From 67a657cb1d41e6ae90e9fdeba9e69fff09a2ea06 Mon Sep 17 00:00:00 2001 From: Corinna Vinschen Date: Sun, 10 Dec 2017 14:11:03 +0100 Subject: [PATCH] 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 --- winsup/cygwin/mmap.cc | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/winsup/cygwin/mmap.cc b/winsup/cygwin/mmap.cc index 4218466f0..d4eddfcb4 100644 --- a/winsup/cygwin/mmap.cc +++ b/winsup/cygwin/mmap.cc @@ -917,6 +917,13 @@ mmap64 (void *addr, size_t len, int prot, int flags, int fd, off_t off) 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) { /* Ensure that fd is open */