On Mon, Nov 03, 2025 at 11:09:22PM +0100, Jiri Olsa 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
>
> Acked-by: Masami Hiramatsu (Google) <[email protected]>
> Signed-off-by: Jiri Olsa <[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)
I lost the plot. Why here and not in asm below?
> #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
