Re: [PATCH 08/14] arm64: simplify access_ok()

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 15/02/2022 à 10:12, Arnd Bergmann a écrit :
> On Tue, Feb 15, 2022 at 9:17 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>> On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>>> From: Arnd Bergmann <arnd@xxxxxxxx>
>>>
>>
>> With set_fs() out of the picture, wouldn't it be sufficient to check
>> that bit #55 is clear? (the bit that selects between TTBR0 and TTBR1)
>> That would also remove the need to strip the tag from the address.
>>
>> Something like
>>
>>      asm goto("tbnz  %0, #55, %2     \n"
>>               "tbnz  %1, #55, %2     \n"
>>               :: "r"(addr), "r"(addr + size - 1) :: notok);
>>      return 1;
>> notok:
>>      return 0;
>>
>> with an additional sanity check on the size which the compiler could
>> eliminate for compile-time constant values.
> 
> That should work, but I don't see it as a clear enough advantage to
> have a custom implementation. For the constant-size case, it probably
> isn't better than a compiler-scheduled comparison against a
> constant limit, but it does hurt maintainability when the next person
> wants to change the behavior of access_ok() globally.
> 
> If we want to get into micro-optimizing uaccess, I think a better target
> would be a CONFIG_CC_HAS_ASM_GOTO_OUTPUT version
> of __get_user()/__put_user as we have on x86 and powerpc.
> 

There is also the user block accesses with 
user_access_begin()/user_access_end() together with unsafe_put_user() 
and unsafe_get_user() which allowed us to optimise user accesses on 
powerpc, especially in the signal code.

Christophe