linux-kernel - Re: [PATCH v2 3/3] coredump: hand a pidfd to the usermode coredump helper

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Message-ID: <20250414141450.GE28345@redhat.com>
Date: Mon, 14 Apr 2025 16:14:50 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, Luca Boccassi <luca.boccassi@...il.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Daan De Meyer <daan.j.demeyer@...il.com>,
	Mike Yuan <me@...dnzj.com>,
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] coredump: hand a pidfd to the usermode coredump
 helper

On 04/14, Christian Brauner wrote:
>
> -static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
> +static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
>  {
>  	struct file *files[2];
>  	struct coredump_params *cp = (struct coredump_params *)info->data;
>  	int err;
>
> +	if (cp->pid) {
> +		struct file *pidfs_file __free(fput) = NULL;
> +
> +		pidfs_file = pidfs_alloc_file(cp->pid, 0);
> +		if (IS_ERR(pidfs_file))
> +			return PTR_ERR(pidfs_file);
> +
> +		/*
> +		 * Usermode helpers are childen of either
> +		 * system_unbound_wq or of kthreadd. So we know that
> +		 * we're starting off with a clean file descriptor
> +		 * table. So we should always be able to use
> +		 * COREDUMP_PIDFD_NUMBER as our file descriptor value.
> +		 */
> +		VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
> +
> +		err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
> +		if (err < 0)
> +			return err;

Yes, but if replace_fd() succeeds we need to nullify pidfs_file
to avoid fput from __free(fput) ?

And I think in this case __free(fput) doesn't buy too much, but
up to you.

Oleg.