Re: [PATCH] Subject: kprobes-x86: correct post-eip value inpost_hander()

[Ingo Molnar]

* Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx> wrote:

> > thanks, i've added your patch to the .26 bucket of x86.git, but it 
> > would be nice to get an Ack/Nack from a kprobes person as well.
>  
> Ingo,
> 
> I've tested Yakov's more comprehensive patch on powerpc too. This has 
> my ack.
> 
> Acked-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>

thanks, i've queued up the x86-only patch below for .26 merging. (that 
is all that is needed for x86, and no .25 urgency, right?)

	Ingo

------------------>
Subject: x86, kprobes: correct post-eip value in post_hander()
From: "Yakov Lerner" <iler.ml@xxxxxxxxx>
Date: Sun, 16 Mar 2008 03:21:21 -0500

I was trying to get the address of instruction to be executed
next after the kprobed instruction.  But regs->eip in post_handler()
contains value which is useless to the user. It's pre-corrected value.
This value is difficult to use without access to resume_execution(), which
is not exported anyway.
I moved the invocation of post_handler() to *after* resume_execution().
Now regs->eip contains meaningful value in post_handler().

I do not think this change breaks any backward-compatibility.
To make meaning of the old value, post_handler() would need access to
resume_execution() which is not exported.  I have difficulty to believe
that previous, uncorrected, regs->eip can be meaningfully used in
post_handler().

Signed-off-by: Yakov Lerner <iler.ml@xxxxxxxxx>
Acked-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 arch/x86/kernel/kprobes.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: linux-x86.q/arch/x86/kernel/kprobes.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/kprobes.c
+++ linux-x86.q/arch/x86/kernel/kprobes.c
@@ -858,15 +858,15 @@ static int __kprobes post_kprobe_handler
 	if (!cur)
 		return 0;
 
+	resume_execution(cur, regs, kcb);
+	regs->flags |= kcb->kprobe_saved_flags;
+	trace_hardirqs_fixup_flags(regs->flags);
+
 	if ((kcb->kprobe_status != KPROBE_REENTER) && cur->post_handler) {
 		kcb->kprobe_status = KPROBE_HIT_SSDONE;
 		cur->post_handler(cur, regs, 0);
 	}
 
-	resume_execution(cur, regs, kcb);
-	regs->flags |= kcb->kprobe_saved_flags;
-	trace_hardirqs_fixup_flags(regs->flags);
-
 	/* Restore back the original saved kprobes variables and continue. */
 	if (kcb->kprobe_status == KPROBE_REENTER) {
 		restore_previous_kprobe(kcb);
--
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