Re: [Xen-devel] [PATCH V2] xen: Check if the range is valid in init_domheap_pages

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.11.13 at 14:34, Julien Grall <julien.grall@xxxxxxxxxx> wrote:

> 
> On 11/13/2013 01:26 PM, Jan Beulich wrote:
>>>>> On 13.11.13 at 14:15, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
>>> On ARM, when an initrd is given to xen by U-boot, it will reserve the memory
>>> in the device tree.
>>> In this case, when xen decides to free unused memory, dt_unreserved_regions
>>> will call init_domheap_pages with the start and the end of range equals. But
>>> the latter assumes that (start > end), if not Xen will hang because the
>>> number of pages is equals to (unsigned)-1.
>>
>> The change is simple enough, so I don't really mind it going in, but
>> I wonder ...
>>
>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
>>> CC: Keir Fraser <keir@xxxxxxx>
>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> ---
>>>      Changes in v2:
>>>          - Change commit title
>>>          - Move the check in init_domheap_pages
>>
>> ... who and why suggested to move it here. After all, I'm considering
>> it an error to call the function with non-page-aligned addresses and/
>> or end < start (I take it that page-aligned, but start == end is not a
>> problem without your change).
> 
> if ps == pe, then emfn == (smfn - 1). This will result to the number of 
> pages of -1.

I'm not following: If ps == pe and they're page aligned, then

    smfn = round_pgup(ps) >> PAGE_SHIFT;
    emfn = round_pgdown(pe) >> PAGE_SHIFT;

produces smfn == emfn. So as said earlier - emfn = smfn - 1 requires
the input to either be not page aligned, or pe < ps, both of which look
invalid to me.

> There is a similar check in init_xenheap_pages, it doesn't seem harmfull 
> to let it here.

In the CONFIG_SEPARATE_XENHEAP case, yes. And it's there for
a good reason - that code doesn't tolerate ps == pe (and if I'm not
mistaken would break even on specific ps < pe cases).

Jan


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