Re: [PATCH] Kwatch: kernel watchpoints using CPU debug registers

[Roland McGrath <roland@redhat.com>]

> I think the best approach will be not to reset dr7 at all.  Then there 
> won't be any need to worry about restoring it.  Leaving a userspace 
> watchpoint enabled while running in the kernel ought not to matter; a 
> system call shouldn't touch any address in userspace more than once or 
> twice.

Hmm.  That sounds reasonable.  But I wonder why the old code clears %dr7.
It's been that way for a long time (since 2.4 at least).

> My idea was to put 4 hwbkpt structures in thread_struct, so they would
> always be available for use by ptrace.  However it turned out not to be
> feasible to replace the debugreg array with something more sophisticated,
> because of conflicting declarations and problems with the ordering of
> #includes.  So instead I have been forced to replace debugreg[] with a
> pointer to a structure which can be allocated as needed.

I think that's preferable anyway.  Most tasks most of the time will never
need that storage, so why not make thread_struct a little smaller?
(There is also the potential for sharing, which I mentioned earlier.)

> This raises the possibility that a PTRACE syscall might fail because the 
> allocation fails.  Hopefully that won't be an issue?

It's not a new issue, anyway, after utrace.  The utrace-based ptrace can
fail for PTRACE_ATTACH because of OOM too, which wasn't possible before.
I think it's survivable.


Thanks,
Roland