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SubjectRe: 2.6.28-rc6-git1 -- BUG: unable to handle kernel paging request at ffff8800be8b0019
On Thu, 27 Nov 2008, Pekka Enberg wrote:
>
> (I'm jumping in as Andrew forwarded the bug to us thinking it's SLUB related.)
>
> On Thu, Nov 27, 2008 at 1:26 AM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> >> [ 3866.841128] RIP [<ffffffff80262f07>] kallsyms_lookup+0x20/0x120
> >> [ 3866.841134] RSP <ffff880073d63da8>
> >> [ 3866.841136] CR2: ffff8800be8b0019
> >> [ 3866.841140] ---[ end trace ebccc2f1a2509fb0 ]---
> >
> > Did that happen after a resume from suspend to RAM, by chance?
>
> Could be. I looked at the oops and I'm pretty sure SLUB is not at
> fault here. Decoding the oopsing code:
>
> [ 3866.841062] Code: df e8 39 d2 ff ff 5b 41 5c c9 c3 55 48 89 e5 41 57 49 89
> cf 41 56 49 89 f6 41 55 49 89 d5 41 54 4d 89 c4 53 48 89 fb 48 83 ec
> 08 <41> c6
> 40 7f 00 41 c6 00 00 48 81 ff 00 90 20 80 72 09 48 81 ff
>
> results in:
>
> 0000000000000000 <.text>:
> 0: 41 c6 40 7f 00 movb $0x0,0x7f(%r8) <<<<----
> 5: 41 c6 00 00 movb $0x0,(%r8)
> 9: 48 81 ff 00 90 20 80 cmp $0xffffffff80209000,%rdi
> 10: 72 09 jb 0x1b
> 12: 48 rex.W
> 13: 81 .byte 0x81
> 14: ff .byte 0xff
>
> which looks like this:
>
> 000000000000023e <kallsyms_lookup>:
> */
> const char *kallsyms_lookup(unsigned long addr,
> unsigned long *symbolsize,
> unsigned long *offset,
> char **modname, char *namebuf)
> {
> 23e: 41 56 push %r14
> 240: 49 89 f6 mov %rsi,%r14
> 243: 41 55 push %r13
> 245: 49 89 d5 mov %rdx,%r13
> 248: 41 54 push %r12
> 24a: 49 89 cc mov %rcx,%r12
> 24d: 55 push %rbp
> 24e: 48 89 fd mov %rdi,%rbp
> 251: 53 push %rbx
> namebuf[KSYM_NAME_LEN - 1] = 0;
> 252: 41 c6 40 7f 00 movb $0x0,0x7f(%r8) <<<<----
> */
>
> That is, we're oopsing because someone is passing a bogus 'namebuf' to
> kallsyms_lookup(). This is further confirmed by looking at the value of R8:
>
> [ 3866.840962] RBP: ffff880073d63dd8 R08: ffff8800be8aff9a R09:
> 0000000000000000
>
> and adding 0x7f to it:
>
> 0xffff8800be8aff9a + 0x7f = 0xffff8800be8b0019
>
> which equals to the faulting address:
>
> [ 3866.840809] BUG: unable to handle kernel paging request at ffff8800be8b0019
>
> Furthermore, the value of KSYM_NAME_LEN is 128 so the offset matches as well
> after subtracting one from it (0x7f).
>
> Looking at the call trace:
>
> [ 3866.841017] Call Trace:
> [ 3866.841020] [<ffffffff8026302f>] sprint_symbol+0x28/0xaa
> [ 3866.841025] [<ffffffff8029e972>] list_locations+0x170/0x2ef
> [ 3866.841031] [<ffffffff8029eb34>] alloc_calls_show+0x1c/0x24
> [ 3866.841036] [<ffffffff8029cbbc>] slab_attr_show+0x23/0x27
> [ 3866.841041] [<ffffffff802efdff>] sysfs_read_file+0xba/0x13c
> [ 3866.841046] [<ffffffff802a4795>] vfs_read+0xa4/0xde
> [ 3866.841052] [<ffffffff802a4893>] sys_read+0x47/0x6e
> [ 3866.841056] [<ffffffff8020b6fb>] system_call_fastpath+0x16/0x1b
>
> we can see that kallsyms_lookup() is being called by sprint_symbol() which is,
> in turn, called by the SLUB code. However, SLUB never touches 'namebuf',
> instead it's being allocated on the stack by sprint_symbol():
>
> /* Look up a kernel symbol and return it in a text buffer. */
> int sprint_symbol(char *buffer, unsigned long address)
> {
> char *modname;
> const char *name;
> unsigned long offset, size;
> char namebuf[KSYM_NAME_LEN];
>
> name = kallsyms_lookup(address, &size, &offset, &modname, namebuf);
>
> Hmm?

I think you're looking at a 2.6.28-rc5 sprint_symbol() there:
the world has moved on since those days. I changed it to use the
supplied "buffer" instead of local "namebuf" in 2.6.28-rc6, so we
have to wonder if my patch is to blame - though I don't see it.

Sorry, I'm eating lunch then about to go out for a couple of hours:
can't look into it now, but maybe this info will help you to make
better sense of what's going on.

Hugh


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