Re: [PATCH v2] build_bug.h: more user friendly error messages in BUILD_BUG_ON_ZERO()

[Kees Cook <kees@kernel.org>]

On Wed, Apr 09, 2025 at 10:17:49AM -0400, Yury Norov wrote:
> On Wed, Apr 09, 2025 at 09:26:41PM +0900, Vincent Mailhol wrote:
> > +To: Yury Norov
> > 
> > On 09/04/2025 at 04:03, Kees Cook wrote:
> > > On Tue, Apr 08, 2025 at 10:23:53PM +0900, Vincent Mailhol wrote:
> > >> On 08/04/2025 at 01:46, Kees Cook wrote:
> > >>> On Sat, Mar 29, 2025 at 01:48:50AM +0900, Vincent Mailhol wrote:
> > >>>> __BUILD_BUG_ON_ZERO_MSG(), as introduced in [1], makes it possible to
> > >>>> do a static assertions in expressions. The direct benefit is to
> > >>>> provide a meaningful error message instead of the cryptic negative
> > >>>> bitfield size error message currently returned by BUILD_BUG_ON_ZERO():
> > >>>>
> > >>>>   ./include/linux/build_bug.h:16:51: error: negative width in bit-field '<anonymous>'
> > >>>>      16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > >>>>         |                                                   ^
> > >>>>
> > >>>> Get rid of BUILD_BUG_ON_ZERO()'s bitfield size hack. Instead rely on
> > >>>> __BUILD_BUG_ON_ZERO_MSG() which in turn relies on C11's
> > >>>> _Static_assert().
> > >>>>
> > >>>> Use some macro magic, similarly to static_assert(), to either use an
> > >>>> optional error message provided by the user or, when omitted, to
> > >>>> produce a default error message by stringifying the tested
> > >>>> expression. With this, for example:
> > >>>>
> > >>>>   BUILD_BUG_ON_ZERO(1 > 0)
> > >>>>
> > >>>> would now throw:
> > >>>>
> > >>>>   ./include/linux/compiler.h:197:62: error: static assertion failed: "1 > 0 is true"
> > >>>
> > >>> This is so much easier to read! Thanks for this. :)
> > >>>
> > >>> If no one else snags it, I can take this via the hardening tree for
> > >>> -next once -rc2 is released.
> > >>
> > >> I discussed about this with Andrew by DM.
> > >>
> > >> Andrew can pick it up but for the next-next release. That is to say,
> > >> wait for [1] to be merged in v6.16 and then take it to target the v6.17
> > >> merge windows.
> > >>
> > >> If you can take it in your hardening-next tree and have it merged in
> > >> v6.16, then this is convenient for me.
> > >>
> > >> Just make sure that you send it to Linus after Yury's bitmap-for-next
> > >> get merged: https://github.com/norov/linux/commits/bitmap-for-next/
> > > 
> > > Could this land via Yury's tree?
> > 
> > Hi Yury,
> > 
> > I have this patch:
> > 
> > https://lore.kernel.org/all/20250329-build_bug-v2-1-1c831e5ddf89@wanadoo.fr/
> > 
> > which depends on commit b88937277df ("drm/i915: Convert REG_GENMASK*()
> > to fixed-width GENMASK_U*()") in your bitmap-for-next tree.
> > 
> > I discussed this with Andrew (by DM) and Kees. Because of the
> > dependency, it would be convenient if this patch went through your tree.
> > 
> > What do you think?
> 
> Sure, I can merge it. Please everyone send your tags before the end of
> week.

Thanks!

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook