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

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jan 05, 2026 at 04:22:20PM -0500, Steven Rostedt wrote:
> On Sun, 4 Jan 2026 22:34:15 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > > 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()?
> 
> Or perhaps: ftrace_partial_regs_update() where you call it if you need to
> update the 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];
> 	}
> }
> 
> -- Steve

lgtm, the full change is below, I'll send new version of the patchset

thanks,
jirka


---
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];
+	}
+}
+
 #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:
-- 
2.52.0