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

On Mon, Aug 10, 2026 at 11:31:10AM +0100, Will Deacon wrote:

On Fri, Aug 07, 2026 at 02:46:11PM -0700, Josh Poimboeuf wrote:

quoted

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.

Clang 21+ omits the landing pad for static functions which can only be
reached by a direct branch.  That's fine for ordinary modules which only
branch to global exported functions.  But livepatch modules use klp
relocations to reference arbitrary kernel symbols, and with
CONFIG_RANDOMIZE_MODULE_REGION_FULL the module is far enough from the
kernel that every R_AARCH64_CALL26 needs a PLT.

RET is exempt from BTI checking, so use it instead of BR when the target
has no landing pad, similar to what ftrace and BPF do.

Hmm, doesn't that somewhat undermine the purpose of using BTI in the
kernel? Now we're going to create PLTs that can branch to arbitrary
addresses.

Yes, but just to clarify:

- Only with livepatch modules loaded (and we can add an
  is_livepatch_module() check).

- Only a small minority of livepatch klp relocations need it.

- There are already other instances of "ret <reg>" in the kernel in
  ftrace, BPF, and kvm.

quoted

This was found by testing with klp-build and Clang 21, but the issue is
not specific to klp-build.  It's inherent to any livepatch module use of
klp relocations.

Previous tests with Clang 20 did not show this problem, as older Clang
unconditionally emits "bti c" for every C function.

Is there an option to restore that behaviour if CONFIG_LIVEPATCH=y?
Otherwise, I think I'd be more inclined to add yet-another dependency
to CONFIG_ARM64_BTI_KERNEL so it's disabled if LIVEPATCH is selected.

Hm, looking deeper, is BTI just fundamentally broken now, independent of
livepatch?

config ARM64_BTI_KERNEL
	...
	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
	depends on !CC_IS_GCC
	...

AFAICT, the reason for the "depends on !CC_IS_GCC" is that GCC was
already doing the exact same thing Clang is now doing: namely, omitting
BTI for static functions that don't have a pointer taken to them.

So Clang 21+ now has the original GCC edge case: an .init.text direct
branching to a .text function which happens to be allocated >= 128MB
away and which doesn't have BTI.

In which case I think to properly support BTI going forward we would
need two "veneers"?  Either that or remove BTI kernel support
altogether.

-- 
Josh