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[newlib-cygwin] cygwin: mmap: fix a fork failure with private, anonymous mappings
- From: Corinna Vinschen <corinna at sourceware dot org>
- To: cygwin-cvs at sourceware dot org
- Date: 10 Dec 2017 13:15:27 -0000
- Subject: [newlib-cygwin] cygwin: mmap: fix a fork failure with private, anonymous mappings
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=67a657cb1d41e6ae90e9fdeba9e69fff09a2ea06
commit 67a657cb1d41e6ae90e9fdeba9e69fff09a2ea06
Author: Corinna Vinschen <corinna@vinschen.de>
Date: Sun Dec 10 14:11:03 2017 +0100
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>
Diff:
---
winsup/cygwin/mmap.cc | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/winsup/cygwin/mmap.cc b/winsup/cygwin/mmap.cc
index 4218466..d4eddfc 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 */