Re: [BUG/RFC 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[Will Deacon <will@kernel.org>]

On Wed, Nov 05, 2025 at 01:59:23PM +0100, Jiri Olsa wrote:
> hi,
> Mahe reported issue with bpf_override_return helper not working
> when executed from kprobe.multi bpf program on arm.
> 
> The problem seems to be that on arm we use alternate storage for
> pt_regs object that is passed to bpf_prog_run and if any register
> is changed (which is the case of bpf_override_return) it's not
> propagated back to actual pt_regs object.
> 
> The change below seems to fix the issue, but I have no idea if
> that's proper fix for arm, thoughts?
> 
> I'm attaching selftest to actually test bpf_override_return helper
> functionality, because currently we only test that we are able to
> attach a program with it, but not the override itself.
> 
> thanks,
> jirka
> 
> 
> ---
>  arch/arm64/include/asm/ftrace.h | 11 +++++++++++
>  include/linux/ftrace.h          |  3 +++
>  kernel/trace/bpf_trace.c        |  1 +
>  3 files changed, 15 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index ba7cf7fec5e9..ad6cf587885c 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -157,6 +157,17 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
>  	return regs;
>  }
>  
> +static __always_inline void
> +ftrace_partial_regs_fix(const struct ftrace_regs *fregs, struct pt_regs *regs)
> +{
> +	struct __arch_ftrace_regs *afregs = arch_ftrace_regs(fregs);
> +
> +	if (afregs->pc != regs->pc) {
> +		afregs->pc = regs->pc;
> +		afregs->regs[0] = regs->regs[0];
> +	}
> +}

This looks a bit grotty to me and presumably other architectures would
need similar treatement. Wouldn't it be cleaner to reuse the existing
API instead? For example, by calling ftrace_regs_set_instruction_pointer()
and ftrace_regs_set_return_value() to update the relevant registers from
the core code?

Will