lkml.org 
[lkml]   [2019]   [Jul]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 20/22] objtool: Fix seg fault on bad switch table entry
On Mon, Jul 15, 2019 at 10:24:24AM -0700, Nick Desaulniers wrote:
> On Sun, Jul 14, 2019 at 5:37 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > In one rare case, Clang generated the following code:
> >
> > 5ca: 83 e0 21 and $0x21,%eax
> > 5cd: b9 04 00 00 00 mov $0x4,%ecx
> > 5d2: ff 24 c5 00 00 00 00 jmpq *0x0(,%rax,8)
> > 5d5: R_X86_64_32S .rodata+0x38
> >
> > which uses the corresponding jump table relocations:
> >
> > 000000000038 000200000001 R_X86_64_64 0000000000000000 .text + 834
> > 000000000040 000200000001 R_X86_64_64 0000000000000000 .text + 5d9
> > 000000000048 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000050 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000058 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000060 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000068 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000070 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000078 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000080 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000088 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000090 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000098 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000a0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000a8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000b0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000b8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000c0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000c8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000d0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000d8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000e0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000e8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000f0 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 0000000000f8 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000100 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000108 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000110 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000118 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000120 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000128 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000130 000200000001 R_X86_64_64 0000000000000000 .text + b96
> > 000000000138 000200000001 R_X86_64_64 0000000000000000 .text + 82f
> > 000000000140 000200000001 R_X86_64_64 0000000000000000 .text + 828
> >
> > Since %eax was masked with 0x21, only the first two and the last two
> > entries are possible.
> >
> > Objtool doesn't actually emulate all the code, so it isn't smart enough
> > to know that all the middle entries aren't reachable. They point to the
> > NOP padding area after the end of the function, so objtool seg faulted
> > when it tried to dereference a NULL insn->func.
> >
> > After this fix, objtool still gives an "unreachable" error because it
> > stops reading the jump table when it encounters the bad addresses:
> >
> > /home/jpoimboe/objtool-tests/adm1275.o: warning: objtool: adm1275_probe()+0x828: unreachable instruction
> >
> > While the above code is technically correct, it's very wasteful of
> > memory -- it uses 34 jump table entries when only 4 are needed. It's
> > also not possible for objtool to validate this type of switch table
> > because the unused entries point outside the function and objtool has no
> > way of determining if that's intentional. Hopefully the Clang folks can
> > fix it.
>
> So this came from
> drivers/hwmon/pmbus/adm1275.c ?
> Any special configuration?

Arnd shared the config and the .o file here:

https://lkml.kernel.org/r/CAK8P3a2beBPP+KX4zTfSfFPwo+7ksWZLqZzpP9BJ80iPecg3zA@mail.gmail.com

I used Arnd's .o file for testing.

--
Josh

\
 
 \ /
  Last update: 2019-07-15 19:30    [W:0.050 / U:5.200 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site