[PATCHv3 2/4] x86/fgraph,bpf: Fix stack ORC unwind from kprobe_multi return probe

Jiri Olsa
2025-11-04T21:54:36Z

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) <mhiramat@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
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