested in
switching to execveat() for service execution, but can't, because the
contents of /proc/pid/comm are the file descriptor which was used,
instead of the path to the binary. This makes the output of tools like
top and ps useless, especially in a world where most fds are opened
CLOEXEC so the number is truly meaningless.

Change exec path to fix up /proc/pid/comm in the case where we have
allocated one of these synthetic paths in bprm_init(). This way the actual
exec machinery is unchanged, but cosmetically the comm looks reasonable to
admins investigating things.

Signed-off-by: Tycho Andersen <tandersen@netflix.com>
Suggested-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
CC: Aleksa Sarai <cyphar@cyphar.com>
Link: https://github.com/uapi-group/kernel-features#set-comm-field-before-exec
---
v2: * drop the flag, everyone :)
    * change the rendered value to f_path.dentry->d_name.name instead of
      argv[0], Eric
v3: * fix up subject line, Eric
---
 fs/exec.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index dad402d55681..9520359a8dcc 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1416,7 +1416,18 @@ int begin_new_exec(struct linux_binprm * bprm)
 		set_dumpable(current->mm, SUID_DUMP_USER);
 
 	perf_event_exec();
-	__set_task_comm(me, kbasename(bprm->filename), true);
+
+	/*
+	 * If fdpath was set, execveat() made up a path that will
+	 * probably not be useful to admins running ps or similar.
+	 * Let's fix it up to be something reasonable.
+	 */
+	if (bprm->fdpath) {
+		BUILD_BUG_ON(TASK_COMM_LEN > DNAME_INLINE_LEN);
+		__set_task_comm(me, bprm->file->f_path.dentry->d_name.name, true);
+	} else {
+		__set_task_comm(me, kbasename(bprm->filename), true);
+	}
 
 	/* An exec changes our domain. We are no longer part of the thread
 	   group */

base-commit: baeb9a7d8b60b021d907127509c44507539c15e5
-- 
2.34.1