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Message-ID: <CA+icZUXo4THf+fcx6wC_Q45p=_e1xpj=pFwNBjeoO1iQVb8YNA@mail.gmail.com>
Date: Tue, 24 Apr 2018 15:46:53 +0200
From: Sedat Dilek <sedat.dilek@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Alan Stern <stern@...land.harvard.edu>,
David Laight <David.Laight@...lab.com>,
Jiri Kosina <jikos@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Tejun Heo <tj@...nel.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Benjamin Tissoires <benjamin.tissoires@...hat.com>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
Andy Lutomirski <luto@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
USB list <linux-usb@...r.kernel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Matthias Kaehlcke <mka@...omium.org>,
Michael Hordijk <hoffbrinkle@...mail.com>, chandlerc@...il.com
Subject: Re: Clang and X86-EFlags (was Re: [PATCH] usbhid: Fix lockdep
unannotated irqs-off warning)
On Tue, Apr 24, 2018 at 3:28 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
> On Mon, Jun 27, 2016 at 10:14 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Mon, Jun 27, 2016 at 12:50 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>>>
>>> $ objdump -S clang-eflag.o
>>>
>>> clang-eflag.o: file format elf64-x86-64
>>>
>>>
>>> Disassembly of section .text:
>>>
>>> 0000000000000000 <bar>:
>>> 0: 55 push %rbp
>>> 1: 48 89 e5 mov %rsp,%rbp
>>> 4: 53 push %rbx
>>> 5: 50 push %rax
>>> 6: e8 00 00 00 00 callq b <bar+0xb>
>>> b: ff 0d 00 00 00 00 decl 0x0(%rip) # 11 <bar+0x11>
>>> 11: 9c pushfq
>>> 12: 5b pop %rbx
>>> 13: e8 00 00 00 00 callq 18 <bar+0x18>
>>> 18: b8 01 00 00 00 mov $0x1,%eax
>>> 1d: 53 push %rbx
>>> 1e: 9d popfq
>>> 1f: 75 07 jne 28 <bar+0x28>
>>
>>
>> Yeah, the above is pure garbage.
>>
>>> So, the issue is still alive.
>>>
>>> What do you mean by "for the kernel we at a minimum need a way to
>>> disable that code generation"?
>>> Can this be fixed in the Linux-kernel?
>>
>> No. This will never be fixed in the kernel. It's a compiler bug.
>>
>> The compiler generates shit code. It's absolutely atrociously bad even
>> if you ignore any kernel issues, because that kind of code just
>> performs badly (the compiler should have used "setcc" or something
>> similar to just set the comparison value, not save and restore eflags.
>>
>> And quite frankly, any compiler writer that thinks it is good code is
>> not somebody I want touching a compiler that the kernel depends on
>> anyway.
>>
>> But it is not just bad code for the kernel, it's actively buggy code,
>> since it corrupts the IF.
>>
>> Until this gets fixed in LLVM, there's no way in hell that we will
>> ever have a kernel compiled with that piece of shit.
>>
>> Really. If the LLVM developers cannot fix their crap code generation,
>> it's not worth touching that shit with a ten-foot pole.
>>
>> I'd love to be able to compile the kernel with LLVM, but the fact that
>> the broken eflags code apparently _still_ hasn't been fixed makes me
>> just go "not worth it".
>>
>> And if the LLVM developers don't see this as an obvious bug, it's even
>> less worth it - because that shows not just that the compiler is
>> broken, but that the developers involved with it are broken too.
>>
>> Linus
>
> [ Changed Subject ]
> [ CC Matthias ]
> [ CC Michael test-case ]
> [ CC Chandler ]
>
> Hi Linus,
>
> Matthias pointed me in [0] to [1] in the LLVM-BTS.
>
> ...and I tried again the test-case from Michael from [3] "[LLVMdev]
> optimizer clobber EFLAGS"...
>
> ...with clang-7 (version:
> 7~svn330207-1~exp1+0~20180417201234.1709~1.gbp6fb10d) from
> <https://apt.llvm.org/>.
>
> [ TEST-CASE ]
>
> [ clang-eflag.c ]
> #include <stdlib.h>
> #include <stdbool.h>
>
> void foo(void);
> int a;
>
> int bar(void)
> {
> foo();
>
> bool const zero = a -= 1;
>
> asm volatile ("" : : : "cc");
> foo();
>
> if (zero) {
> return EXIT_FAILURE;
> }
>
> foo();
>
> return EXIT_SUCCESS;
> }
> - EOF -
>
> $ clang-7 -O2 -c -o clang-eflag.o clang-eflag.c
>
> [ OBJDUMP ]
>
> $ objdump -S clang-eflag.o
>
> clang-eflag.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <bar>:
> 0: 53 push %rbx
> 1: e8 00 00 00 00 callq 6 <bar+0x6>
> 6: 83 05 00 00 00 00 ff addl $0xffffffff,0x0(%rip) #
> d <bar+0xd>
> d: 0f 95 c3 setne %bl
> 10: e8 00 00 00 00 callq 15 <bar+0x15>
> 15: b8 01 00 00 00 mov $0x1,%eax
> 1a: f6 c3 ff test $0xff,%bl
> 1d: 74 02 je 21 <bar+0x21>
> 1f: 5b pop %rbx
> 20: c3 retq
> 21: e8 00 00 00 00 callq 26 <bar+0x26>
> 26: 31 c0 xor %eax,%eax
> 28: 5b pop %rbx
> 29: c3 retq
>
> Does this now look good?
>
> Thanks.
>
> Kind regards,
> - Sedat -
>
> [0] https://marc.info/?l=linux-kernel&m=152450535720279&w=2
> [1] https://bugs.llvm.org/show_bug.cgi?id=36028
> [2] http://lists.llvm.org/pipermail/llvm-dev/2015-July/088766.html
> [3] https://marc.info/?l=linux-kernel&m=152457089205170&w=2
For the sake of completeness the two fixes in LLVM upstream.
>From [4]:
Chandler Carruth 2018-04-09 23:41:58 PDT
This should finally be resolved in its entirety with r329657 (and r329673).
"[x86] Introduce a pass to begin more systematically fixing PR36028
and similar issues"
https://github.com/llvm-mirror/llvm/commit/21a0c18174343502c9f2b546a01333d1c351d9c0
svn -r329657
"[x86] Model the direction flag (DF) separately from the rest of EFLAGS."
https://github.com/llvm-mirror/llvm/commit/9f8f7c5e8ab12afcc92f51d0ed596ac0867eb0fa
svn-r329673
- Sedat -
[4] https://bugs.llvm.org/show_bug.cgi?id=36028#c2
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