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

[Steven Rostedt]

On Wed, 7 Jan 2026 09:23:01 +0100
Jiri Olsa <olsajiri at gmail.com> wrote:

> ---
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 1621c84f44b3..177c7bbf3b84 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -157,6 +157,30 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
>  	return regs;
>  }
>  
> +/*
> + * ftrace_partial_regs_update - update the original ftrace_regs from regs
> + * @fregs: The ftrace_regs to update from @regs
> + * @regs: The partial regs from ftrace_partial_regs() that was updated
> + *
> + * Some architectures have the partial regs living in the ftrace_regs
> + * structure, whereas other architectures need to make a different copy
> + * of the @regs. If a partial @regs is retrieved by ftrace_partial_regs() and
> + * if the code using @regs updates a field (like the instruction pointer or
> + * stack pointer) it may need to propagate that change to the original @fregs
> + * it retrieved the partial @regs from. Use this function to guarantee that
> + * update happens.
> + */

I think the kernel doc should go into the generic function, and this just say:

  /* See description of this function in the generic header */

or something similar.

Also, should probably have the description in the general function say:

  *
  * For architectures that need to update @fregs from @regs, this function needs to be
  * defined.

Or something similar.

-- Steve

> +static __always_inline void
> +ftrace_partial_regs_update(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 770f0dc993cc..ae22559b4099 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -213,6 +213,9 @@ ftrace_partial_regs(struct ftrace_regs *fregs, struct pt_regs *regs)
>  	return regs;
>  }
>  
> +static __always_inline void
> +ftrace_partial_regs_update(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 6e076485bf70..3a17f79b20c2 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_update(fregs, bpf_kprobe_multi_pt_regs_ptr());
>  	rcu_read_unlock();
>  
>   out:
> --