[PATCH] Cygwin: mmap: fix mapping beyond EOF on 64 bit
Mon Jul 20 14:23:03 GMT 2020
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
> 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?
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