Re: [PATCH 3/7] x86/dom0: placate GCC 12 compile-time errors with UBSAN and PVH_GUEST

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Mar 14, 2025 at 09:10:59AM +0100, Jan Beulich wrote:
> On 13.03.2025 16:30, Roger Pau Monne wrote:
> > When building Xen with GCC 12 with UBSAN and PVH_GUEST both enabled the
> > compiler emits the following errors:
> > 
> > arch/x86/setup.c: In function '__start_xen':
> > arch/x86/setup.c:1504:19: error: 'consider_modules' reading 40 bytes from a 
> > region of size 4 [-Werror=stringop-overread]
> >  1504 |             end = consider_modules(s, e, reloc_size + mask,
> >       |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >  1505 |                                    bi->mods, bi->nr_modules, -1);
> >       |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > arch/x86/setup.c:1504:19: note: referencing argument 4 of type 'const 
> > struct boot_module[0]'
> > arch/x86/setup.c:686:24: note: in a call to function 'consider_modules'
> >   686 | static uint64_t __init consider_modules(
> >       |                        ^~~~~~~~~~~~~~~~
> > arch/x86/setup.c:1535:19: error: 'consider_modules' reading 40 bytes from a 
> > region of size 4 [-Werror=stringop-overread]
> >  1535 |             end = consider_modules(s, e, size, bi->mods,
> >       |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >  1536 |                                    bi->nr_modules + relocated, j);
> >       |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > arch/x86/setup.c:1535:19: note: referencing argument 4 of type 'const 
> > struct boot_module[0]'
> > arch/x86/setup.c:686:24: note: in a call to function 'consider_modules'
> >   686 | static uint64_t __init consider_modules(
> >       |                        ^~~~~~~~~~~~~~~~
> > 
> > This seems to be the result of some function manipulation done by UBSAN
> > triggering GCC stringops related errors.  Placate the errors by declaring
> > the function parameter as `const struct *boot_module` instead of `const
> > struct boot_module[]`.
> > 
> > Note that GCC 13 seems to be fixed, and doesn't trigger the error when
> > using `[]`.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/setup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> > index 4a32d8491186..bde5d75ea6ab 100644
> > --- a/xen/arch/x86/setup.c
> > +++ b/xen/arch/x86/setup.c
> > @@ -684,7 +684,7 @@ static void __init noinline move_xen(void)
> >  #undef BOOTSTRAP_MAP_LIMIT
> >  
> >  static uint64_t __init consider_modules(
> > -    uint64_t s, uint64_t e, uint32_t size, const struct boot_module mods[],
> > +    uint64_t s, uint64_t e, uint32_t size, const struct boot_module *mods,
> >      unsigned int nr_mods, unsigned int this_mod)
> >  {
> >      unsigned int i;
> 
> While I'm okay-ish with the change, how are we going to make sure it won't be
> re-introduced? Or something similar be introduced elsewhere?

I'm afraid I don't have a good response, as I don't even know exactly
why the error triggers.  We will rely on the CI to start doing
randconfig builds with UBSAN enabled (see patch 7/7).

Thanks, Roger.