Re: [PATCH bpf-next 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[Will Deacon <will@kernel.org>]

On Wed, Jan 07, 2026 at 10:32:55AM +0100, Jiri Olsa wrote:
> Mahe reported issue with bpf_override_return helper not working when
> executed from kprobe.multi bpf program on arm.
> 
> The problem is 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.
> 
> Fixing this by introducing and calling ftrace_partial_regs_update function
> to propagate the values of changed registers (ip and stack).
> 
> Fixes: b9b55c8912ce ("tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs")
> Reported-by: Mahe Tardy <mahe.tardy@gmail.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> v1 changes:
>  - used ftrace_partial_regs_update with comments from Steven
> 
>  arch/arm64/include/asm/ftrace.h | 24 ++++++++++++++++++++++++
>  include/linux/ftrace.h          |  3 +++
>  kernel/trace/bpf_trace.c        |  1 +
>  3 files changed, 28 insertions(+)
> 
> 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.
> + */
> +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];
> +	}
> +}

I still don't understand why we need anything new in the arch code for this.

We've selected HAVE_ARCH_FTRACE_REGS and we implement
ftrace_regs_set_instruction_pointer() and ftrace_regs_set_return_value()
so the core code already has everything it needs to make this work
without additional arch support.

Will