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SubjectRe: perf_fuzzer compiled for x32 causes reboot
On Thu, 27 Feb 2014 20:34:34 -0500 (EST)
Vince Weaver <vincent.weaver@maine.edu> wrote:


> > I would actually suggest we do the equivalent on i386 as well.
> >
> > Vince, could you try this patch as an experiment?
>
> OK with your patch applied it does not segfault.
>

Vince, Great! Can you remove Peter's patch, and try this one. It
removes the crud to save the cr2 from entry_64.S and makes both i386
and x86_64 do the same thing in regards to cr2 handling.

-- Steve

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 1e96c36..937cb8d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1854,29 +1854,11 @@ end_repeat_nmi:
call save_paranoid
DEFAULT_FRAME 0

- /*
- * Save off the CR2 register. If we take a page fault in the NMI then
- * it could corrupt the CR2 value. If the NMI preempts a page fault
- * handler before it was able to read the CR2 register, and then the
- * NMI itself takes a page fault, the page fault that was preempted
- * will read the information from the NMI page fault and not the
- * origin fault. Save it off and restore it if it changes.
- * Use the r12 callee-saved register.
- */
- movq %cr2, %r12
-
/* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
movq %rsp,%rdi
movq $-1,%rsi
call do_nmi

- /* Did the NMI take a page fault? Restore cr2 if it did */
- movq %cr2, %rcx
- cmpq %rcx, %r12
- je 1f
- movq %r12, %cr2
-1:
-
testl %ebx,%ebx /* swapgs needed? */
jnz nmi_restore
nmi_swapgs:
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 6fcb49c..f1a6294 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -443,7 +443,6 @@ enum nmi_states {
NMI_LATCHED,
};
static DEFINE_PER_CPU(enum nmi_states, nmi_state);
-static DEFINE_PER_CPU(unsigned long, nmi_cr2);

#define nmi_nesting_preprocess(regs) \
do { \
@@ -452,14 +451,11 @@ static DEFINE_PER_CPU(unsigned long, nmi_cr2);
return; \
} \
this_cpu_write(nmi_state, NMI_EXECUTING); \
- this_cpu_write(nmi_cr2, read_cr2()); \
} while (0); \
nmi_restart:

#define nmi_nesting_postprocess() \
do { \
- if (unlikely(this_cpu_read(nmi_cr2) != read_cr2())) \
- write_cr2(this_cpu_read(nmi_cr2)); \
if (this_cpu_dec_return(nmi_state)) \
goto nmi_restart; \
} while (0)
@@ -512,8 +508,21 @@ static inline void nmi_nesting_postprocess(void)
dotraplinkage notrace __kprobes void
do_nmi(struct pt_regs *regs, long error_code)
{
+ unsigned long cr2;
+
nmi_nesting_preprocess(regs);

+ /*
+ * Save off the CR2 register. If we take a page fault in the NMI then
+ * it could corrupt the CR2 value. If the NMI preempts a page fault
+ * handler before it was able to read the CR2 register, and then the
+ * NMI itself takes a page fault, the page fault that was preempted
+ * will read the information from the NMI page fault and not the
+ * origin fault. Save it off and restore it if it changes.
+ * Use the r12 callee-saved register.
+ */
+ cr2 = read_cr2();
+
nmi_enter();

inc_irq_stat(__nmi_count);
@@ -523,6 +532,10 @@ do_nmi(struct pt_regs *regs, long error_code)

nmi_exit();

+ /* Reads are cheaper than writes */
+ if (unlikely(cr2 != read_cr2()))
+ write_cr2(cr2);
+
/* On i386, may loop back to preprocess */
nmi_nesting_postprocess();
}

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