Re: CLONE_PTRACE implementation incomplete

Mike Coleman (mkc@kc.net)
24 Dec 1999 01:21:37 -0600

Alon Ziv <alonz@cs.technion.ac.il> writes:
> IIRC, Linus has already claimed once that he doesn't very much like the
> whole reparenting mess introduced for debugging. He said that instead of
> fixing some other bugs in this code, ptrace should be fixed so the ppid of
> a process isn't changed just because it's being debugged. (Actually, he
> proposed that the connection between debugger and debuggee shouldn't be
> made using the p_pptr field, but through another specific field; I believe
> this is even quite easy to achieve, as it's very similar to the CLONE_WAIT
> flag's implementation...)
>
> So, if this is fixed, CLONE_PTRACE can truly become standalone (it will
> just copy the new p_dbptr field)...
>
> Any takers? (I can only start on this when I have my new comp, which will
> take a few more weeks...)

I've thought about doing something like this. I'd really, really, really like
to have some way to trace processes without the Heisenberg effect. Because of
the reparenting games it plays, ptrace messes up the semantics of the
interaction between a traced process and its parent. These alterations
greatly limit the usefulness of ptrace. In a proper solution, any changes
should be absolutely minimal (e.g., if a process watched itself carefully, it
might notice that system calls run slower while it's being traced).

(I'm not sure, but Solaris' /proc/<pid>/ctl interface looks like it might have
this property. See
'http://docs.sun.com:80/ab2/coll.40.5/REFMAN4/@Ab2PageView/69221?DwebQuery=PCSENTRY&Ab2Lang=C&Ab2Enc=iso-8859-1'.)

A second thing I'd like to have is the ability to have multiple tracers (as
opposed the single tracer currently allowed by ptrace). This would allow, for
example, a user to run 'strace' and 'gdb' on the same process. This can
probably mostly be faked in user mode, but it looks painful.

--Mike

-- 
Any sufficiently adverse technology is indistinguishable from Microsoft.


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