linux-kernel - Re: [PATCH -tip 0/5] kprobes: Fix stacktrace in kretprobes

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Date:   Mon, 8 Mar 2021 19:19:45 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Daniel Xu <dxu@...uu.xyz>, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org, kuba@...nel.org,
        mingo@...hat.com, ast@...nel.org, tglx@...utronix.de,
        kernel-team@...com, yhs@...com
Subject: Re: [PATCH -tip 0/5] kprobes: Fix stacktrace in kretprobes

On Mon, Mar 08, 2021 at 11:52:10AM +0900, Masami Hiramatsu wrote:
> So at the kretprobe handler, we have 2 issues.
> 1) the return address (caller_func+0x15) is not on the stack.
>    this can be solved by searching from current->kretprobe_instances.
> 2) the stack frame size of kretprobe_trampoline is unknown
>    Since the stackframe is fixed, the fixed number (0x98) can be used.
> 
> However, those solutions are only for the kretprobe handler. The stacktrace
> from interrupt handler hit in the kretprobe_trampoline still doesn't work.
> 
> So, here is my idea;
> 
> 1) Change the trampline code to prepare stack frame at first and save
>    registers on it, instead of "push". This will makes ORC easy to setup
>    stackframe information for this code.
> 2) change the return addres fixup timing. Instead of using return value
>    of trampoline handler, before removing the real return address from
>    current->kretprobe_instances.
> 3) Then, if orc_find() finds the ip is in the kretprobe_trampoline, it
>    checks the contents of the end of stackframe (at the place of regs->sp)
>    is same as the address of it. If it is, it can find the correct address
>    from current->kretprobe_instances. If not, there is a correct address.
> 
> I need to find how the ORC info is prepared for 1), maybe UNWIND_HINT macro
> may help?

Hi Masami,

If I understand correctly, for #1 you need an unwind hint which treats
the instruction *after* the "pushq %rsp" as the beginning of the
function.

I'm thinking this should work:


diff --git a/arch/x86/include/asm/unwind_hints.h b/arch/x86/include/asm/unwind_hints.h
index 8e574c0afef8..8b33674288ea 100644
--- a/arch/x86/include/asm/unwind_hints.h
+++ b/arch/x86/include/asm/unwind_hints.h
@@ -52,6 +52,11 @@
 	UNWIND_HINT sp_reg=ORC_REG_SP sp_offset=8 type=UNWIND_HINT_TYPE_FUNC
 .endm
 
+#else
+
+#define UNWIND_HINT_FUNC \
+	UNWIND_HINT(ORC_REG_SP, 8, UNWIND_HINT_TYPE_FUNC, 0)
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_X86_UNWIND_HINTS_H */
diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index df776cdca327..38ff1387f95d 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -767,6 +767,7 @@ 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"
@@ -790,7 +791,6 @@ asm(
 	".size kretprobe_trampoline, .-kretprobe_trampoline\n"
 );
 NOKPROBE_SYMBOL(kretprobe_trampoline);
-STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
 
 
 /*