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

[Ard Biesheuvel]

On Thu, 13 Aug 2026, at 09:34, Ard Biesheuvel wrote:
> On Thu, 13 Aug 2026, at 00:52, Josh Poimboeuf wrote:
>> 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@xxxxxxxxxx>
>>> ---
>>>  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".
>>
>
> ... when kernel mode BTI is enabled.
>
> I have no insight into which pieces of livepatch for arm64 are actually
> upstream. Is it just the tooling that is missing? In this case, though,
> I think HAVE_LIVEPATCH should depend on !ARM64_BTI_KERNEL, rather than
> the other way around. I can add that in v2.
>

BTW we may still try and get the compiler folks to add a command line
option to force BTI landing pads to be emitted for static functions, and
enable it for vmlinux only when livepatch is enabled. That way, we can
re-enable kernel mode BTI for all toolchains for the common case, and
have livepatch on arm64 (with BTI) depend on recent versions that implement
this option.