linux-kernel - Re: [PATCH 1/2] fcntl: add fcntl(F_CHECK_ORIGINAL

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Message-ID: <20230901-lockt-erbfolge-e1f9a26f0d63@brauner>
Date:   Fri, 1 Sep 2023 14:56:14 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Michal Clapinski <mclapinski@...gle.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jeff Xu <jeffxu@...gle.com>, Aleksa Sarai <cyphar@...har.com>,
        Daniel Verkamp <dverkamp@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] fcntl: add fcntl(F_CHECK_ORIGINAL_MEMFD)

On Thu, Aug 31, 2023 at 10:36:46PM +0200, Michal Clapinski wrote:
> Add a way to check if an fd points to the memfd's original open fd
> (the one created by memfd_create).
> Useful because only the original open fd can be both writable and
> executable.
> 
> Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
> ---
>  fs/fcntl.c                 | 3 +++
>  include/uapi/linux/fcntl.h | 9 +++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index e871009f6c88..301527e07a4d 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -419,6 +419,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>  	case F_SET_RW_HINT:
>  		err = fcntl_rw_hint(filp, cmd, arg);
>  		break;
> +	case F_CHECK_ORIGINAL_MEMFD:
> +		err = !(filp->f_mode & FMODE_WRITER);
> +		break;

Honestly, make this an ioctl on memfds. This is so specific that it
really doesn't belong into fcntl().