From 8cf4b738f6d84fdd8d7ff1e8d0e2298ded3e4153 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 3 Apr 2025 16:09:03 +0200 Subject: [PATCH] pidfd: improve uapi when task isn't found We currently report EINVAL whenever a struct pid has no tasked attached anymore thereby conflating two concepts: (1) The task has already been reaped. (2) The caller requested a pidfd for a thread-group leader but the pid actually references a struct pid that isn't used as a thread-group leader. This is causing issues for non-threaded workloads as in [1]. This patch tries to allow userspace to distinguish between (1) and (2). This is racy of course but that shouldn't matter. Link: https://github.com/systemd/systemd/pull/36982 [1] Link: https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-3-a123b6ed6716@kernel.org Reviewed-by: Oleg Nesterov Signed-off-by: Christian Brauner --- kernel/fork.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 182ec2e9087d..4a2080b968c8 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2108,10 +2108,27 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re */ int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret) { - bool thread = flags & PIDFD_THREAD; + int err = 0; - if (!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID)) - return -EINVAL; + if (!(flags & PIDFD_THREAD)) { + /* + * If this is struct pid isn't used as a thread-group + * leader pid but the caller requested to create a + * thread-group leader pidfd then report ENOENT to the + * caller as a hint. + */ + if (!pid_has_task(pid, PIDTYPE_TGID)) + err = -ENOENT; + } + + /* + * If this wasn't a thread-group leader struct pid or the task + * got reaped in the meantime report -ESRCH to userspace. + */ + if (!pid_has_task(pid, PIDTYPE_PID)) + err = -ESRCH; + if (err) + return err; return __pidfd_prepare(pid, flags, ret); } -- 2.25.1