Re: [PATCHv2 bpf-next 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[Will Deacon <will@kernel.org>]

On Fri, Jan 09, 2026 at 10:34:53AM +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).
> 
> Reported-by: Mahe Tardy <mahe.tardy@gmail.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> v2 changes:
> - moved ftrace_partial_regs_update to generic code [Will]
> 
>  include/linux/ftrace_regs.h | 25 +++++++++++++++++++++++++
>  kernel/trace/bpf_trace.c    |  1 +
>  2 files changed, 26 insertions(+)
> 
> diff --git a/include/linux/ftrace_regs.h b/include/linux/ftrace_regs.h
> index 15627ceea9bc..f9a7c009cdae 100644
> --- a/include/linux/ftrace_regs.h
> +++ b/include/linux/ftrace_regs.h
> @@ -33,6 +33,31 @@ struct ftrace_regs;
>  #define ftrace_regs_get_frame_pointer(fregs) \
>  	frame_pointer(&arch_ftrace_regs(fregs)->regs)
>  
> +static __always_inline void
> +ftrace_partial_regs_update(const struct ftrace_regs *fregs, struct pt_regs *regs) { }
> +
> +#else
> +
> +/*
> + * 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)
> +{
> +	ftrace_regs_set_instruction_pointer(fregs, instruction_pointer(regs));
> +	ftrace_regs_set_return_value(fregs, regs_return_value(regs));
> +}

I think the AI thingy is right about dropping the const qualifier here
but overall I prefer this to the previous revisions. Thanks for sticking
with it!

Acked-by: Will Deacon <will@kernel.org>

Will