Re: General Protection Fault in prelist_remove()

On Mon, Sep 16, 2013 at 05:27:30PM +0200, Hans Petter Selasky wrote:
> Hi,
> 
> I caught a General protection fault in prelist_remove. Any clues what 
> this might be?

Any chance you were creating or destroying interfaces around the time
this crash happened?

There is no locking in the code which manipulates the prefix list (or
any of the global NDP data structures), so it's possible to get crashes
if, for instance, the prefix expiry callout races with in6_ifdetach() to
delete a prefix. It's not clear what caused your crash (not without a
crash dump at least), but I imagine it has something to do with this.

I've partially fixed this at work by adding a rw lock to protect access
to the the prefix, default router, and DAD lists. The patch is here:
http://people.freebsd.org/~markj/patches/ndp-locking.diff

If anyone can review or test this patch, please let me know.

Thanks,
-Mar

> 
> FreeBSD-10 from one month back approx.
> 
> > ffffffff80a95810 <prelist_remove>:
> > ffffffff80a95810:       55                      push   %rbp
> > ffffffff80a95811:       48 89 e5                mov    %rsp,%rbp
> > ffffffff80a95814:       41 57                   push   %r15
> > ffffffff80a95816:       41 56                   push   %r14
> > ffffffff80a95818:       53                      push   %rbx
> > ffffffff80a95819:       48 83 ec 38             sub    $0x38,%rsp
> > ffffffff80a9581d:       49 89 ff                mov    %rdi,%r15
> > ffffffff80a95820:       48 8b 04 25 c0 de 3b    mov    0xffffffff813bdec0,%rax
> > ffffffff80a95827:       81
> > ffffffff80a95828:       48 89 45 e0             mov    %rax,-0x20(%rbp)
> > ffffffff80a9582c:       49 c7 47 44 00 00 00    movq   $0x0,0x44(%r15)
> > ffffffff80a95833:       00
> > ffffffff80a95834:       41 f6 47 6c 01          testb  $0x1,0x6c(%r15)
> > ffffffff80a95839:       74 4d                   je     ffffffff80a95888 <prelist_remove+0x78>
> > ffffffff80a9583b:       4c 89 ff                mov    %r15,%rdi
> > ffffffff80a9583e:       e8 fd 00 00 00          callq  ffffffff80a95940 <nd6_prefix_offlink>
> > ffffffff80a95843:       41 89 c6                mov    %eax,%r14d
> > ffffffff80a95846:       45 85 f6                test   %r14d,%r14d
> > ffffffff80a95849:       74 3d                   je     ffffffff80a95888 <prelist_remove+0x78>
> > ffffffff80a9584b:       8b 04 25 c4 a6 56 81    mov    0xffffffff8156a6c4,%eax
> > ffffffff80a95852:       85 c0                   test   %eax,%eax
> > ffffffff80a95854:       74 32                   je     ffffffff80a95888 <prelist_remove+0x78>
> > ffffffff80a95856:       49 8d 77 20             lea    0x20(%r15),%rsi
> > ffffffff80a9585a:       48 8d 7d b0             lea    -0x50(%rbp),%rdi
> > ffffffff80a9585e:       e8 1d fa fd ff          callq  ffffffff80a75280 <ip6_sprintf>
> > ffffffff80a95863:       41 0f b6 4f 78          movzbl 0x78(%r15),%ecx
> > ffffffff80a95868:       4d 8b 07                mov    (%r15),%r8
> > ffffffff80a9586b:       49 83 c0 28             add    $0x28,%r8
> > ffffffff80a9586f:       bf 03 00 00 00          mov    $0x3,%edi
> > ffffffff80a95874:       48 c7 c6 c6 17 fa 80    mov    $0xffffffff80fa17c6,%rsi
> > ffffffff80a9587b:       48 89 c2                mov    %rax,%rdx
> > ffffffff80a9587e:       45 89 f1                mov    %r14d,%r9d
> > ffffffff80a95881:       30 c0                   xor    %al,%al
> > ffffffff80a95883:       e8 08 f0 e5 ff          callq  ffffffff808f4890 <log>
> > ffffffff80a95888:       41 83 7f 7c 00          cmpl   $0x0,0x7c(%r15)
> > ffffffff80a9588d:       7f 6c                   jg     ffffffff80a958fb <prelist_remove+0xeb>
> > ffffffff80a9588f:       49 8b 47 08             mov    0x8(%r15),%rax
> > ffffffff80a95893:       48 85 c0                test   %rax,%rax
> > ffffffff80a95896:       74 0a                   je     ffffffff80a958a2 <prelist_remove+0x92>
> > ffffffff80a95898:       49 8d 4f 08             lea    0x8(%r15),%rcx
> > ffffffff80a9589c:       48 39 48 10             cmp    %rcx,0x10(%rax)
> ^^^^ crash here
> > ffffffff80a958a0:       75 72                   jne    ffffffff80a95914 <prelist_remove+0x104>
> > ffffffff80a958a2:       49 8b 4f 10             mov    0x10(%r15),%rcx
> 
> Non-reproducable.
> 
> --HPS
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