From: Josh Poimboeuf <jpoimboe@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, will@kernel.org,
catalin.marinas@arm.com, mark.rutland@arm.com,
Mark Brown <broonie@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH 1/2] arm64: module: Emit BTI veneers for cross-section calls
Date: Wed, 12 Aug 2026 19:25:32 -0700 [thread overview]
Message-ID: <an0l-wSimIc8C38S@jpoimboe> (raw)
In-Reply-To: <anzo2Q-TxbB0PUhi@jpoimboe>
On Wed, Aug 12, 2026 at 02:52:04PM -0700, Josh Poimboeuf wrote:
> On Wed, Aug 12, 2026 at 06:21:00PM +0200, Ard Biesheuvel wrote:
> > --- 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.
Hm, does the omitted BTI also break long intra-vmlinux branches for
vmlinux text size > 128MB?
The linker adds veneers for that case, but doesn't make them
BTI-friendly unless GNU_PROPERTY_AARCH64_FEATURE_1_BTI is set on *all*
.o files, which is not currently happening for .S files.
And when I force that bit with "-z force-bti" on a 700MB text
allyesconfig vmlinux with 22k linker-created veneers, it's crashes the
GNU linker :-/
Seems kernel BTI is borked all around...
--
Josh
next prev parent reply other threads:[~2026-08-13 2:25 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-12 16:20 [PATCH 0/2] arm64: Implement support for BTI veneers Ard Biesheuvel
2026-08-12 16:21 ` [PATCH 1/2] arm64: module: Emit BTI veneers for cross-section calls Ard Biesheuvel
2026-08-12 21:52 ` Josh Poimboeuf
2026-08-13 2:25 ` Josh Poimboeuf [this message]
2026-08-12 16:21 ` [PATCH 2/2] DONOTMERGE: arm64: module: Test module for BTI veneers Ard Biesheuvel
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