Gitweb:     
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b506a9d08bae9336ff9223c8a46a37bf27bd924a
Commit:     b506a9d08bae9336ff9223c8a46a37bf27bd924a
Parent:     3f4380a1e0ea44bc1062ca55e8e479ddcda369fc
Author:     Quentin Barnes <[EMAIL PROTECTED]>
AuthorDate: Wed Jan 30 13:32:32 2008 +0100
Committer:  Ingo Molnar <[EMAIL PROTECTED]>
CommitDate: Wed Jan 30 13:32:32 2008 +0100

    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 <[EMAIL PROTECTED]>
    Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
    Signed-off-by: Thomas Gleixner <[EMAIL PROTECTED]>
    Tested-by: Ananth N Mavinakayahanalli <[EMAIL PROTECTED]>
    Acked-by: Ananth N Mavinakayahanalli <[EMAIL PROTECTED]>
---
 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 a72e02b..711fec8 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;
-
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