On Tue, Jan 01, 2008 at 04:19:43PM +0100, Ingo Molnar wrote: > > * 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]>
Tested on x86. Acked-by: Ananth N Mavinakayahanalli <[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; > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [EMAIL PROTECTED] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
