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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2025 09:27, Roger Pau Monné wrote:
> 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.

One option might be to amend ./CODING_STYLE for dis-encourage [] notation
in function parameters. I wouldn't be happy about us doing so, as I think
that serves a documentation purpose, but compiler deficiencies getting in
the way is certainly higher priority here.

Trying to abstract this (vaguely along the lines of gcc11_wrap()), otoh,
wouldn't be desirable imo, as it would still lose the doc effect, at least
to some degree.

>  We will rely on the CI to start doing
> randconfig builds with UBSAN enabled (see patch 7/7).

Right. Just that randconfig is, well, random in what it covers.

Jan