linux-kernel - Re: [PATCH] fs/file.c: __fget() and dup2() atomicity rules

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Message-ID: <1435640524.4110.116.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 30 Jun 2015 07:02:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs/file.c: __fget() and dup2() atomicity rules

On Mon, 2015-06-29 at 18:46 +0100, Al Viro wrote:
> On Mon, Jun 29, 2015 at 05:10:30PM +0200, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > __fget() makes sure a file refcount is not zero before
> > taking a reference. It should also fetch again file pointer
> > in order to respect dup2() atomicity requirements.
> > 
> > It should either read a NULL pointer or a file on which
> > a refcount can be taken.
> 
> Hmm...   The problem is real, but I wonder if one could trigger a long
> spin there...

I do not believe we can spin a long time.

By the time do_dup2() calls filp_close(tofree, files), we already have a
stable fdt->fd[fd], because of spin_unlock(&files->file_lock) was called
before filp_close()

A loop would require threads doing dup2() calls like crazy on same
destination fd.





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