On Mon, 27 Oct 2025 14:13:53 +0100 Jiri Olsa <[email protected]> wrote:
> Currently we don't get stack trace via ORC unwinder on top of fgraph exit > handler. We can see that when generating stacktrace from kretprobe_multi > bpf program which is based on fprobe/fgraph. > > The reason is that the ORC unwind code won't get pass the return_to_handler > callback installed by fgraph return probe machinery. > > Solving this by creating stack frame in return_to_handler expected by > ftrace_graph_ret_addr function to recover original return address and > continue with the unwind. > > Also updating the pt_regs data with cs/flags/rsp which are needed for > successful stack retrieval from ebpf bpf_get_stackid helper. > - in get_perf_callchain we check user_mode(regs) so CS has to be set > - in perf_callchain_kernel we call perf_hw_regs(regs), so EFLAGS/FIXED > has to be unset > > Note I feel like it'd be better to set those directly in return_to_handler, > but Steven suggested setting them later in kprobe_multi code path. Yeah, stacktrace is not always used from the return handler. > > Signed-off-by: Jiri Olsa <[email protected]> Thanks for the fix. This looks good to me. Acked-by: Masami Hiramatsu (Google) <[email protected]> > --- > arch/x86/include/asm/ftrace.h | 5 +++++ > arch/x86/kernel/ftrace_64.S | 8 +++++++- > include/linux/ftrace.h | 10 +++++++++- > 3 files changed, 21 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h > index 93156ac4ffe0..b08c95872eed 100644 > --- a/arch/x86/include/asm/ftrace.h > +++ b/arch/x86/include/asm/ftrace.h > @@ -56,6 +56,11 @@ arch_ftrace_get_regs(struct ftrace_regs *fregs) > return &arch_ftrace_regs(fregs)->regs; > } > > +#define arch_ftrace_partial_regs(regs) do { \ > + regs->flags &= ~X86_EFLAGS_FIXED; \ > + regs->cs = __KERNEL_CS; \ > +} while (0) > + > #define arch_ftrace_fill_perf_regs(fregs, _regs) do { \ > (_regs)->ip = arch_ftrace_regs(fregs)->regs.ip; \ > (_regs)->sp = arch_ftrace_regs(fregs)->regs.sp; \ > diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S > index 367da3638167..823dbdd0eb41 100644 > --- a/arch/x86/kernel/ftrace_64.S > +++ b/arch/x86/kernel/ftrace_64.S > @@ -354,12 +354,17 @@ SYM_CODE_START(return_to_handler) > UNWIND_HINT_UNDEFINED > ANNOTATE_NOENDBR > > + /* Restore return_to_handler value that got eaten by previous ret > instruction. */ > + subq $8, %rsp > + UNWIND_HINT_FUNC > + > /* Save ftrace_regs for function exit context */ > subq $(FRAME_SIZE), %rsp > > movq %rax, RAX(%rsp) > movq %rdx, RDX(%rsp) > movq %rbp, RBP(%rsp) > + movq %rsp, RSP(%rsp) > movq %rsp, %rdi > > call ftrace_return_to_handler > @@ -368,7 +373,8 @@ SYM_CODE_START(return_to_handler) > movq RDX(%rsp), %rdx > movq RAX(%rsp), %rax > > - addq $(FRAME_SIZE), %rsp > + addq $(FRAME_SIZE) + 8, %rsp > + > /* > * Jump back to the old return address. This cannot be JMP_NOSPEC rdi > * since IBT would demand that contain ENDBR, which simply isn't so for > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h > index 7ded7df6e9b5..07f8c309e432 100644 > --- a/include/linux/ftrace.h > +++ b/include/linux/ftrace.h > @@ -193,6 +193,10 @@ static __always_inline struct pt_regs > *ftrace_get_regs(struct ftrace_regs *fregs > #if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || \ > defined(CONFIG_HAVE_FTRACE_REGS_HAVING_PT_REGS) > > +#ifndef arch_ftrace_partial_regs > +#define arch_ftrace_partial_regs(regs) do {} while (0) > +#endif > + > static __always_inline struct pt_regs * > ftrace_partial_regs(struct ftrace_regs *fregs, struct pt_regs *regs) > { > @@ -202,7 +206,11 @@ ftrace_partial_regs(struct ftrace_regs *fregs, struct > pt_regs *regs) > * Since arch_ftrace_get_regs() will check some members and may return > * NULL, we can not use it. > */ > - return &arch_ftrace_regs(fregs)->regs; > + regs = &arch_ftrace_regs(fregs)->regs; > + > + /* Allow arch specific updates to regs. */ > + arch_ftrace_partial_regs(regs); > + return regs; > } > > #endif /* !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS || > CONFIG_HAVE_FTRACE_REGS_HAVING_PT_REGS */ > -- > 2.51.0 > -- Masami Hiramatsu (Google) <[email protected]>
