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

[Jiri Olsa]

ping, thanks

jirka

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];
> +	}
> +}
> +
>  #define arch_ftrace_fill_perf_regs(fregs, _regs) do {		\
>  		(_regs)->pc = arch_ftrace_regs(fregs)->pc;			\
>  		(_regs)->regs[29] = arch_ftrace_regs(fregs)->fp;		\
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 7ded7df6e9b5..4cb1315522bb 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -205,6 +205,9 @@ ftrace_partial_regs(struct ftrace_regs *fregs, struct pt_regs *regs)
>  	return &arch_ftrace_regs(fregs)->regs;
>  }
>  
> +static __always_inline void
> +ftrace_partial_regs_fix(struct ftrace_regs *fregs, struct pt_regs *regs) { }
> +
>  #endif /* !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS || CONFIG_HAVE_FTRACE_REGS_HAVING_PT_REGS */
>  
>  #ifdef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a795f7afbf3d..7b5768ced9b3 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2564,6 +2564,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>  	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
>  	err = bpf_prog_run(link->link.prog, regs);
>  	bpf_reset_run_ctx(old_run_ctx);
> +	ftrace_partial_regs_fix(fregs, bpf_kprobe_multi_pt_regs_ptr());
>  	rcu_read_unlock();
>  
>   out:
> -- 
> 2.51.1
>