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

Ken Brown kbrown@cornell.edu
Mon Jul 20 13:34:42 GMT 2020

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


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 ());
       len = roundup2 (len, wincap.is_wow64 () ? wincap.allocation_granularity ()
 					      : wincap.page_size ());

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