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

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

On Tue, 11 Aug 2026, at 16:23, Will Deacon wrote:

> Author: Josh Poimboeuf <jpoimboe@kernel.org>
> Date:   Tue Aug 11 14:11:44 2026 +0000
>
>     arm64: bti: Disable in-kernel BTI with recent versions of Clang
>    
>     The following BTI exception was seen when loading a livepatch module:
>    
>       Internal error: Oops - BTI: 0000000036000001 [#1]  SMP
>       pstate: 634004c9 (nZCv daIF +PAN -UAO +TCO +DIT -SSBS BTYPE=jc)
>       pc : kill_orphaned_pgrp+0x0/0x150
>       lr : do_exit+0x498/0xaf0 [livepatch_combined]
>    
>     The problem is that the patch module's do_exit() is branching to a
>     static function in vmlinux using a module PLT veneer (indirect branch),
>     but the target function doesn't have a BTI landing pad.
>    

I had missed this bit before, i.e., the fact that the target is a static
function inside vmlinux.

The lack of a BTI landing pad suggests that the address of this function
is never taken either, and so the symbol in question is not exported to
modules, right?

Does KLP make any special accommodations to ensure that this is safe? Otherwise,
BTI is not the only potential issue: the compiler may apply interprocedural
optimizations, and the function in question may not be AAPCS64 compliant as
a result.

It also means treating cross-section jumps as indirect calls in the BTI
landing pad elision heuristic would not work around the issue here.