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Date:   Fri, 1 Mar 2019 16:07:42 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     hpa@zytor.com
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        "linux-alpha@vger.kernel.org" <linux-arm-kernel@lists.infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: [RFC][PATCH] objtool: STAC/CLAC validation
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On Mon, Feb 25, 2019 at 02:21:03PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 25, 2019 at 12:47:00AM -0800, hpa@zytor.com wrote:
> > It doesn't have to understand the contents of the memop, but it seems
> > that the presence of a modrm with mode ≠ 3 should be plenty. It needs
> > to know that much in order to know the length of instructions anyway.
> > For extra credit, ignore LEA or hinting instructions.
> 
> A little something like so then?


$ ./objtool check --no-fp --backtrace ../../defconfig-build/arch/x86/lib/usercopy_64.o
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0x3: UACCESS disable without MEMOPs: __clear_user()
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: __clear_user()+0x3a: (alt)
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: __clear_user()+0x2e: (branch)
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: __clear_user()+0x18: (branch)
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0xffffffffffffffff: (branch)
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: __clear_user()+0x5: (alt)
../../defconfig-build/arch/x86/lib/usercopy_64.o: warning: objtool: __clear_user()+0x0: <=== (func)


0000000000000000 <__clear_user>:
   0:   e8 00 00 00 00          callq  5 <__clear_user+0x5>
   1: R_X86_64_PLT32       __fentry__-0x4
   5:   90                      nop
   6:   90                      nop
   7:   90                      nop
   8:   48 89 f0                mov    %rsi,%rax
   b:   48 c1 ee 03             shr    $0x3,%rsi
   f:   83 e0 07                and    $0x7,%eax
  12:   48 89 f1                mov    %rsi,%rcx
  15:   48 85 c9                test   %rcx,%rcx
  18:   74 0f                   je     29 <__clear_user+0x29>
  1a:   48 c7 07 00 00 00 00    movq   $0x0,(%rdi)
  21:   48 83 c7 08             add    $0x8,%rdi
  25:   ff c9                   dec    %ecx
  27:   75 f1                   jne    1a <__clear_user+0x1a>
  29:   48 89 c1                mov    %rax,%rcx
  2c:   85 c9                   test   %ecx,%ecx
  2e:   74 0a                   je     3a <__clear_user+0x3a>
  30:   c6 07 00                movb   $0x0,(%rdi)
  33:   48 ff c7                inc    %rdi
  36:   ff c9                   dec    %ecx
  38:   75 f6                   jne    30 <__clear_user+0x30>
  3a:   90                      nop
  3b:   90                      nop
  3c:   90                      nop
  3d:   48 89 c8                mov    %rcx,%rax
  40:   c3                      retq


Seems correct. Not sure you want to go fix that though. Let me know if
you want more output.