linux-kernel - Re: [PATCH] proc: nommu: /proc/<pid>/maps: release mmap read lock

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Message-ID: <ZQNDHXyl8c6YZ4Q6@dell-precision-5540>
Date:   Thu, 14 Sep 2023 13:30:05 -0400
From:   Ben Wolsieffer <ben.wolsieffer@...ring.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Greg Ungerer <gerg@...inux.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Giulio Benetti <giulio.benetti@...ettiengineering.com>
Subject: Re: [PATCH] proc: nommu: /proc/<pid>/maps: release mmap read lock

On Thu, Sep 14, 2023 at 10:02:03AM -0700, Andrew Morton wrote:
> On Thu, 14 Sep 2023 12:30:20 -0400 Ben Wolsieffer <ben.wolsieffer@...ring.com> wrote:
> 
> > The no-MMU implementation of /proc/<pid>/map doesn't normally release
> > the mmap read lock, because it uses !IS_ERR_OR_NULL(_vml) to determine
> > whether to release the lock. Since _vml is NULL when the end of the
> > mappings is reached, the lock is not released.
> > 
> 
> Thanks.  Is this bug demonstrable from userspace?  If so, how?

Yes, run "cat /proc/1/maps" twice. You should observe that the
second run hangs.