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

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Date:   Mon, 15 Jul 2019 12:29:34 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [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@...hat.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