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

On Tue, Aug 11, 2026 at 04:55:37PM +0200, Ard Biesheuvel wrote:

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

quoted

Author: Josh Poimboeuf [off-list ref]
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.

Yes, the livepatch module creation process takes interprocedural
optimizations into account.

With kpatch-build and the new in-tree klp-build (x86 only, arm64 coming
soon), it does a binary diff between the old and new kernels, and
extracts the changed functions.  So if the interface between caller and
callee changes, they both get patched.

-- 
Josh