linux-kernel - Re: [PATCH] pidfd: getfd should always report ESRCH if a task is exiting

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Message-ID: <20240206192553.GC3593@redhat.com>
Date: Tue, 6 Feb 2024 20:25:54 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Tycho Andersen <tycho@...ho.pizza>
Cc: Christian Brauner <brauner@...nel.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Tycho Andersen <tandersen@...flix.com>
Subject: Re: [PATCH] pidfd: getfd should always report ESRCH if a task is
 exiting

On 02/06, Tycho Andersen wrote:

> On Tue, Feb 06, 2024 at 07:06:07PM +0100, Oleg Nesterov wrote:
> > Or we can check task->files != NULL rather than PF_EXITING.
> >
> > To me this looks even better, but looks more confusing without a comment.
> > OTOH, imo this needs a comment anyway ;)
>
> I thought about this, but I didn't really understand the null check in
> exit_files();

I guess task->files can be NULL at least if it was cloned with
kernel_clone_args->no_files == T

> if it can really be called more than once,

I don't think this is possible. Well, unless the exiting task hits
a BUG() after exit_files() and calls do_exit() recursively.

> are there
> other cases where task->files == NULL that we really should report
> EBADF?

I don't think so...

If nothing else, sys_close() dereferences current->files without any
checks, so I think task->files == NULL is simply impossible if this
task is a userspace process/thread until it exits.

But Tycho, I won't insist. If you prefer to check PF_EXITING, I am fine.

Oleg.