Tuesday, March 23, 2010

Incompatible change in Mach exception handler behavior on Mac OS X 10.6

On Mac OS X, Factor uses Mach exceptions rather than Unix signals to receive notifications of illegal memory accesses and arithmetic exceptions from the operating system. This is used to catch datastack underflows, division by zero, and the like. The code implementing this can be found in vm/mach_signal.c in the Factor repository. This file is based on code from the GNU libsigsegv project, with special permission to use it under a BSD license instead of the restrictive GPL.

It seems that as of Mac OS X 10.6, exceptions raised by child processes are now reported to the parent if the parent has an exception handler thread. This caused a problem in Factor if a child process crashed with an access violation; Factor would think the access violation occurred in Factor code, and die with an assertion failure when attempting to map the thread ID back into a Factor VM object. It seems the simplest way is to add the following clause to the catch_exception_raise() function:

if(task != mach_task_self()) return KERN_FAILURE;

This fix should be applicable to the original libsigsegv code as well, along with other projects, such as CLisp, that use this library.


Anonymous said...

Did you receive explicit permission from Paolo Bonzini and Bruno Haible for a GPL exception? The mailing list message you link to only gives permission to SBCL.

Anonymous said...

SBCL is BSD licensed. handler-macos.c could be copied from SBCL by anyone under a BSD license.