Re: [Valgrind-users] Re: [uml-devel] Re: UML and valgrind

[Jeff Dike <jdike@addtoit.com>]

thh@cyberscience.com said:
> The problem is that the virtual environment seen by a program running
> under valgrind may not be the same as the real environment - things
> like signal handlers, signal masks, resource limits and son. That
> might be confusing for the thread that isn't running under valgrind
> and might even lead to that thread damaging the process state. I'm not
> saying it will happen, just that it's something to beware of. 

Yeah, that's why I was concerned about the process data existing in its
native state.  As for the rest, in the absense of CLONE_SIGHAND, it is all
private to the thread, so it and valgrind won't trip over each other, assuming
that the child is sufficiently careful not to inherit stuff that it doesn't 
expect.

> It's still switch from the simulated CPU to the real CPU though - the
> thread of control arrives at the clone and then branches with one
> branch continuing on the real CPU and one on the simulated CPU. That
> sounds to me just the same (for the new thread) as the existing thread
> of control continuing on the real CPU.

It feels to me more like the valgrind thread runs a system call with no
perceptible effect, and there's this new thread all of a sudden which runs
happily in this address space because it can't tell that valgrind is there.

> You probably really need Jeremy's thoughts on the matter - he's more
> up on this stuff and I think he was involved in dropping the support
> for switching back to the real CPU. 

Yeah, he was responsible for that fragment I posted as well.

				Jeff



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