* Quentin Barnes <[EMAIL PROTECTED]> wrote:
> Since people are discussing some x86 Kprobes code cleanup, I thought I
> would contribute a small change as well. When developing the Kprobes
> arch code for ARM, I ran across some code found in x86 and s390
> Kprobes arch code which I didn't consider as good as it could be.
>
> Once I figured out what the code was doing, I changed the code for ARM
> Kprobes to work the way I felt was more appropriate. I've tested the
> code this way in ARM for about a year and would like to push the same
> change to the other affected architectures.
thanks Quentin, it looks good to me and i've applied the x86 bit to
x86.git. (find the merged patch attached below)
small note:
> @@ -654,12 +655,12 @@ int __kprobes kprobe_exceptions_notify(struct
> notifier_block *self,
> ret = NOTIFY_STOP;
your email client apparently line-wrapped this portion of the patch - i
fixed it up manually (wasnt a big issue). Please see
Documentation/email-clients.txt about how to set up your email client.
Ingo
-------------------->
Subject: Code clarification patch to Kprobes arch code
From: "Quentin Barnes" <[EMAIL PROTECTED]>
When developing the Kprobes arch code for ARM, I ran across some code
found in x86 and s390 Kprobes arch code which I didn't consider as
good as it could be.
Once I figured out what the code was doing, I changed the code
for ARM Kprobes to work the way I felt was more appropriate.
I've tested the code this way in ARM for about a year and would
like to push the same change to the other affected architectures.
The code in question is in kprobe_exceptions_notify() which
does:
====
/* kprobe_running() needs smp_processor_id() */
preempt_disable();
if (kprobe_running() &&
kprobe_fault_handler(args->regs, args->trapnr))
ret = NOTIFY_STOP;
preempt_enable();
====
For the moment, ignore the code having the preempt_disable()/
preempt_enable() pair in it.
The problem is that kprobe_running() needs to call smp_processor_id()
which will assert if preemption is enabled. That sanity check by
smp_processor_id() makes perfect sense since calling it with preemption
enabled would return an unreliable result.
But the function kprobe_exceptions_notify() can be called from a
context where preemption could be enabled. If that happens, the
assertion in smp_processor_id() happens and we're dead. So what
the original author did (speculation on my part!) is put in the
preempt_disable()/preempt_enable() pair to simply defeat the check.
Once I figured out what was going on, I considered this an
inappropriate approach. If kprobe_exceptions_notify() is called
from a preemptible context, we can't be in a kprobe processing
context at that time anyways since kprobes requires preemption to
already be disabled, so just check for preemption enabled, and if
so, blow out before ever calling kprobe_running(). I wrote the ARM
kprobe code like this:
====
/* To be potentially processing a kprobe fault and to
* trust the result from kprobe_running(), we have
* be non-preemptible. */
if (!preemptible() && kprobe_running() &&
kprobe_fault_handler(args->regs, args->trapnr))
ret = NOTIFY_STOP;
====
The above code has been working fine for ARM Kprobes for a year.
So I changed the x86 code (2.6.24-rc6) to be the same way and ran
the Systemtap tests on that kernel. As on ARM, Systemtap on x86
comes up with the same test results either way, so it's a neutral
external functional change (as expected).
This issue has been discussed previously on linux-arm-kernel and the
Systemtap mailing lists. Pointers to the by base for the two
discussions:
http://lists.arm.linux.org.uk/lurker/message/20071219.223225.1f5c2a5e.en.html
http://sourceware.org/ml/systemtap/2007-q1/msg00251.html
Signed-off-by: Quentin Barnes <[EMAIL PROTECTED]>
Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
Signed-off-by: Thomas Gleixner <[EMAIL PROTECTED]>
---
arch/x86/kernel/kprobes.c | 11 +++++++----
1 file changed, 7 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
@@ -44,6 +44,7 @@
#include <linux/ptrace.h>
#include <linux/string.h>
#include <linux/slab.h>
+#include <linux/hardirq.h>
#include <linux/preempt.h>
#include <linux/module.h>
#include <linux/kdebug.h>
@@ -951,12 +952,14 @@ int __kprobes kprobe_exceptions_notify(s
ret = NOTIFY_STOP;
break;
case DIE_GPF:
- /* kprobe_running() needs smp_processor_id() */
- preempt_disable();
- if (kprobe_running() &&
+ /*
+ * To be potentially processing a kprobe fault and to
+ * trust the result from kprobe_running(), we have
+ * be non-preemptible.
+ */
+ if (!preemptible() && kprobe_running() &&
kprobe_fault_handler(args->regs, args->trapnr))
ret = NOTIFY_STOP;
- preempt_enable();
break;
default:
break;
--
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