Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying shstk

[Andrew Cooper]

On 05/10/2022 23:47, Edgecombe, Rick P wrote:
> On Wed, 2022-10-05 at 02:43 +0000, Andrew Cooper wrote:
>> On 29/09/2022 23:29, Rick Edgecombe wrote:
>>> diff --git a/arch/x86/include/asm/special_insns.h
>>> b/arch/x86/include/asm/special_insns.h
>>> index 35f709f619fb..f096f52bd059 100644
>>> --- a/arch/x86/include/asm/special_insns.h
>>> +++ b/arch/x86/include/asm/special_insns.h
>>> @@ -223,6 +223,19 @@ static inline void clwb(volatile void *__p)
>>>                : [pax] "a" (p));
>>>   }
>>>   
>>> +#ifdef CONFIG_X86_SHADOW_STACK
>>> +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
>>> +{
>>> +     asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
>>> +                       _ASM_EXTABLE(1b, %l[fail])
>>> +                       :: [addr] "r" (addr), [val] "r" (val)
>>> +                       :: fail);
>> "1: wrssq %[val], %[addr]\n"
>> _ASM_EXTABLE(1b, %l[fail])
>> : [addr] "+m" (*addr)
>> : [val] "r" (val)
>> :: fail
>>
>> Otherwise you've failed to tell the compiler that you wrote to *addr.
>>
>> With that fixed, it's not volatile because there are no unexpressed
>> side
>> effects.
> Ok, thanks!

On further consideration, it should be "=m" not "+m", which is even less
constrained, and even easier for an enterprising optimiser to try and do
something useful with.

~Andrew