Re: [PATCH 1/2] arm64: module: Emit BTI veneers for cross-section calls

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Wed, Aug 12, 2026 at 06:21:00PM +0200, Ard Biesheuvel wrote:
> The compiler is permitted to omit BTI landing pads from static functions
> that never have their address taken, but are only called directly, even
> if those calls originate from other code sections.
> 
> This means that calls into a module's .text section from .init.text,
> which may need to be routed via a PLT if .text is out of direct
> branching range, may result in BTI exceptions due to the indirect calls
> performed by the PLT veneers. (Note that calls to .init.text from .text
> are not allowed.)
> 
> The 'solution' is to emit yet another veneer - this is what the ELF
> psABI for AArch64 mandates in this case.
> 
> So derive an upper bound for the number of veneers that may be needed in
> the core module region to ensure that any call from init code that ends
> up needing a PLT can be directed at a veneer with a BTI landing pad, and
> allocate the additional space.
> 
> Then, emit these veneers as needed, i.e., only when emitting a PLT entry
> for a call from an init code section to a normal code section in the
> same module. In practice, this only occurs when a module's .init.text
> happens to be allocated far away from its .text section, which might
> happen when the initial 128M 'near' module region runs out of space
> between allocating the core module and allocating its init region.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/Kconfig                  |   2 -
>  arch/arm64/include/asm/module.h     |  12 ++
>  arch/arm64/include/asm/module.lds.h |   3 +
>  arch/arm64/kernel/module-plts.c     | 128 +++++++++++++++++++-
>  4 files changed, 138 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b3afe0688919..25fa80b5591d 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2114,8 +2114,6 @@ config ARM64_BTI_KERNEL
>  	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
>  	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
>  	depends on !CC_IS_GCC || GCC_VERSION >= 100100
> -	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
> -	depends on !CC_IS_GCC

This doesn't work for livepatch though, and removing the "depends on
!CC_IS_GCC" is a livepatch regression as it broadly increases the
likelihood of ARM64_BTI_KERNEL (default y) getting enabled.

So "livepatch broken on arm64 clang 21+" now becomes "livepatch broken on arm64".

So either ARM64_BTI_KERNEL needs to depend on !LIVEPATCH (which will
keep BTI disabled for most distros), or we need a different approach
which uses __vmalloc_node_range() to allocate a veneer within 128MB of
the target.  I can try to write that up.

-- 
Josh