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Sun, 04 Jan 2026 03:56:18 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sun, 4 Jan 2026 12:56:17 +0100 To: Will Deacon Cc: Masami Hiramatsu , Steven Rostedt , Peter Zijlstra , bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, x86@kernel.org, Yonghong Song , Song Liu , Andrii Nakryiko , Mark Rutland , Mahe Tardy Subject: Re: [BUG/RFC 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run Message-ID: References: <20251105125924.365205-1-jolsa@kernel.org> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Jan 02, 2026 at 02:52:25PM +0000, Will Deacon 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 knew I forgot some change.. thanks for replying ftrace_partial_regs is overloaded in arm64 and because of that we need to propagate the change to pt_regs, so I think the ftrace_partial_regs_fix code is arm64 specific, so can't see that in core code also wrt ftrace_partial_regs_fix name, I was thinking it might be better to have begin/end functions, like: ftrace_partial_regs_begin ftrace_partial_regs_end thanks, jirka --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -2560,10 +2560,11 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link, } rcu_read_lock(); - regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr()); + regs = ftrace_partial_regs_begin(fregs, bpf_kprobe_multi_pt_regs_ptr()); 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_end(fregs, bpf_kprobe_multi_pt_regs_ptr()); rcu_read_unlock(); out: