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

[Denys Vlasenko]

On Fri, 2011-09-09 at 14:15 +0100, Pedro Alves wrote:
> On Friday 09 September 2011 13:28:55, Denys Vlasenko wrote:
> > On Fri, 2011-09-09 at 12:12 +0100, Pedro Alves wrote:
> > > On Thursday 08 September 2011 19:22:04, Denys Vlasenko wrote:
> > > > Make PTRACE_SEIZE set ptrace options specified in 'data' parameter
> > > >     
> > > > This can be used to close a few corner cases in strace where we get
> > > > unwanted racy behavior after attach, but before we have a chance
> > > > to set options (the notorious post-execve SIGTRAP comes to mind),
> > > 
> > > I'm still confused on why you're raising the SIGTRAP argument.  Did you see
> > > 
> > >  https://lkml.org/lkml/2011/9/8/7
> > > 
> > > From previous discussions, I understood that PTRACE_SEIZE _always_
> > > disables
> > > the post-execve SIGTRAP, so I don't believe that race actually exists.
> > > Or is that not the case?
> > 
> > 
> > I believe it is not the case. And I object to making it the case.
> 
> Well, if you'll remember, back in <https://lkml.org/lkml/2011/5/19/704> I
> raised this exact problem with that pesky racy post-execve SIGTRAP showing
> through on SEIZE, and Tejun a few replies later mentioned that the SIGTRAP
> was to be removed on SEIZE.   I'm sure it was said before even, but I'm
> not finding the emails now.
> 
> > My sense of taste says the approach "you need to use SEIZE to affect
> > feature <foo>" for various unrelated <foo> makes ptrace API ugly.
> 
> Yes, very much agreed!
> 
> > Especially that in this case, we already have a method in API
> > to suppress post-execve SIGTRAP.
> 
> Right, but we end up with no way to make the tracee _not ever
> stop_ at execve if the tracer wants to, like you can make
> the tracee not ever stop on forks or syscalls, by not enabling
> the corresponding PTRACE_O_FOO or not PTRACE_SYSCALL.  Not
> specifying PTRACE_O_TRACEEXEC coupled with `SEIZE not stopping
> tracees for that magic SIGTRAP' got you that.  In a way, it looked to
> me to make the API a bit less ugly.

This would be a _very_ minor improvement, so tiny it's not worth
bothering with. Let me show you the real-world code (part of strace
source) which skips over unneeded PTRACE_EVENT_EXEC:

                if ((status >> 16) != 0)
                        /* Ptrace event (we ignore all of them for now) */
                        goto restart_tracee_with_sig_0;

Yes. That is all.
It probably compiles into just two assembly instructions.

-- 
vda


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