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Tue, 20 Jan 2026 08:17:09 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Tue, 20 Jan 2026 17:17:08 +0100 To: Jiri Olsa Cc: Steven Rostedt , Masami Hiramatsu , Josh Poimboeuf , Mahe Tardy , Peter Zijlstra , bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, x86@kernel.org, Yonghong Song , Song Liu , Andrii Nakryiko Subject: Re: [PATCH bpf-next 2/4] x86/fgraph,bpf: Switch kprobe_multi program stack unwind to hw_regs path Message-ID: References: <20260112214940.1222115-1-jolsa@kernel.org> <20260112214940.1222115-3-jolsa@kernel.org> <20260112170757.4e41c0d8@gandalf.local.home> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 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