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SubjectRe: [PATCH] fs/file.c: __fget() and dup2() atomicity rules
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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@google.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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