linux-kernel - Re: [PATCH 1/3] compiler_types: Introduce __counted_by

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Message-ID: <1D53887A-B277-4813-A3E6-9367CCFC8759@kernel.org>
Date: Thu, 23 Oct 2025 06:45:31 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
CC: Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
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Subject: Re: [PATCH 1/3] compiler_types: Introduce __counted_by_ptr()

On October 23, 2025 1:01:23 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Wed, Oct 22, 2025 at 05:47:43PM -0700, Kees Cook wrote:
>> On Tue, Oct 21, 2025 at 12:24:05PM -0700, Kees Cook wrote:
>> > On Tue, Oct 21, 2025 at 11:54:47AM +0200, Peter Zijlstra wrote:
>> > > > [...]
>> > > > Unfortunately, this annotation cannot be used for "void *" members
>> > > > (since such a member is considered a pointer to an incomplete type,
>> > > > and neither Clang nor GCC developers could be convinced otherwise[1],
>> > > > even in the face of the GNU extension that "void *" has size "1 byte"
>> > > > for pointer arithmetic). For "void *" members, we must use the coming
>> > > > "sized_by" attribute.
>> > > 
>> > > So why do we need both __counted_by_ptr() and this __sized_by(), won't
>> > > one be good enough?
>> > [...]
>> > Let me take another stab at it...
>> 
>> It seems this will be acceptable as long as it is gated by GNU
>> extensions.
>
>Excellent!
>
>> GCC patch in progress. Clang PR here:
>> https://github.com/llvm/llvm-project/pull/163698
>
>I think you've got your link mixed up, this appears to be arm-kcfi (also
>good ofc). Either that, or I need copy/paste lessons.
>
>This one?
>
>  https://github.com/llvm/llvm-project/pull/164737

Whoops, yes, that's the one! Seems I'm the one needing those lessons. ;)

-- 
Kees Cook