Re: [PATCH] misra: consider conversion from UL or (void*) to function pointer as safe

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-08-21 10:01, Jan Beulich wrote:
> On 19.08.2025 20:55, Dmytro Prokopchuk1 wrote:
>> Rule 11.1 states as following: "Conversions shall not be performed
>> between a pointer to a function and any other type."
>> 
>> The conversion from unsigned long or (void *) to a function pointer
>> is safe in Xen because the architectures it supports (e.g., x86 and
>> ARM) guarantee compatible representations between these types.
> 
> I think we need to be as precise as possible here. The architectures
> guarantee nothing, they only offer necessary fundamentals. In the
> Windows x86 ABI, for example, you can't convert pointers to/from longs
> without losing data. What we build upon is what respective ABIs say,
> possibly in combination of implementation specifics left to compilers.
> 

+1, a mention of the compilers and targets this deviation relies upon is 
needed.

>> --- a/docs/misra/deviations.rst
>> +++ b/docs/misra/deviations.rst
>> @@ -370,6 +370,16 @@ Deviations related to MISRA C:2012 Rules:
>>         to store it.
>>       - Tagged as `safe` for ECLAIR.
>> 
>> +   * - R11.1
>> +     - The conversion from unsigned long or (void \*) to a function 
>> pointer does
>> +       not lose any information or violate type safety assumptions if 
>> unsigned
>> +       long or (void \*) type is guaranteed to be the same bit size 
>> as a
>> +       function pointer. This ensures that the function pointer can 
>> be fully
>> +       represented without truncation or corruption. The macro 
>> BUILD_BUG_ON is
>> +       integrated into xen/common/version.c to confirm conversion 
>> compatibility
>> +       across all target platforms.
>> +     - Tagged as `safe` for ECLAIR.
> 
> Why the escaping of * here, when ...
> 
>> --- a/docs/misra/rules.rst
>> +++ b/docs/misra/rules.rst
>> @@ -431,7 +431,13 @@ maintainers if you want to suggest a change.
>>       - All conversions to integer types are permitted if the 
>> destination
>>         type has enough bits to hold the entire value. Conversions to 
>> bool
>>         and void* are permitted. Conversions from 'void noreturn 
>> (*)(...)'
>> -       to 'void (*)(...)' are permitted.
>> +       to 'void (*)(...)' are permitted. Conversions from unsigned 
>> long or
>> +       (void \*) to a function pointer are permitted if the source 
>> type has
>> +       enough bits to restore function pointer without truncation or 
>> corruption.
>> +       Example::
>> +
>> +           unsigned long func_addr = (unsigned long)&some_function;
>> +           void (*restored_func)(void) = (void (*)(void))func_addr;
> 
> ... context here suggests they work fine un-escaped, and you even add 
> some un-
> escaped instances as well. Perhaps I'm simply unaware of some 
> peculiarity?
> 

This is a literal rst block, while the other is not (* acts as a bullet 
point in rst iirc)

> Jan

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253