linux-kernel - Re: [PATCH 1/2] io_uring: fix big-endian compat signal mask handling

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Date:   Mon, 25 Mar 2019 17:13:05 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hannes Reinecke <hare@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Nikolay Borisov <nborisov@...e.com>,
        linux-block <linux-block@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] io_uring: fix big-endian compat signal mask handling

On Mon, 2019-03-25 at 17:24 +0100, Arnd Bergmann wrote:
> On Mon, Mar 25, 2019 at 5:19 PM James Bottomley
> <James.Bottomley@...senpartnership.com> wrote:
> 
> > > --- a/fs/io_uring.c
> > > +++ b/fs/io_uring.c
> > > @@ -1968,7 +1968,15 @@ static int io_cqring_wait(struct
> > > io_ring_ctx
> > > *ctx, int min_events,
> > >               return 0;
> > > 
> > >       if (sig) {
> > > -             ret = set_user_sigmask(sig, &ksigmask, &sigsaved,
> > > sigsz);
> > > +#ifdef CONFIG_COMPAT
> > > +             if (in_compat_syscall())
> > > +                     ret = set_compat_user_sigmask((const
> > > compat_sigset_t __user *)sig,
> > > +                                                   &ksigmask,
> > > &sigsaved, sigsz);
> > > +             else
> > > +#endif
> > 
> > This looks a bit suboptimal: shouldn't in_compat_syscall() be hard
> > coded to return 0 if CONFIG_COMPAT isn't defined?  That way the
> > compiler can do the correct optimization and we don't have to
> > litter #ifdefs and worry about undefined variables and other
> > things.
> 
> The check can be outside of the #ifdef, but set_compat_user_sigmask
> is not declared then.

Right, but shouldn't it be declared?  I thought BUILD_BUG_ON had nice
magic that allowed it to work here (meaning if the compiler doesn't
eliminate the branch we get a build bug).

> I think for the future we can consider just moving the compat logic
> into set_user_sigmask(), which would simplify most of the callers,
> but that seemed to invasive as a bugfix for 5.1.

Well, that too.  I've just been on a recent bender to stop #ifdefs
after I saw what some people were doing with them.

James