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

Ken Brown kbrown@cornell.edu
Mon Jul 20 14:49:26 GMT 2020


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.

Ken


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