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SubjectRe: doublefault debugging (was Re: Linux v2.5.62 --- spontaneous reboots)

On Wed, 19 Feb 2003, Zwane Mwaikambo wrote:
> Here is a triple fault case (2.5.62-pgcl) and since i'm not a Real
> Man i had to use a simulator ;) Unfortunately i can't unwind the stack.

Well, the reason you can't unwind the stack is the same reason you got the
double fault: the stack pointer is crap.

> Freeing unused kernel memory: 100k freed
> double fault, gdt at c0268020 [255 bytes]
> double fault, tss at c027d800
> eip = c01181c4, esp = f7f9bf90
> eax = c0003dfc, ebx = ffffffff, ecx = 0000007b, edx = f7f9c04c
> esi = 00000003, edi = c01181b0

Whee. So the double-fault patch actually ends up being useful? It didn't
help with Chris' problem, but hey, if it helps with something else..

Anyway, that %esp is crap, which also explains this:

> 0xc01181c4 <do_page_fault+20>: mov %eax,0xc(%esp,1)

Took a page fault because 0xc(%esp) wasn't there, and the page fault
couldn't write the fault trace to the stack (same reason), so you got a
double fault.

Anyway, it's hard to try to re-create any state from the above. Very few
clues about why the stack pointer is so messed up, but _usually_ a messed
up stack pointer is because the stack itself got hammered, and then the
stack pointer gets corrupted when somebody restores it off the stack (ie
the normal

movl %ebp,%esp
popl %ebp

kind of epilogue thing).

You could try to make the double-fault handler print out more information,
suggested starting point something like the following: the stack pointer
is corrupted, but we know what the original top-of-stack was (esp0), so we
could print out part of that stack to get a guess about what it was doing
when it all went south..



===== arch/i386/kernel/doublefault.c 1.1 vs edited =====
--- 1.1/arch/i386/kernel/doublefault.c Wed Feb 19 17:48:55 2003
+++ edited/arch/i386/kernel/doublefault.c Wed Feb 19 20:50:47 2003
@@ -33,13 +33,26 @@

if (ptr_ok(tss)) {
struct tss_struct *t = (struct tss_struct *)tss;
+ unsigned long esp0 = t->esp0;

printk("eip = %08lx, esp = %08lx\n", t->eip, t->esp);

printk("eax = %08lx, ebx = %08lx, ecx = %08lx, edx = %08lx\n",
t->eax, t->ebx, t->ecx, t->edx);
- printk("esi = %08lx, edi = %08lx\n",
- t->esi, t->edi);
+ printk("esi = %08lx, edi = %08lx, %ebp = %08lx\n",
+ t->esi, t->edi, t->ebp);
+ /*
+ * We could print out the stack contents here: esp0
+ * is the beginning of the stack, we could print out
+ * all the code points we can find underneath it or
+ * something..
+ */
+ /* This might be a point to try to kill the process and clean up */
+ t->esp = esp0;
+ t->eip = (unsigned long) do_exit;
+ asm volatile("iret");

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