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

[Corinna Vinschen]

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?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer