On 09/24/2015 09:11 AM, Oleg Nesterov wrote:
> On 09/15, Sasha Levin wrote:
>>
>> I've modified my tests to stress the exit path of processes with many vmas,
>> and hit the following NULL ptr deref (not sure if it's related to the original issue):
>
> I am shy to ask. Looks like I am the only stupid one who needs
> more info...
>
>> [1181047.935563] kasan: GPF could be caused by NULL-ptr deref or user memory accessgeneral protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN
>
> 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.
>> [1181047.937223] Modules linked in:
>> [1181047.937772] CPU: 4 PID: 21912 Comm: trinity-c341 Not tainted 4.3.0-rc1-next-20150914-sasha-00043-geddd763-dirty #2554
>> [1181047.939387] task: ffff8804195c8000 ti: ffff880433f00000 task.ti: ffff880433f00000
>> [1181047.940533] RIP: unmap_vmas (mm/memory.c:1337)
>
> I do not know which tree/branch do you use. In Linus's tree mm/memory.c:1337 is
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);
}
> but this doesn't match the asm below,
>
>> 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.
Looking at the assembly, the address we were trying to access was:
RDI: 00000000000004c8
> Any chance you can tell us where exactly we hit NULL-deref in unmap_vmas?
I hope the information above helped, please let me know if it didn't and you
need anything else.
Thanks,
Sasha
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