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Subject: Re: [PATCH] x86/uaccess: fix code generation in put_user()
To:     Andy Lutomirski <luto@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
CC:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>
From:   hpa@zytor.com
Message-ID: <AD6F6BCA-B614-4218-9C0C-D2B8A1D7F338@zytor.com>
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On October 23, 2020 1:42:39 PM PDT, Andy Lutomirski <luto@kernel=2Eorg> wro=
te:
>On Fri, Oct 23, 2020 at 1:32 PM Rasmus Villemoes
><linux@rasmusvillemoes=2Edk> wrote:
>>
>> Quoting
>> https://gcc=2Egnu=2Eorg/onlinedocs/gcc/Local-Register-Variables=2Ehtml:
>>
>>   You can define a local register variable and associate it with a
>>   specified register=2E=2E=2E
>>
>>   The only supported use for this feature is to specify registers for
>>   input and output operands when calling Extended asm (see Extended
>>   Asm)=2E This may be necessary if the constraints for a particular
>>   machine don't provide sufficient control to select the desired
>>   register=2E
>>
>> On 32-bit x86, this is used to ensure that gcc will put an 8-byte
>> value into the %edx:%eax pair, while all other cases will just use
>the
>> single register %eax (%rax on x86-64)=2E While the _ASM_AX actually
>just
>> expands to "%eax", note this comment next to get_user() which does
>> something very similar:
>>
>>  * The use of _ASM_DX as the register specifier is a bit of a
>>  * simplification, as gcc only cares about it as the starting point
>>  * and not size: for a 64-bit value it will use %ecx:%edx on 32 bits
>>  * (%ecx being the next register in gcc's x86 register sequence), and
>>  * %rdx on 64 bits=2E
>>
>> However, getting this to work requires that there is no code between
>> the assignment to the local register variable and its use as an input
>> to the asm() which can possibly clobber any of the registers involved
>> - including evaluation of the expressions making up other inputs=2E
>
>This looks like the patch is an improvement, but this is still IMO
>likely to be very fragile=2E  Can we just do the size-dependent "a" vs
>"A" selection method instead?  Sure, it's a little more code, but it
>will be Obviously Correct=2E  As it stands, I can easily see our code
>failing on some gcc or clang version and the compiler authors telling
>us that we're relying on unsupportable behavior=2E

Yeah, the reason get_user hacks it is because there is no equivalent to "A=
" for other register pairs, but not using it for dx:ax is just silly=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E