linux-kernel - Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

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Message-ID: <CAKgNAkisD7zDRoqJd6Gk1JMCZ8+Huj5QPV04nh2JXHMA+_R0-A@mail.gmail.com>
Date:   Wed, 20 Dec 2017 17:17:46 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Linux API <linux-api@...r.kernel.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Waychison <mikew@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

Hello Michal,

On 20 December 2017 at 10:20, Michal Hocko <mhocko@...nel.org> wrote:
> On Tue 19-12-17 17:45:40, Michael Kerrisk wrote:
>> But, is
>> there a pressing reason to make the change? (Okay, I guess iterating
>> using *_STAT is nicer than parsing /proc/sysvipc/*.)
>
> The reporter of this issue claims that "Reading /proc/sysvipc/shm is way
> slower than executing the system call." I haven't checked that but I can
> imagine that /proc/sysvipc/shm can take quite some time when there are
> _many_ segments registered.

Yes, that makes sense.

> So they would like to use the syscall but
> the interacting parties do not have compatible permissions.

So, I don't think there is any security issue, since the same info is
available in /proc/sysvipc/*. The only question would be whether
change in the *_STAT behavior might surprise some applications into
behaving differently. I presume the chances of that are low, but if it
was a concert, one could add new shmctl/msgctl/semctl *_STAT_ALL (or
some such) operations that have the desired behavior.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/