Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension

[Julien Grall <julien@xen.org>]

Hi Jan,

On 01/12/2020 07:55, Jan Beulich wrote:
> On 01.12.2020 00:27, Oleksandr wrote:
>> On 30.11.20 23:03, Volodymyr Babchuk wrote:
>>> Oleksandr Tyshchenko writes:
>>>> --- a/xen/include/asm-arm/traps.h
>>>> +++ b/xen/include/asm-arm/traps.h
>>>> @@ -83,6 +83,30 @@ static inline bool VABORT_GEN_BY_GUEST(const struct cpu_user_regs *regs)
>>>>            (unsigned long)abort_guest_exit_end == regs->pc;
>>>>    }
>>>>    
>>>> +/* Check whether the sign extension is required and perform it */
>>>> +static inline register_t sign_extend(const struct hsr_dabt dabt, register_t r)
>>>> +{
>>>> +    uint8_t size = (1 << dabt.size) * 8;
>>>> +
>>>> +    /*
>>>> +     * Sign extend if required.
>>>> +     * Note that we expect the read handler to have zeroed the bits
>>>> +     * outside the requested access size.
>>>> +     */
>>>> +    if ( dabt.sign && (r & (1UL << (size - 1))) )
>>>> +    {
>>>> +        /*
>>>> +         * We are relying on register_t using the same as
>>>> +         * an unsigned long in order to keep the 32-bit assembly
>>>> +         * code smaller.
>>>> +         */
>>>> +        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
>>>> +        r |= (~0UL) << size;
>>> If `size` is 64, you will get undefined behavior there.
>> I think, we don't need to worry about undefined behavior here. Having
>> size=64 would be possible with doubleword (dabt.size=3). But if "r"
>> adjustment gets called (I mean Syndrome Sign Extend bit is set) then
>> we deal with byte, halfword or word operations (dabt.size<3). Or I
>> missed something?
> 
> At which point please put in a respective ASSERT(), possibly amended
> by a brief comment.

ASSERT()s are only meant to catch programatic error. However, in this 
case, the bigger risk is an hardware bug such as advertising a sign 
extension for either 64-bit (or 32-bit) on Arm64 (resp. Arm32).

Actually the Armv8 spec is a bit more blurry when running in AArch32 
state because they suggest that the sign extension can be set even for 
32-bit access. I think this is a spelling mistake, but it is probably 
better to be cautious here.

Therefore, I would recommend to rework the code so it is only called 
when len < sizeof(register_t).

Cheers,

-- 
Julien Grall