Consider exposing mmap_is_attached_or_noreserve
- Date: Wed, 27 Feb 2019 16:38:10 +0100
- From: "E. Madison Bray" <erik.m.bray@xxxxxxxxx>
- Subject: Consider exposing mmap_is_attached_or_noreserve
A very technical request regarding Cygwin internals: In mmap.c there
is a function mmap_is_attached_or_noreserve(void *addr, size_t len)
which is called from Cygwin's exception handler in the case of a
This is called in case an access violation occurs in memory that was
allocated with Cygwin's mmap() with the MAP_NORESERVE flag, and allows
us to commit the relevant pages when they are accessed.
After a successful call of mmap_is_attached_or_noreserve(), the Cygwin
exception handler returns with ExceptionContinueExecution.
Unfortunately, if the application happens to have a Vectored Continue
Handler registered which happens to do something in the case of
STATUS_ACCESS_VIOLATION (see ) there is no obvious way to tell if
we're handling this sort of case.
Normally this isn't too much of a problem: E.g. we could just check
the address that caused the access violation and see if its status is
now MEM_COMMIT (i.e. Cygwin ran its exception handler and all is
good). However, due to the bug described in , if an exception
occurs in code running on a sigaltstack, the Cygwin exception handler
This makes for a tricky to handle use case: What if some code in a
signal handler function tries to access uncommitted memory in a
MAP_NORESERVE mmap? It's probably an unusual, undesirable case, and I
haven't personally encountered it *yet*, but I could imagine some
cases where it might happen.
In order to handle such a case it might be nice if
mmap_is_attached_or_noreserve were able to be called by user code,
perhaps as a new cygwin_internal(...) call. I'd happily provide a
patch, but I fear this might be an X/Y problem that I'm not seeing.
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