Commits
Josh Poimboeuf committed e65050b94d8
objtool: Fix seg fault on bad switch table entry
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.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/a9db88eec4f1ca089e040989846961748238b6d8.1563413318.git.jpoimboe@redhat.com