Re: [PATCH -tip v8 05/13] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe_trampoline code

On Sat, Jul 10, 2021 at 10:41:04AM +0900, Masami Hiramatsu wrote:

Hi Ingo and Josh,

On Sat, 10 Jul 2021 00:31:40 +0900
Masami Hiramatsu [off-list ref] wrote:

quoted

quoted

quoted

+STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
+#undef UNWIND_HINT_FUNC
+#define UNWIND_HINT_FUNC
+#endif
 /*
  * When a retprobed function returns, this code saves registers and
  * calls trampoline_handler() runs, which calls the kretprobe's handler.

@@ -1031,6 +1044,7 @@ asm(
 	/* We don't bother saving the ss register */
 #ifdef CONFIG_X86_64
 	"	pushq %rsp\n"
+	UNWIND_HINT_FUNC
 	"	pushfq\n"
 	SAVE_REGS_STRING
 	"	movq %rsp, %rdi\n"

@@ -1041,6 +1055,7 @@ asm(
 	"	popfq\n"
 #else
 	"	pushl %esp\n"
+	UNWIND_HINT_FUNC
 	"	pushfl\n"
 	SAVE_REGS_STRING
 	"	movl %esp, %eax\n"

Why not provide an appropriate annotation method in <asm/unwind_hints.h>, 
so that other future code can use it too instead of reinventing the wheel?

I think I got what you meant. Let me summarize the issue.

In case of CONFIG_FRAME_POINTER=n, it is OK just adding UNWIND_HINT_FUNC.

In case of CONFIG_FRAME_POINTER=y, without STACK_FRAME_NON_STANDARD(),
the objtool complains that a CALL instruction without the frame pointer.
---
  arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x25: call without frame pointer save/setup
---

If we just add STACK_FRAME_NON_STANDARD() with UNWIND_HINT_FUNC macro,
the objtool complains that non-standard function has unwind hint.
---
arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x1: BUG: why am I validating an ignored function?

I'm thinking this latter warning indicates an objtool bug (as the BUG
warning claims).  If a function is ignored with
STACK_FRAME_NON_STANDARD() then objtool should probably also ignore its
hints.  Then we should be able to get rid of the #undef/#ifdef
UNWIND_HINT_FUNC silliness.

The other warning is correct and STACK_FRAME_NON_STANDARD() still needs
to be behind '#ifdef CONFIG_FRAME_POINTER' since the function is missing
a frame pointer.  So maybe we can make a STACK_FRAME_NON_STANDARD_FP()
or similar.

I'll post a few patches.

-- 
Josh