[PATCH] Cygwin: mmap: fix mapping beyond EOF on 64 bit

[Ken Brown]

On 7/20/2020 10:49 AM, Ken Brown via Cygwin-patches wrote:
> On 7/20/2020 10:43 AM, Ken Brown via Cygwin-patches wrote:
>> On 7/20/2020 10:23 AM, Corinna Vinschen wrote:
>>> On Jul 20 09:34, Ken Brown via Cygwin-patches wrote:
>>>> Commit 605bdcd410384dda6db66b9b8cd19e863702e1bb enabled mapping beyond
>>>> EOF in 64 bit environments.  But the variable 'orig_len' did not get
>>>> rounded up to a multiple of 64K.  This rounding was done on 32 bit
>>>> only.  Fix this by rounding up orig_len on 64 bit, in the same place
>>>> where 'len' is rounded up.
>>>>
>>>> One consequence of this bug is that orig_len could be slightly smaller
>>>> than len.  Since these are both unsigned values, the statement
>>>> 'orig_len -= len' would then cause orig_len to be huge, and mmap would
>>>> fail with errno EFBIG.
>>>>
>>>> I observed this failure while debugging the problem reported in
>>>>
>>>>    https://sourceware.org/pipermail/cygwin/2020-July/245557.html.
>>>>
>>>> The failure can be seen by running the test case in that report under
>>>> gdb or strace.
>>>> ---
>>>>   winsup/cygwin/mmap.cc | 1 +
>>>>   1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/winsup/cygwin/mmap.cc b/winsup/cygwin/mmap.cc
>>>> index feb9e5d0e..a08d00f83 100644
>>>> --- a/winsup/cygwin/mmap.cc
>>>> +++ b/winsup/cygwin/mmap.cc
>>>> @@ -1144,6 +1144,7 @@ go_ahead:
>>>>        ends in, but there's nothing at all we can do about that. */
>>>>   #ifdef __x86_64__
>>>>         len = roundup2 (len, wincap.allocation_granularity ());
>>>> +      orig_len = roundup2 (orig_len, wincap.allocation_granularity ());
>>>
>>> Wouldn't it be simpler to just check for
>>>
>>> -      if (orig_len - len)
>>> +      if (orig_len > len)
>>>
>>> in the code following this #if/#else/#endif snippet?
>>
>> I don't think so, because we also want to use the rounded-up value of orig_len 
>> further down when we set sigbus_page_len
> 
> Actually we first modify orig_len in 'orig_len -= len;' and then use it to set 
> sigbus_page_len.  In any case, I think it needs to be rounded up before being used.

If you agree, maybe I should modify the commit message to make this point clear.

Ken