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

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Thu, Aug 18, 2016 at 03:43:54AM -0600, Jan Beulich wrote:
> >>> 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

Yep.

> of interest really is entry_size > sizeof(*mmap_src), and that's
> what mmap_src[i] above doesn't handle correctly.

Ahhh... I have missed that. So, we can fix it in that way:
  mmap_src = (void *)mmap_src + i * get_mb2_data(tag, mmap, entry_size);
  mmap_dst[i].base_addr_low = (u32)mmap_src.addr;
  ...

Does it make sense?

Daniel

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