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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. Rounding up is needed to make sigbus_page_len a multiple of the allocation granularity. In addition, failing to round up could cause orig_len to be smaller than len. Since these are both unsigned values, the statement 'orig_len -= len' could 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.
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