linux-kernel - Re: [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation

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Message-ID: <CAK8P3a09h4jhJWckxVUMYLoUp8=vAJ5NXuMTzSmghRxuk2_PTQ@mail.gmail.com>
Date:   Thu, 23 Apr 2020 11:24:49 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     io-uring@...r.kernel.org,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Aleksa Sarai <cyphar@...har.com>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest
 available fd" allocation

On Wed, Apr 22, 2020 at 7:19 AM Josh Triplett <josh@...htriplett.org> wrote:
>
> Some applications want to prevent the usual "lowest available fd"
> allocation from allocating certain file descriptors. For instance, they
> may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> dup2/dup3, or reserve some low file descriptors for other purposes.
>
> Add a prctl to increase the minimum fd and return the previous minimum.
>
> System calls that allocate a specific file descriptor, such as
> dup2/dup3, ignore this minimum.
>
> exec resets the minimum fd, to prevent one program from interfering with
> another program's expectations about fd allocation.

Have you considered making this a separate system call rather than
a part of prctl()?

At the moment, there are certain classes of things controlled by prctl,
e.g. capabilities, floating point handling and timer behavior, but nothing
that relates to file descriptors as such, so it's not an obvious decision.

Another option would be prlimit(), as it already controls the maximum
file descriptor number with RLIMIT_NOFILE, and adding a minimum
there would let you set min/max atomically.

     Arnd