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TEST RELEASE: Cygwin 2.1.0-0.4
- From: Corinna Vinschen <corinna-cygwin at cygwin dot com>
- To: cygwin-announce at cygwin dot com
- Date: Sun, 5 Jul 2015 23:34:17 +0200
- Subject: TEST RELEASE: Cygwin 2.1.0-0.4
- Authentication-results: sourceware.org; auth=none
- Reply-to: The Cygwin Mailing List <cygwin at cygwin dot com>
Hi Cygwin friends and users,
I released another TEST version of Cygwin. The version number is
This test release needs some good testing!
While the changes are still mostly interesting for developers, the
under-the-hood changes will potentially impact existing applications.
I'd like to release 2.1.0-1 in about two weeks, if possible.
==================================== tl;dr ==================================
- Handle pthread stacksizes as in GLibc: Default to RLIMIT_STACK resource.
Allow to set RLIMIT_STACK via setrlimit. Default RLIMIT_STACK to value
from executable header as described on
Default stacksize to 2 Megs in case RLIMIT_STACK is set to RLIM_INFINITY.
- First cut of an implementation to allow signal handlers running on an
alternate signal stack.
- New API sigaltstack, plus definitions for SA_ONSTACK, SS_ONSTACK, SS_DISABLE,
- New API: sethostname.
- Enable non-SA_RESTART behaviour on threads other than main thread.
- Try to handle concurrent close on socket more gracefully
- Fix fork failing after the parent recovered from a stack overflow.
- Fix a crash on 64 bit XP/2003 when opening /proc/$PID/maps.
Changes compared to the previous test release:
- getrlimit/setrlimit RLIMIT_STACK handling has been improved considerably.
The old implementation returned wrong values and was generally useless.
After some discussion with collegues working on glibc and the Linux
kernel, I now implemented RLIMIT_STACK more closely aligned with
the behaviour on Linux/glibc:
- By default, getrlimit(RLIMIT_STACK) returns the value of the default
stacksize taken from the Windows executable file header in rlim_cur,
only slightly changed to account for guard pages.
- setrlimit(RLIMIT_STACK) now works and stores the new values for later
- The rlim_cur value is now used as default stacksize when creating
pthreads. If rlim_cur is RLIM_INFINITY, the fallback stacksize is
set to 2 Megs.
- So far, if the application set the guardpage size via
pthread_attr_setguardsize(), the thread stack was set up fully
commited with a NOACCESS guardpage at the stack bottom. This approach
wasted physical memory and it failed to trigger normal stack overflow
Cygwin now sets up a Windows-typical stack with only few commited
pages and movable guardpages. However, this requires OS support if
the guardpage area requested by pthread_attr_setguardsize() differs
from the default OS guardpage size.
This OS support is only available starting with Windows 2003 and 64
bit XP. 32 bit XP will still use the fully commited stack setup in
this case. Another nail in XP's coffin...
- If the pthread stack is not provided by the application (which is
unusual anyway), the newly created stack will use a Windows compatible
guardpage setup reflecting the setting from pthread_attr_setguardsize(),
or the default OS-specific guardpage size.
- When running a signal handler on the alternate signal stack, the
handler is now called via a wrapper function. This wrapper function
checkr if the SEGV was triggered by a STATUS_STACK_OVERFLOW.
If so, it restores the last set of guard pages on the primary thread
stack. If we don't do that, and if the handler longjmps, the stack
stays broken and another stack overflow exits the process immediately
with no chance to recover.
If the handler simply returns, the wrapper restores the "broken"
stack state to allow accessing the stack, then the exception handling
triggers a SIG_DFL action for SIGSEGV: Create a stackdump and exit.
- The alternate signal stack installed via sigaltstack is only valid for
the current thread. Each thread must call its own sigaltstack. On
pthread_create, the alternate signal stack setting of the calling
thread is *not* propagated to the newly created thread. This follows
current Linux semantics.
- The alternate signal stack is a minimal stack. Certain datastructures
used by Cygwin (_cygtls area) and Windows (on 32 bit: exception
records) are not copied over to the alternate signal stack. The stack
settings in the Thread Environment Block (TEB) are not reflecting the
current alternate stack while running the signal handler. The TEB
will still point to the original thread stack. This seems to work
nicely in my testing, but there may be Windows functions which stop
working in this scenario.
- The volatile registers and the original stack registers are stored at
the base of the alternate stack. If you screw this up while running
the signal handler, your thread is doomed on return to the caller.
I'd be grateful if curious developers would give this new sigaltstack
implementation and the changed RLIMIT_STACK handling a whirl and report
back if it's working for them as desired/expected. And if not, simple
reproducers in plain C are most welcome in this case. Discussing
aspects of this implementation may be best handled on the
cygwin-developers mailing list or the #cygwin-developers IRC channel on
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com