[kaffe] CVS kaffe (guilhem): Implemented stack overflow

Timothy Stack stack at cs.utah.edu
Fri Apr 23 12:53:03 PDT 2004

> On Thu, 2004-04-22 at 23:08, Timothy Stack wrote:
> > > Implemented stack overflow detection.
> > 
> > Can you give us a more detailed explanation of what this all is?
> > 
> Sure. I was writing some comments in the code now. I wanted to be able
> to detect stack overflows like null pointer exception. There were two
> problems for that: detect the real boundaries of the stack and be able
> to run the signal handlers on another stack.

So, I actually implemented most of this in the JanosVM (see the 
kaffe/kaffevm/systems/unix-jthreads2 directory) and that implementation 
touched quite a few things that didn't seem to show up in your checkin.
Of course, you might still have other things to checkin or I'm 
misunderstanding the scope of your work.

> detectStackBoundaries() sets up a temporary signal handler for SIGSEGV,
> places a jump point and tries to overflow the stack using a recursive
> function (infiniteLoop). This is the slowest part of the code indeed
> because if the stack is very large you may have to wait some time before
> reaching the end of it...

Many operating systems only allocate stack pages when the program attempts 
an access, so its very possible that this will allocate a lot of memory 
that doesn't get freed and is never used except for this detection.

> Anyway, when the stack end is reached a
> sigsegv is raised. This gives us the state of the stack pointer when the
> segv occured and so one of the boundary of the stack. With getrlimit you
> get the other boundary of the stack using its size.

Its usually safe to assume that the very top/bottom of memory is the other 
end of the stack...

> Now to detect stack overflows while running programs you also only have
> to check the stack pointer. If it is outside of the boundaries while a
> segv is raised we have to throw a StackOverflowError, in the other case
> it is NullPointerException.

But this will only work for the main thread...  You need to setup guard
pages on all of the other threads to cause the SIGSEGV.  Also, SIGSEGV is
synchronous and does not block the async signals (e.g. timer), so you need
multiple signal stacks to handle the possibility of multiple threads doing
a null deref.  In the JanosVM implementation, SIGBUS/SIGSEGV are captured
on an alternate signal stack and then passed off to a separate per-thread
stack where code can continue as normal for a little while (e.g. allocate
and throw the exception).

There is also the matter of native code that cannot be rewound like java.  
For example, in the JanosVM impl I added checks to all of the non-trivial 
soft_ methods to make sure there was sufficient stack, otherwise an 
overflow was thrown.  But, I think there were still a few cases that I 
did not cover, especially for JNI...

> This code is entirely optional, in case sigaltstack, SA_ONSTACK or
> STACK_POINTER is not defined, the old buggy behaviour is used.

How is it buggy?

> Cheers,
> Guilhem.


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