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

Ken Brown kbrown@cornell.edu
Mon Jul 20 14:43:53 GMT 2020


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.

Ken


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