Re: [PATCH 20/22] objtool: Fix seg fault on bad switch table entry

From: Nick Desaulniers
Date: Mon Jul 15 2019 - 13:24:39 EST


On Sun, Jul 14, 2019 at 5:37 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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?
--
Thanks,
~Nick Desaulniers