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

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2025-08-22 18:34, Dmytro Prokopchuk1 wrote:
> On 8/21/25 11:25, Nicola Vetrini wrote:
> > 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.
> Maybe with this wording:
>
> This deviation is based on the guarantees provided by the specific ABIs
> (e.g., ARM AAPCS) and compilers (e.g., GCC) supported in Xen. These 
> ABIs
s/supported in/supported by/

> guarantee compatible representations for 'void *', 'unsigned long' and
> function pointers for the supported target platforms. This behavior is
It's not just about the guarantees of the ABIs: it's the behavior of the 
compiler for those ABIs that makes this safe or unsafe. If present, such 
documentation should be included
> architecture-specific and may not be portable outside of supported
> environments.
>
> > > > --- 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)
> This is how "sphinx-build" tool interprets this.
> 1. * inside single quotes '' -> looks normal, e.g. ‘void (*)(…)’
> 2. * without quotes -> warning
> deviations.rst:369: WARNING: Inline emphasis start-string without
> end-string. [docutils]
> 3. \* -> looks normal, e.g. (void *)
>
> Because that we need such format: \*
>
> Dmytro.
>
> > > Jan
--
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253