Re: [Xen-devel] [PATCH v4 09/19] x86: add multiboot2 protocol support

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.08.16 at 11:23, <daniel.kiper@xxxxxxxxxx> wrote:
> On Wed, Aug 17, 2016 at 09:39:58AM -0600, Jan Beulich wrote:
>> >>> On 06.08.16 at 01:04, <daniel.kiper@xxxxxxxxxx> wrote:
> 
> [...]
> 
>> > +        case MULTIBOOT2_TAG_TYPE_MMAP:
>> > +            mbi_out->flags |= MBI_MEMMAP;
>> > +            mbi_out->mmap_length = get_mb2_data(tag, mmap, size);
>> > +            mbi_out->mmap_length -= sizeof(multiboot2_tag_mmap_t);
>> > +            mbi_out->mmap_length /= get_mb2_data(tag, mmap, entry_size);
>>
>> Okay, here you use the entry size as provided by grub, allowing
>> compatibility even when the boot loader uses a newer layout (with
>> extra fields added to the end). However ...
>>
>> > +            mbi_out->mmap_length *= sizeof(memory_map_t);
>> > +
>> > +            mbi_out->mmap_addr = alloc_mem(mbi_out->mmap_length);
>> > +
>> > +            mmap_src = get_mb2_data(tag, mmap, entries);
>> > +            mmap_dst = (memory_map_t *)mbi_out->mmap_addr;
>> > +
>> > +            for ( i = 0; i < mbi_out->mmap_length / sizeof(memory_map_t); 
>> > i++ 
> )
>> > +            {
>> > +                /* Init size member properly. */
>> > +                mmap_dst[i].size = sizeof(memory_map_t);
>> > +                mmap_dst[i].size -= sizeof(((memory_map_t){0}).size);
>> > +                /* Now copy a given region data. */
>> > +                mmap_dst[i].base_addr_low = (u32)mmap_src[i].addr;
>> > +                mmap_dst[i].base_addr_high = (u32)(mmap_src[i].addr >> 
>> > 32);
>> > +                mmap_dst[i].length_low = (u32)mmap_src[i].len;
>> > +                mmap_dst[i].length_high = (u32)(mmap_src[i].len >> 32);
>>
>> ... here you index an array of type multiboot2_memory_map_t.
> 
> All calculations looks correct, so, I am not sure what is wrong here.
> 
>> Also note that in any event you should check that
>> entry_size >= sizeof(*mmap_src) (or, if you don't want [or need]
>> to go with the flexible model, ==).
> 
> This make sense to some extent. However, I am not sure what we should do
> if entry_size < sizeof(*mmap_src) (I think that we should assume flexible
> model). Just do not fill memory map? Probably yes...

Perhaps you misunderstood my comment?
entry_size < sizeof(*mmap_src) is a case we simply can't deal with,
so you should (as you say) not obtain the memory map, which I
assume is equivalent to not failing the boot altogether. The point
of interest really is entry_size > sizeof(*mmap_src), and that's
what mmap_src[i] above doesn't handle correctly.

Jan


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