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. Will --->8 Author: Josh Poimboeuf <[email protected]> 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. Clang 21+ omits the landing pad for static functions which can only be reached by a direct branch. That's normally fine for ordinary modules which only branch to global exported functions, but Mark Brown points out [1] that this isn't guaranteed if the module branches between sections. Futhermore, livepatch modules use klp relocations to reference arbitrary kernel symbols, so with CONFIG_RANDOMIZE_MODULE_REGION_FULL the module is far enough from the kernel that every R_AARCH64_CALL26 needs a PLT. Put Clang 21+ in the naughty corner alongside GCC, which suffers from the same issue, by disabling CONFIG_ARM64_BTI_KERNEL until we have a version of the toolchain with the problem resolved. Cc: Ard Biesheuvel <[email protected]> Link: https://lore.kernel.org/r/[email protected] [1] Fixes: fd1e0fd71f65 ("arm64: Implement HAVE_LIVEPATCH") Signed-off-by: Josh Poimboeuf <[email protected]> [will: Stitched together commit message, diff and bug number] Signed-off-by: Will Deacon <[email protected]> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index b3afe0688919..fc57d90d92c1 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -2116,6 +2116,8 @@ config ARM64_BTI_KERNEL depends on !CC_IS_GCC || GCC_VERSION >= 100100 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671 depends on !CC_IS_GCC + # https://github.com/llvm/llvm-project/issues/215547 + depends on !CC_IS_CLANG || CLANG_VERSION < 210000 depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_ARGS) help Build the kernel with Branch Target Identification annotations
