Re: possible deadlock in proc_pid_syscall (2)

[peterz@infradead.org]

On Sun, Aug 30, 2020 at 07:31:39AM -0500, Eric W. Biederman wrote:
> peterz@infradead.org writes:

> > Could we check privs twice instead?
> >
> > Something like the completely untested below..
> 
> That might work.
> 
> I am thinking that for cases where we want to do significant work it
> might be better to ask the process to pause at someplace safe (probably
> get_signal) and then do all of the work when we know nothing is changing
> in the process.
> 
> I don't really like the idea of checking and then checking again.  We
> might have to do it but it feels like the model is wrong somewhere.

Another possible aproach might be to grab a copy of the cred pointer and
have the final install check that. It means we need to allow
perf_install_in_context() to fail though. That might be a little more
work.

> I had not realized before this how much setting up tracing in
> perf_even_open looks like attaching a debugger in ptrace_attach.

Same problem; once you've attached a perf event you can observe much of
what the task does.