Re: [PATCH v2 16/17] xen/riscv: implement mfn_valid() and page reference, ownership handling helpers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.07.2025 12:09, Jan Beulich wrote:
> On 10.06.2025 15:05, Oleksii Kurochko wrote:
>> @@ -613,3 +612,91 @@ void __iomem *ioremap(paddr_t pa, size_t len)
>>  {
>>      return ioremap_attr(pa, len, PAGE_HYPERVISOR_NOCACHE);
>>  }
>> +
>> +int page_is_ram_type(unsigned long mfn, unsigned long mem_type)
>> +{
>> +    ASSERT_UNREACHABLE();
>> +
>> +    return 0;
>> +}
>> +
>> +static struct domain *page_get_owner_and_nr_reference(struct page_info 
>> *page,
>> +                                                      unsigned long nr)
>> +{
>> +    unsigned long x, y = page->count_info;
>> +    struct domain *owner;
>> +
>> +    /* Restrict nr to avoid "double" overflow */
>> +    if ( nr >= PGC_count_mask )
>> +    {
>> +        ASSERT_UNREACHABLE();
>> +        return NULL;
>> +    }
> 
> I question the validity of this, already in the Arm original: I can't spot
> how the caller guarantees to stay below that limit. Without such an
> (attempted) guarantee, ASSERT_UNREACHABLE() is wrong to use. All I can see
> is process_shm_node() incrementing shmem_extra[].nr_shm_borrowers, without
> any limit check.
> 
>> +    do {
>> +        x = y;
>> +        /*
>> +         * Count ==  0: Page is not allocated, so we cannot take a 
>> reference.
>> +         * Count == -1: Reference count would wrap, which is invalid.
>> +         */
> 
> May I once again ask that you look carefully at comments (as much as at code)
> you copy. Clearly this comment wasn't properly updated when the bumping by 1
> was changed to bumping by nr.
> 
>> +        if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
>> +            return NULL;
>> +    }
>> +    while ( (y = cmpxchg(&page->count_info, x, x + nr)) != x );
>> +
>> +    owner = page_get_owner(page);
>> +    ASSERT(owner);
>> +
>> +    return owner;
>> +}

There also looks to be a dead code concern here (towards the "nr" parameters
here and elsewhere, when STATIC_SHM=n). Just that apparently we decided to
leave out Misra rule 2.2 entirely.

Jan