Re: [lkp-robot] [x86/asm] f5caf621ee: PANIC:double_fault

[Josh Poimboeuf]

On Thu, Sep 28, 2017 at 09:21:07AM -0700, Linus Torvalds wrote:
> On Thu, Sep 28, 2017 at 12:47 AM, kernel test robot
> <xiaolong.ye@xxxxxxxxx> wrote:
> >
> > [   10.587519] RIP: 0010:compat_sock_ioctl+0xfea/0x103e
> > [   10.587974] RSP: 0000:0000000000277d78 EFLAGS: 00010283
> > [   10.588448] RAX: 0000000000277d78 RBX: 0000000000008933 RCX: ffff8800141a8000
> > [   10.589103] RDX: 0000000000000020 RSI: 00000000fffbea00 RDI: 00000000fffbea50
> > [   10.589757] RBP: ffffc90000277e18 R08: fffbea50fffbea34 R09: ffffffff814a68c9
> > [   10.590407] R10: ffffff9c00000002 R11: 00000000fffbea50 R12: 0000000000000000
> > [   10.591056] R13: ffff880012c8c880 R14: 00000000fffbea50 R15: 00000000fffbea00
> > [   10.591708] FS:  0000000000000000(0000) GS:ffff880019a00000(0063) knlGS:00000000f7fab9a0
> > [   10.592446] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
> > [   10.592973] CR2: 0000000000277d68 CR3: 000000001807f000 CR4: 00000000000006b0
> > [   10.593623] Call Trace:
> > [   10.593858] Code: 02 0f ff 65 48 8b 04 25 80 d1 00 00 48 8b 80 28 25 00 00 48 83 e8 20 49 39 c7 77 34 89 e0 4c 89 f7 4c 89 fe ba 20 00 00 00 89 c4 <e8> b3 52 05 00 85 c0 74 22 eb 1a 4c 89 fa 89 de 4c 89 ef e8 c6
> > [   10.595705] Kernel panic - not syncing: Machine halted.
> 
> That is some _funky_ code, and yes, this may well be triggered by the
> inline asm changes.
> 
> The code decodes to (after ignoring a few bytes at the beginning that
> were in the middle of an instruction)
> 
>    0: 65 48 8b 04 25 80 d1 mov    %gs:0xd180,%rax
>    7: 00 00
>    9: 48 8b 80 28 25 00 00 mov    0x2528(%rax),%rax
>   10: 48 83 e8 20          sub    $0x20,%rax
>   14: 49 39 c7              cmp    %rax,%r15
>   17: 77 34                ja     0x4d
>   19: 89 e0                mov    %esp,%eax
>   1b: 4c 89 f7              mov    %r14,%rdi
>   1e: 4c 89 fe              mov    %r15,%rsi
>   21: ba 20 00 00 00        mov    $0x20,%edx
>   26: 89 c4                mov    %eax,%esp
>   28:* e8 b3 52 05 00        callq  0x552e0 <-- trapping instruction
>   2d: 85 c0                test   %eax,%eax
>   2f: 74 22                je     0x53
>   31: eb 1a                jmp    0x4d
>   33: 4c 89 fa              mov    %r15,%rdx
>   36: 89 de                mov    %ebx,%esi
>   38: 4c 89 ef              mov    %r13,%rdi
> 
> and it's worth noting that insane
> 
>      mov    %eax,%esp
> 
> instruction, and how RAX (and RSP) both have that bad value of
> 0000000000277d78 in them.
> 
> So double fault is correct - we've corrupted the stack.
> 
> And NOTE! It's reloading 32 bits, not 64 bits, and that's the basic bug there.
> 
> I do note that when I build a kernel, I do see that pattern of
> 
>     movl    $32, %edx
>     call <something>
> 
> and in every case it's a a call to a user copy. One is "call
> _copy_from_user", while the other ones are all the
> alternative_call_2() in copy_user_generic().
> 
> Judging by the offset within the function, and judging by the bug,
> it's almost certainly that alternative_call_2() case.
> 
> So it does sound like the clang fix has now introduced a gcc regression.
> 
> And yes, in both cases it seems to be a compiler bug, but I'm not
> convinced it's a good idea to fix a clang bug by introducing a gcc
> one.
> 
> Anyway, I think the real hint here is that 32-bit reload.
> 
> Lookie here:
> 
>   register unsigned int __asm_call_sp asm("esp");
>   #define ASM_CALL_CONSTRAINT "+r" (__asm_call_sp)
> 
> yeah, that's just garbage. It sure as hell should not be "unsigned int".
> 
> Yeah. yeah, gcc shouldn't do that insane reload in the first place,
> but once that gcc bug has triggered, then the "unsigned int" is what
> makes the code go really bad.
> 
> I bet that changing it to "unsigned long" will just fix things.
> 
> Josh?

Agreed, changing it to "unsigned long" and "rsp" will probably fix it.

I had made it "unsigned int" because of a clang issue with "unsigned
long":

    CC      arch/x86/entry/vdso/vdso32/vclock_gettime.o
  In file included from arch/x86/entry/vdso/vdso32/vclock_gettime.c:32:
  In file included from arch/x86/entry/vdso/vdso32/../vclock_gettime.c:15:
  In file included from ./arch/x86/include/asm/vgtod.h:5:
  In file included from ./include/linux/clocksource.h:12:
  In file included from ./include/linux/timex.h:56:
  In file included from ./include/uapi/linux/timex.h:56:
  In file included from ./include/linux/time.h:5:
  In file included from ./include/linux/seqlock.h:35:
  In file included from ./include/linux/spinlock.h:50:
  In file included from ./include/linux/preempt.h:10:
  In file included from ./include/linux/list.h:8:
  In file included from ./include/linux/kernel.h:10:
  In file included from ./include/linux/bitops.h:37:
  In file included from ./arch/x86/include/asm/bitops.h:16:
  In file included from ./arch/x86/include/asm/alternative.h:9:
  ./arch/x86/include/asm/asm.h:142:42: error: register 'rsp' unsuitable for global register variables on this target
  register unsigned long __asm_call_sp asm("rsp");

And I think we saw the same error in the realmode code.

So we may need to tweak the macro a bit.

-- 
Josh