Re: [PATCH v1 1/3] x86: x86_emulate: address violations of MISRA C Rule 19.1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2025 00:54, Stefano Stabellini wrote:
> On Tue, 29 Apr 2025, Jan Beulich wrote:
>> On 29.04.2025 03:27, Stefano Stabellini wrote:
>>> On Mon, 28 Apr 2025, Jan Beulich wrote:
>>>> On 26.04.2025 01:42, victorm.lira@xxxxxxx wrote:
>>>>> From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
>>>>>
>>>>> Rule 19.1 states: "An object shall not be assigned or copied
>>>>> to an overlapping object". Since the "call" and "compat_call" are
>>>>
>>>> Was this taken from patch 2 without editing?
>>>>
>>>>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>>>>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>>>>> @@ -526,9 +526,19 @@ static inline void put_loop_count(
>>>>>           */                                                             \
>>>>>          if ( !amd_like(ctxt) && mode_64bit() && ad_bytes == 4 )         \
>>>>>          {                                                               \
>>>>> +            uint64_t tmp;                                               \
>>>>> +                                                                        \
>>>>>              _regs.r(cx) = 0;                                            \
>>>>> -            if ( extend_si ) _regs.r(si) = _regs.esi;                   \
>>>>> -            if ( extend_di ) _regs.r(di) = _regs.edi;                   \
>>>>> +            if ( extend_si )                                            \
>>>>> +            {                                                           \
>>>>> +                tmp = _regs.esi;                                        \
>>>>> +                _regs.r(si) = tmp;                                      \
>>>>> +            }                                                           \
>>>>> +            if ( extend_di )                                            \
>>>>> +            {                                                           \
>>>>> +                tmp = _regs.edi;                                        \
>>>>> +                _regs.r(di) = tmp;                                      \
>>>>> +            }                                                           \
>>>>
>>>> See commit 7225f13aef03 for how we chose to address similar issues 
>>>> elsewhere
>>>> in the emulator. I think we want to be consistent there. This will then 
>>>> also
>>>> eliminate ...
>>>>
>>>>> @@ -2029,7 +2039,12 @@ x86_emulate(
>>>>>          switch ( op_bytes )
>>>>>          {
>>>>>          case 2: _regs.ax = (int8_t)_regs.ax; break; /* cbw */
>>>>> -        case 4: _regs.r(ax) = (uint32_t)(int16_t)_regs.ax; break; /* 
>>>>> cwde */
>>>>> +        case 4:
>>>>> +            {
>>>>> +                uint32_t tmp = (uint32_t)(int16_t)_regs.ax;
>>>>> +                _regs.r(ax) = tmp;
>>>>> +                break; /* cwde */
>>>>> +            }
>>>>
>>>> ... the odd brace placement here, as well as the inconsistency in the types
>>>> you used for the temporary variables (both really could have been unsigned
>>>> int; no need for a fixed-width type).
>>>
>>> Is this what you have in mind?
>>
>> No, and that's also not what the referenced commit did in a similar 
>> situation.
>>
>>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>>> @@ -527,8 +527,8 @@ static inline void put_loop_count(
>>>          if ( !amd_like(ctxt) && mode_64bit() && ad_bytes == 4 )         \
>>>          {                                                               \
>>>              _regs.r(cx) = 0;                                            \
>>> -            if ( extend_si ) _regs.r(si) = _regs.esi;                   \
>>> -            if ( extend_di ) _regs.r(di) = _regs.edi;                   \
>>> +            if ( extend_si ) _regs.r(si) = (uint64_t)_regs.esi;         \
>>> +            if ( extend_di ) _regs.r(di) = (uint64_t)_regs.edi;         \
>>
>>             if ( extend_si ) _regs.r(si) = (uint32_t)_regs.r(si);       \
>>             if ( extend_di ) _regs.r(di) = (uint32_t)_regs.r(di);       \
>>
>> After all what the rule requires is that we use _the same_ field on both 
>> sides.
> 
> I see, thanks Jan. Yes I did try this version and worked as expected.

Except that ...

> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -527,8 +527,8 @@ static inline void put_loop_count(
>          if ( !amd_like(ctxt) && mode_64bit() && ad_bytes == 4 )         \
>          {                                                               \
>              _regs.r(cx) = 0;                                            \
> -            if ( extend_si ) _regs.r(si) = _regs.esi;                   \
> -            if ( extend_di ) _regs.r(di) = _regs.edi;                   \
> +            if ( extend_si ) _regs.r(si) = (uint32_t)_regs.r(si);        \
> +            if ( extend_di ) _regs.r(di) = (uint32_t)_regs.r(di);        \
>          }                                                               \
>          goto complete_insn;                                             \
>      }                                                                   \
> @@ -2029,7 +2029,7 @@ x86_emulate(
>          switch ( op_bytes )
>          {
>          case 2: _regs.ax = (int8_t)_regs.ax; break; /* cbw */
> -        case 4: _regs.r(ax) = (uint32_t)(int16_t)_regs.ax; break; /* cwde */
> +        case 4: _regs.r(ax) = (int16_t)_regs.r(ax); break; /* cwde */

... the change in casts here renders this wrong now, afaict. We'd sign-
extend from 16 to 64 bits, rather than sign-extending to 32 bits and
then zero-extending to 64.

Jan