linux-kernel - [PATCH] pidfs: never refuse ppid == 0 in PIDFD_GET

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Message-ID: <20250604150238.42664-1-me@yhndnzj.com>
Date: Wed, 04 Jun 2025 15:03:42 +0000
From: Mike Yuan <me@...dnzj.com>
To: linux-fsdevel@...r.kernel.org
Cc: Mike Yuan <me@...dnzj.com>, linux-kernel@...r.kernel.org, Christian Brauner <brauner@...nel.org>, Luca Boccassi <luca.boccassi@...il.com>, stable@...r.kernel.org
Subject: [PATCH] pidfs: never refuse ppid == 0 in PIDFD_GET_INFO

In systemd we spotted an issue after switching to ioctl(PIDFD_GET_INFO)
for obtaining pid number the pidfd refers to, that for processes
with a parent from outer pidns PIDFD_GET_INFO unexpectedly yields
-ESRCH [1]. It turned out that there's an arbitrary check blocking
this, which is not really sensible given getppid() happily returns
0 for such processes. Just drop the spurious check and userspace
ought to handle ppid == 0 properly everywhere.

[1] https://github.com/systemd/systemd/issues/37715

Fixes: cdda1f26e74b ("pidfd: add ioctl to retrieve pid info")
Signed-off-by: Mike Yuan <me@...dnzj.com>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Luca Boccassi <luca.boccassi@...il.com>
Cc: <stable@...r.kernel.org>
---
 fs/pidfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index c1f0a067be40..69919be1c9d8 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -366,7 +366,7 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 	kinfo.pid = task_pid_vnr(task);
 	kinfo.mask |= PIDFD_INFO_PID;
 
-	if (kinfo.pid == 0 || kinfo.tgid == 0 || (kinfo.ppid == 0 && kinfo.pid != 1))
+	if (kinfo.pid == 0 || kinfo.tgid == 0)
 		return -ESRCH;
 
 copy_out:

base-commit: 5abc7438f1e9d62e91ad775cc83c9594c48d2282
-- 
2.49.0