On Tue, Jan 13, 2026 at 12:43:39PM +0100, Jiri Olsa wrote:
> On Mon, Jan 12, 2026 at 05:07:57PM -0500, Steven Rostedt wrote:
> > On Mon, 12 Jan 2026 22:49:38 +0100
> > Jiri Olsa <[email protected]> wrote:
> >
> > > To recreate same stack setup for return probe as we have for entry
> > > probe, we set the instruction pointer to the attached function address,
> > > which gets us the same unwind setup and same stack trace.
> > >
> > > With the fix, entry probe:
> > >
> > > # bpftrace -e 'kprobe:__x64_sys_newuname* { print(kstack)}'
> > > Attaching 1 probe...
> > >
> > > __x64_sys_newuname+9
> > > do_syscall_64+134
> > > entry_SYSCALL_64_after_hwframe+118
> > >
> > > return probe:
> > >
> > > # bpftrace -e 'kretprobe:__x64_sys_newuname* { print(kstack)}'
> > > Attaching 1 probe...
> > >
> > > __x64_sys_newuname+4
> > > do_syscall_64+134
> > > entry_SYSCALL_64_after_hwframe+118
> >
> > But is this really correct?
> >
> > The stack trace of the return from __x86_sys_newuname is from offset "+4".
> >
> > The stack trace from entry is offset "+9". Isn't it confusing that the
> > offset is likely not from the return portion of that function?
>
> right, makes sense.. so standard kprobe actualy skips attached function
> (__x86_sys_newuname) on return probe stacktrace.. perhaps we should do
> the same for kprobe_multi
>
> I managed to get that with the change below, but it's wrong wrt arch code,
> note the ftrace_regs_set_stack_pointer(fregs, stack + 8) .. will try to
> figure out better way when we agree on the solution
>
> thanks,
> jirka
>
>
> ---
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index c56e1e63b893..b0e8ce4934e7 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -71,6 +71,9 @@ arch_ftrace_get_regs(struct ftrace_regs *fregs)
> #define ftrace_regs_set_instruction_pointer(fregs, _ip) \
> do { arch_ftrace_regs(fregs)->regs.ip = (_ip); } while (0)
>
> +#define ftrace_regs_set_stack_pointer(fregs, _sp) \
> + do { arch_ftrace_regs(fregs)->regs.sp = (_sp); } while (0)
> +
>
> static __always_inline unsigned long
> ftrace_regs_get_return_address(struct ftrace_regs *fregs)
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 6279e0a753cf..b1510c412dcb 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -717,7 +717,8 @@ int function_graph_enter_regs(unsigned long ret, unsigned
> long func,
> /* Retrieve a function return address to the trace stack on thread info.*/
> static struct ftrace_ret_stack *
> ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
> - unsigned long frame_pointer, int *offset)
> + unsigned long *stack, unsigned long frame_pointer,
> + int *offset)
> {
> struct ftrace_ret_stack *ret_stack;
>
> @@ -762,6 +763,7 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace,
> unsigned long *ret,
>
> *offset += FGRAPH_FRAME_OFFSET;
> *ret = ret_stack->ret;
> + *stack = (unsigned long) ret_stack->retp;
> trace->func = ret_stack->func;
> trace->overrun = atomic_read(¤t->trace_overrun);
> trace->depth = current->curr_ret_depth;
> @@ -810,12 +812,13 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs,
> unsigned long frame_pointe
> struct ftrace_ret_stack *ret_stack;
> struct ftrace_graph_ret trace;
> unsigned long bitmap;
> + unsigned long stack;
> unsigned long ret;
> int offset;
> int bit;
> int i;
>
> - ret_stack = ftrace_pop_return_trace(&trace, &ret, frame_pointer,
> &offset);
> + ret_stack = ftrace_pop_return_trace(&trace, &ret, &stack,
> frame_pointer, &offset);
>
> if (unlikely(!ret_stack)) {
> ftrace_graph_stop();
> @@ -824,8 +827,11 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs,
> unsigned long frame_pointe
> return (unsigned long)panic;
> }
>
> - if (fregs)
> - ftrace_regs_set_instruction_pointer(fregs, trace.func);
> + if (fregs) {
> + ftrace_regs_set_instruction_pointer(fregs, ret);
> + ftrace_regs_set_stack_pointer(fregs, stack + 8);
actually looks like this might be better solution.. storing the proper
rsp value directly to the regs in return_to_handler
jirka
---
diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index a132608265f6..971883045b75 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -368,13 +368,16 @@ SYM_CODE_START(return_to_handler)
subq $8, %rsp
UNWIND_HINT_FUNC
+ movq %rsp, %rdi
+ addq $8, %rdi
+
/* 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 %rdi, RSP(%rsp)
movq %rsp, %rdi
call ftrace_return_to_handler
