Received-SPF: pass (google.com: domain of linux-kernel+bounces-62524-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Feedback-ID: i21f147d5:Fastmail
Date: Mon, 12 Feb 2024 16:14:45 -0700
From: Tycho Andersen <tycho@tycho.pizza>
To: Kees Cook <keescook@chromium.org>
Cc: Arnd Bergmann <arnd@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] samples: user-trap: fix strict-aliasing warning
Message-ID: <ZcqmZdZ6x5w54nTI@tycho.pizza>
References: <20240212111737.917428-1-arnd@kernel.org>
 <170776334482.2557794.95461406898503535.b4-ty@chromium.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <170776334482.2557794.95461406898503535.b4-ty@chromium.org>

On Mon, Feb 12, 2024 at 10:42:27AM -0800, Kees Cook wrote:
> On Mon, 12 Feb 2024 12:17:31 +0100, Arnd Bergmann wrote:
> > I started getting warnings for this one file, though I can't see what changed
> > since it was originally introduced in commit fec7b6690541 ("samples: add an
> > example of seccomp user trap").
> > 
> > samples/seccomp/user-trap.c: In function 'send_fd':
> > samples/seccomp/user-trap.c:50:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >    50 |         *((int *)CMSG_DATA(cmsg)) = fd;
> >       |          ~^~~~~~~~~~~~~~~~~~~~~~~
> > samples/seccomp/user-trap.c: In function 'recv_fd':
> > samples/seccomp/user-trap.c:83:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >    83 |         return *((int *)CMSG_DATA(cmsg));
> >       |                 ~^~~~~~~~~~~~~~~~~~~~~~~
> > 
> > [...]
> 
> Applied to for-next/seccomp, thanks!
> 
> [1/1] samples: user-trap: fix strict-aliasing warning
>       https://git.kernel.org/kees/c/9ad28ca5238d
> 
> Take care,

If you happen to update the trailers, looks good to me too:

Acked-by: Tycho Andersen <tandersen@netflix.com>

I also don't understand what changed, or why this really fixes it.
We're still "violating" strict aliasing as far as I can tell, since we
just introduce `int *fd_ptr` insted of memcpy()-ing out the fd into
an int?

But whatever shuts the compiler up works for me.

Tycho