Re: [PATCH bpf-next 2/4] x86/fgraph,bpf: Switch kprobe_multi program stack unwind to hw_regs path

[Jiri Olsa <olsajiri@gmail.com>]

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 <jolsa@kernel.org> 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(&current->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