Re: [PATCH v7] arm64: implement support for static call trampolines

[Sami Tolvanen <samitolvanen@google.com>]

On Fri, Mar 13, 2026 at 06:18:52AM +0000, Carlos Llamas wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Implement arm64 support for the 'unoptimized' static call variety, which
> routes all calls through a single trampoline that is patched to perform a
> tail call to the selected function.
> 
> Since static call targets may be located in modules loaded out of direct
> branching range, we need to use a ADRP/ADD pair to load the branch target
> into R16 and use a branch-to-register (BR) instruction to perform an
> indirect call. Unlike on x86, there is no pressing need on arm64 to avoid
> indirect calls at all cost, but hiding it from the compiler as is done
> here does have some benefits:
> - the literal is located in .rodata, which gives us the same robustness
>   advantage that code patching does;
> - no performance hit on CFI enabled Clang builds that decorate compiler
>   emitted indirect calls with branch target validity checks.
> 
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Carlos Llamas <cmllamas@google.com>

Does this need a Co-developed-by tag as well?

> ---
> v7:
>   - Took Ard's v3 patch (as it leaves the code patching logic out) and
>     rebased it on top  of mainline 7.0-rc3.
>   - Dropped the changes to arch/arm64/lib/insn.c and instead switched to
>     the (now) existing aarch64_insn_write_literal_u64().
>   - Added the RET0 trampoline define which points to the generic stub
>     __static_call_return0.
>   - Made the HAVE_STATIC_CALL conditional on CFI as suggested by Ard.
>   - Added .type and .size sections to the trampoline definition to
>     support ABI tools.
> 
>  arch/arm64/Kconfig                   |  1 +
>  arch/arm64/include/asm/static_call.h | 33 ++++++++++++++++++++++++++++
>  arch/arm64/kernel/Makefile           |  1 +
>  arch/arm64/kernel/static_call.c      | 20 +++++++++++++++++
>  arch/arm64/kernel/vmlinux.lds.S      |  1 +
>  5 files changed, 56 insertions(+)
>  create mode 100644 arch/arm64/include/asm/static_call.h
>  create mode 100644 arch/arm64/kernel/static_call.c
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 38dba5f7e4d2..9ea19b74b6c3 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -252,6 +252,7 @@ config ARM64
>  	select HAVE_RSEQ
>  	select HAVE_RUST if RUSTC_SUPPORTS_ARM64
>  	select HAVE_STACKPROTECTOR
> +	select HAVE_STATIC_CALL if CFI
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_KPROBES
>  	select HAVE_KRETPROBES
> diff --git a/arch/arm64/include/asm/static_call.h b/arch/arm64/include/asm/static_call.h
> new file mode 100644
> index 000000000000..331580542fd4
> --- /dev/null
> +++ b/arch/arm64/include/asm/static_call.h
> @@ -0,0 +1,33 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_STATIC_CALL_H
> +#define _ASM_STATIC_CALL_H
> +
> +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, target)			    \
> +	asm("	.pushsection .static_call.text, \"ax\"			\n" \
> +	    "	.align	3						\n" \
> +	    "	.globl	" STATIC_CALL_TRAMP_STR(name) "			\n" \
> +	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
> +	    "	hint	34	/* BTI C */				\n" \

It doesn't really matter either way, but do we still support toolchains
that don't understand "bti c"?

Otherwise looks good to me, and definitely a much better way to solve
this issue:

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>

Sami