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...
> [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
struct mm_struct *mm = vma->vm_mm;
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?
Any chance you can tell us where exactly we hit NULL-deref in unmap_vmas?
Oleg.
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