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cygwin: mmap: fix a fork failure with private, anonymous mappings 

Rounddown incoming addr on a page boundary. Without this, we may end
up with a fork error for private, anonymous maps.  The reason is, we
use VirtualAlloc in this case which will potentially overcommit if
addr is not on a page boundary.  This isn't taken into account in
bookkeeping, but fixup_mmaps_after_fork will eventually stumble over
this when trying to reproduce the copy-on-write pages: VirtualQuery
returns a region reaching beyond the supposedly allocated address
range and from there it goes downhill.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen 2017-12-10 14:11:03 +01:00
parent c6f14b3c81
commit 67a657cb1d
1 changed files with 7 additions and 0 deletions
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7
winsup/cygwin/mmap.cc
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@ -917,6 +917,13 @@ mmap64 (void *addr, size_t len, int prot, int flags, int fd, off_t off)
goto out; goto out;
} }
/* POSIX: When MAP_FIXED is not set, the implementation uses addr in an
implementation-defined manner to arrive at pa [the return address].
Given that we refuse addr if it's not exactly at a page boundary, we
can just make sure addr does so indiscriminately. Just round down
to the next lower page boundary. */
addr = (void *) rounddown ((uintptr_t) addr, pagesize);
if (!anonymous (flags) && fd != -1) if (!anonymous (flags) && fd != -1)
{ {
/* Ensure that fd is open */ /* Ensure that fd is open */