2015-09-24 20:26 GMT+03:00 Oleg Nesterov <oleg@xxxxxxxxxx>:
> On 09/24, Sasha Levin wrote:
>>
>> On 09/24/2015 09:11 AM, Oleg Nesterov wrote:
>> >
>> > Well, I know absolutely nothing about kasan, to the point I can't even
>> > unserstand where does this message come from. grep didn't help. But this
>> > doesn't matter...
>>
>> The reason behind this message is that NULL ptr derefs when using kasan are
>> manifested as GFPs. This is because in order to validate an access to a given
>> memory address kasan would check (shadow_base + (mem_offset >> 3)), so in the case of
>> a NULL it would try to access shadow_base + 0, which would GFP.
>
> OK, so this just means the kernele derefs the NULL pointer,
>
>> I'm running -next + Kirill's THP patchset.
>>
>> > struct mm_struct *mm = vma->vm_mm;
>>
>> void unmap_vmas(struct mmu_gather *tlb,
>> struct vm_area_struct *vma, unsigned long start_addr,
>> unsigned long end_addr)
>> {
>> struct mm_struct *mm = vma->vm_mm;
>>
>> mmu_notifier_invalidate_range_start(mm, start_addr, end_addr);
>> for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next)
>> unmap_single_vma(tlb, vma, start_addr, end_addr, NULL); <--- this
>> mmu_notifier_invalidate_range_end(mm, start_addr, end_addr);
>> }
>
> And I do not see any dereference at this line,
>
I noticed, that addr2line sometimes doesn't work reliably on
compiler-instrumented code.
I've seen couple times that it points to the next line of code.
>> >> 0: 08 80 3c 02 00 0f or %al,0xf00023c(%rax)
>> >> 6: 85 22 test %esp,(%rdx)
>> >> 8: 01 00 add %eax,(%rax)
>> >> a: 00 48 8b add %cl,-0x75(%rax)
>> >> d: 43 rex.XB
>> >> e: 40 rex
>> >> f: 48 8d b8 c8 04 00 00 lea 0x4c8(%rax),%rdi
>> >> 16: 48 89 45 d0 mov %rax,-0x30(%rbp)
>> >> 1a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
>> >> 21: fc ff df
>> >> 24: 48 89 fa mov %rdi,%rdx
>> >> 27: 48 c1 ea 03 shr $0x3,%rdx
>> >> 2b:* 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction
>> >> 2f: 0f 85 ee 00 00 00 jne 0x123
>> >> 35: 48 8b 45 d0 mov -0x30(%rbp),%rax
>> >> 39: 48 83 b8 c8 04 00 00 cmpq $0x0,0x4c8(%rax)
>> >> 40: 00
>> >
>> > And I do not see anything similar in "objdump -d". So could you at least
>> > show mm/memory.c:1337 in your tree?
>> >
>> > Hmm. movabs $0xdffffc0000000000,%rax above looks suspicious, this looks
>> > like kasan_mem_to_shadow(). So perhaps this code was generated by kasan?
>> > (I can't check, my gcc is very old). Or what?
>>
>> This is indeed kasan code. 0xdffffc0000000000 is the shadow base, and you see
>> kasan trying to access shadow base + (ptr >> 3), which is why we get GFP.
>
> and thus this asm can't help, right?
>
I think it can.
> So how can we figure out where exactly the kernel hits NULL ? And what
> exactly it tries to dereference?
So we tried to dereference 0x4c8. That 0x4c8 is probably offset in some struct.
The only big struct here is mm_struct.
So I think that we tried to derefernce null mm, and this asm:
> cmpq $0x0,0x4c8(%rax)
is likely from inlined mm_has_notifiers():
static inline int mm_has_notifiers(struct mm_struct *mm)
{
return unlikely(mm->mmu_notifier_mm);
}
Sasha, could you confirm that in your kernel mmu_notifier_mm field has
0x4c8 offset?
I would use gdb for that:
gdb vmlinux
(gdb) p/x &(((struct mm_struct*)0)->mmu_notifier_mm)
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