Re: [PATCH] arm64/module: Fix livepatch BTI exceptions with Clang 21+

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 11 Aug 2026, at 18:52, Nick Desaulniers wrote:
> On Tue, Aug 11, 2026 at 9:41 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>>
>> On Tue, Aug 11, 2026 at 09:27:28AM -0700, Nick Desaulniers wrote:
>> > On Tue, Aug 11, 2026 at 7:23 AM Will Deacon <will@kernel.org> wrote:
>> > >
>> > > On Tue, Aug 11, 2026 at 03:18:17PM +0200, Ard Biesheuvel wrote:
>> > > >
>> > > > On Tue, 11 Aug 2026, at 14:02, Ard Biesheuvel wrote:
>> > > > > On Tue, 11 Aug 2026, at 11:44, Will Deacon wrote:
>> > > > >> On Mon, Aug 10, 2026 at 09:41:35AM -0700, Nick Desaulniers wrote:
>> > > > >
>> > > > >>> Sure, but let's replace this with a link to a bug report in llvm's
>> > > > >>> issue tracker?
>> > > > >>
>> > > > >> Yes, please! I can apply the patch once we have the bug number.
>> > > > >>
>> > > > >
>> > > > > I can look into that.
>> > > >
>> > > > https://github.com/llvm/llvm-project/issues/215547
>> > >
>> > > Thanks, Ard. I've ended up with the patch below.
>> >
>> > What's the extent that HAVE_LIVEPATCH will be used?
>> >
>> > Does this penalize kernels built with BTI in favor of HAVE_LIVEPATCH?
>> >
>> > For example, with my Android hat on, if we have a kernel that's built
>> > with ARM64_BTI_KERNEL=y, and _dont_ use HAVE_LIVEPATCH, we're going to
>> > turn off BTI for a feature we're not using?  But maybe we _do_ intend
>> > to use HAVE_LIVEPATCH, and my assumption was wrong?
>>
>> The problem isn't specific to livepatch, livepatch just makes it easier
>> to hit.
>
> Rereading Ard's report, let me see if I can articulate _how_ and tell
> me if I'm still not getting it.
>
> Let's say we have code in .init.text that calls code in .text. If
> .text was placed large enough away from .init.text, we get a veneer
> inserted by _the kernel_ (not the linker).  This veneer has an
> indirect call to the destination. No landing pad. Boom.
>
> Is that precise?
>

Yes.