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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan