Re: [XEN PATCH v2 2/3] x86/uaccess: replace __{get,put}_user_bad() with STATIC_ASSERT_UNREACHABLE()

[Federico Serafini <federico.serafini@xxxxxxxxxxx>]

On 07/02/24 08:38, Jan Beulich wrote:
> On 07.02.2024 02:08, Stefano Stabellini wrote:
> > On Tue, 6 Feb 2024, Jan Beulich wrote:
> > > On 26.01.2024 11:05, Federico Serafini wrote:
> > > > @@ -208,7 +205,7 @@ do {                                                        
> > > >                \
> > > >       case 8:                                                                \
> > > >           put_unsafe_asm(x, ptr, grd, retval, "q",  "", "ir", errret);       \
> > > >           break;                                                             \
> > > > -    default: __put_user_bad();                                             \
> > > > +    default: STATIC_ASSERT_UNREACHABLE();                                  \
> > > >       }                                                                      \
> > > >       clac();                                                                \
> > > >   } while ( false )
> > > > @@ -227,7 +224,7 @@ do {                                                        
> > > >                \
> > > >       case 2: get_unsafe_asm(x, ptr, grd, retval, "w", "=r", errret); break; \
> > > >       case 4: get_unsafe_asm(x, ptr, grd, retval, "k", "=r", errret); break; \
> > > >       case 8: get_unsafe_asm(x, ptr, grd, retval,  "", "=r", errret); break; \
> > > > -    default: __get_user_bad();                                             \
> > > > +    default: STATIC_ASSERT_UNREACHABLE();                                  \
> > > >       }                                                                      \
> > > >       clac();                                                                \
> > > >   } while ( false )
> > > Related to my remark on patch 1 - how is one to know the macro this was
> > > invoked from, when seeing the resulting diagnostic?
> > I am not sure what do you mean here... we do get an error like the
> > following (I added a STATIC_ASSERT_UNREACHABLE for case 4):
> >
> > ./arch/x86/include/asm/uaccess.h:262: Error: static assertion failed: 
> > unreachable
> Right - and how do I know what _user_ of the macro actually triggered
> it? ISTR suggesting to use one or more of __FILE__ / __LINE__ /
> __FUNCTION__ here, for that specific purpose ...
To test the macro and its diagnostics,
I modified the first "git grep" occurrence of ASSERT_UNREACHABLE()
on the x86 code with STATIC_ASSERT_UNREACHABLE(),
that is in file arch/x86/alternative.c, line 312,
function _apply_alternatives().

What I got is the following build error:

...
arch/x86/alternative.c: Assembler messages:
arch/x86/alternative.c:312: Error: static assertion failed: unreachable
  CC      arch/x86/copy_page.o
make[2]: *** [Rules.mk:247: arch/x86/alternative.o] Error 1
make[2]: *** Waiting for unfinished jobs....
...

If I understood your requests correctly,
the only thing missing is the function name but I didn't find a way
to make __FUNCTION__ or __func__ work with the .error directive.
Do you know any tricks to make it work?

--
Federico Serafini, M.Sc.

Software Engineer, BUGSENG (http://bugseng.com)