diff options
author | Quentin Barnes <qbarnes@gmail.com> | 2008-01-30 13:32:32 +0100 |
---|---|---|
committer | Ingo Molnar <mingo@elte.hu> | 2008-01-30 13:32:32 +0100 |
commit | b506a9d08bae9336ff9223c8a46a37bf27bd924a (patch) | |
tree | 4beb31c17d1a69531ae90046d43f6da45bf75944 /arch/x86 | |
parent | 3f4380a1e0ea44bc1062ca55e8e479ddcda369fc (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')
-rw-r--r-- | arch/x86/kernel/kprobes.c | 11 |
1 files changed, 7 insertions, 4 deletions
diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c index a72e02bf113..711fec8f637 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; |