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authorQuentin Barnes <qbarnes@gmail.com>2008-01-30 13:32:32 +0100
committerIngo Molnar <mingo@elte.hu>2008-01-30 13:32:32 +0100
commitb506a9d08bae9336ff9223c8a46a37bf27bd924a (patch)
tree4beb31c17d1a69531ae90046d43f6da45bf75944 /arch/x86/kernel/kprobes.c
parent3f4380a1e0ea44bc1062ca55e8e479ddcda369fc (diff)
x86: code clarification patch to Kprobes arch code
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 <qbarnes@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Ananth N Mavinakayahanalli <ananth@in.ibm.com> Acked-by: Ananth N Mavinakayahanalli <ananth@in.ibm.com>
Diffstat (limited to 'arch/x86/kernel/kprobes.c')
-rw-r--r--arch/x86/kernel/kprobes.c11
1 files changed, 7 insertions, 4 deletions
diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index a72e02bf1135..711fec8f6379 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/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(struct notifier_block *self,
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;