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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 2 Jan 2026 14:52:25 +0000
Will Deacon <will@kernel.org> wrote:

> 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?

I agreed with using the generic APIs. Also, ftrace_partial_regs_fix() is
not self-explained. Maybe ftrace_regs_set_by_regs()?

Thank you,

> 
> Will


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>