linux-kernel - Re: [PATCH v3] Make PTRACE_SEIZE set ptrace options specified in 'data'

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Message-Id: <201109101219.30942.pedro@codesourcery.com>
Date:	Sat, 10 Sep 2011 12:19:30 +0100
From:	Pedro Alves <pedro@...esourcery.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Make PTRACE_SEIZE set ptrace options specified in 'data'

On Friday 09 September 2011 21:03:10, Denys Vlasenko wrote:
> On Friday 09 September 2011 19:09, Pedro Alves wrote:

> No need to shout.

Sorry.

> execve is such a rare syscall the one extra stop on it is not
> going to be a problem.
> 
> > And about not needing to handle the magic unadorned SIGTRAP.
> > The magic unadorned post-exec SIGTRAP does not have `status & 0xff00'
> > set, it is not a ptrace event!
> 
> What SIGTRAP? With PTRACE_O_TRACEEXEC, there is no SIGTRAP.

But _without_ PTRACE_O_TRACEEXEC there is.  You've raised its
existence as justification for needing to be able to set
options directly on PTRACE_SEIZE.  Point is, if we don't get rid
of the SIGTRAP when PTRACE_O_TRACEEXEC is _not_ in effect, then
_everyone_ will always pass PTRACE_O_TRACEEXEC to SEIZE.
If that is true, you might as well make it default...  But
I'm claiming that a tracer may not want to see exec events at
all, so making it so that when you don't specify
PTRACE_O_TRACEEXEC, then you also don't get the magic SIGTRAP,
is more useful, and eliminates your justification too.  Oleg already
showed it's a super trivial patch too.  If you want to be
able to specify options directly on SEIZE, fine, I can see
that it is useful (of course gdb does the same "I still need to
set options on this child" dance as strace does).

> > If we don't disable the magic SIGTRAP, there's no way for a
> > tracer to do a very non-invasive SEIZE, say, a GDB mode that
> > only cares to let the tracer run free to catch SIGSEGVs
> > in some child, while later on during the run, the user remembers
> > to set a breakpoint.  At that point the tracer needs to catch
> > exec events, so it'd enable TRACE_O_EVENTEXEC.  Getting rid of
> > the SIGTRAP gets rid of the spurious stops when TRACE_O_EVENTEXEC
> > is not enabled.
> 
> This part I don't understand.

Say, you run the whole of gcc's testsuite under gdb, and
let it run until one of the children SIGSEGVs.  You do "gdb make; run".
Currently, all the children stop momentarily for fork/vfork/exec,
which slows down the run significantly (there are thousands of
forks/execs).  We should be able to only SEIZE the shell that runs
"make" (gdb runs the child through the shell, like "sh -c make"),
and let all its children run free, the least invasive way possible.
When a SIGSEGV happens, gdb can sync up about the process that crashed
from /proc.

We can't get rid of the magic SIGTRAP on PTRACE_ATTACH/PTRACE_TRACEME
for backwards compatibility reasons, but SEIZE is new.

> (btw, PTRACE_O_TRACEEXEC, not TRACE_O_EVENTEXEC).

Thanks.

-- 
Pedro Alves
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