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

[Eric Dumazet]

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@xxxxxxxxxx>
> > 
> > __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.





--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/