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SubjectUgly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)
Hi,

I've got the following oopsie two times now when hibernating - this
means, I don't get it everytime I hibernate but only sometimes, say once
in a blue moon.

And yeah, I couldn't catch it over serial console so I had to make ugly
pictures. By the way, the numbers in the filenames increment as I scroll
down the whole oops (yep, it hadn't completely frozen and I still could
do Shift->PgUp or Shift->PgDn on the console):

http://www.kernel.org/pub/linux/kernel/people/bp/

So, here's what I could decipher from the oopsie, someone else who's
more knowledgeable in mm, rmap and anon_vma's list traversal should be
able to tell what goes wrong there.

EIP is at page_referenced+0xee

which is

<disasm>
10c4: 41 01 c4 add %eax,%r12d
10c7: 83 7d cc 00 cmpl $0x0,-0x34(%rbp)
10cb: 74 19 je 10e6 <page_referenced+0xff>
10cd: 4d 8b 6d 20 mov 0x20(%r13),%r13
10d1: 49 83 ed 20 sub $0x20,%r13

10d5: 49 8b 45 20 mov 0x20(%r13),%rax <--------------
10d9: 0f 18 08 prefetcht0 (%rax)
10dc: 49 8d 45 20 lea 0x20(%r13),%rax
10e0: 48 39 45 80 cmp %rax,-0x80(%rbp)
</disasm>


Corresponding asm:

<asm>
.loc 1 496 0
movq 32(%r13), %r13 # <variable>.same_anon_vma.next, __mptr.451
.LVL295:
subq $32, %r13 #, avc
.LVL296:
.L184:
.LBE1278:
movq 32(%r13), %rax # <variable>.same_anon_vma.next, <variable>.same_anon_vma.next <----------------
prefetcht0 (%rax) # <variable>.same_anon_vma.next
leaq 32(%r13), %rax #, tmp97
cmpq %rax, -128(%rbp) # tmp97, %sfp
jne .L187 #,
.L186:
.loc 1 514 0
movq %r14, %rdi # anon_vma,
call page_unlock_anon_vma #
</asm>

and the NULL pointer in question is being written into %r13 and then 32
is subtracted from it (I'm guessing container_of()). This is consistent
with the register snapshot - %r13 contains 0xffffffffffffffe0 which is
-32 and with the code dump in the oops, in CIMG1640.JPG code points to
opcode 49 8b 45 20.
Which is the following piece of code in <mm/rmap.c:page_referenced_anon()>.

<source>

mapcount = page_mapcount(page);
list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
struct vm_area_struct *vma = avc->vma;
unsigned long address = vma_address(page, vma);
if (address == -EFAULT)
continue;
</source>

which tells us that same_anon_vma.next is NULL. Hmm...

--
Regards/Gruss,
Boris.


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