Re: [PATCH 2.5.73] Signal stack fixes #1 introduce PF_SS_ACTIVE

[ebiederm@xmission.com (Eric W. Biederman)]

Linus Torvalds <torvalds@osdl.org> writes:

> On Fri, 4 Jul 2003, Jörn Engel wrote:
> > 
> > So some application has it's signal handler on the signal stack and
> > instead of returning to the kernel, it detect where it left off before
> > the signal, mangles the last two stack frames, and goes back directly?
> 
> Yeah, basically a lot of old threading stuff did the equivalent of 
> longjump by hand.
> 
> It is entirely possible that they do not do this out of signal handlers, 
> since that has its own set of problems anyway, and one of the reasons for 
> doing co-operative user level threading is to not need locking, and thus 
> you never want to do any thread switching asynchronously (eg from a signal 
> context).
> 
> So I'm not saying that your patch will necessarily break stuff, I'm just 
> pointing out that it was actually done the way it is done on purpose.

I would have to double check but I am pretty certain dosemu does
this when running dpmi applications.  An alternative stack is setup
for signals so we get a stack we can control, and if we want to
return to dosemu instead of the dpmi application we must change the
stack we return to. 

Eric