Re: [PATCH v7 1/2] fork: extend clone3() to support setting a PID

[Adrian Reber]

On Mon, Nov 11, 2019 at 09:41:39PM +0100, Rasmus Villemoes wrote:
> On 11/11/2019 14.17, Adrian Reber wrote:
> > The main motivation to add set_tid to clone3() is CRIU.
> > 
> > To restore a process with the same PID/TID CRIU currently uses
> > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > ns_last_pid and then (quickly) does a clone(). This works most of the
> > time, but it is racy. It is also slow as it requires multiple syscalls.
> > 
> > Extending clone3() to support *set_tid makes it possible restore a
> > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > race free (as long as the desired PID/TID is available).
> > 
> > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > on clone3() with *set_tid as they are currently in place for ns_last_pid.
> > 
> > The original version of this change was using a single value for
> > set_tid. At the 2019 LPC, after presenting set_tid, it was, however,
> > decided to change set_tid to an array to enable setting the PID of a
> > process in multiple PID namespaces at the same time. If a process is
> > created in a PID namespace it is possible to influence the PID inside
> > and outside of the PID namespace. Details also in the corresponding
> > selftest.
> > 
> 
> >  	/*
> >  	 * Verify that higher 32bits of exit_signal are unset and that
> >  	 * it is a valid signal
> > @@ -2556,8 +2561,17 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> >  		.stack		= args.stack,
> >  		.stack_size	= args.stack_size,
> >  		.tls		= args.tls,
> > +		.set_tid	= kargs->set_tid,
> > +		.set_tid_size	= args.set_tid_size,
> >  	};
> 
> This is a bit ugly. And is it even well-defined? I mean, it's a bit
> similar to the "i = i++;". So it would be best to avoid.
> 
> > +	for (i = 0; i < args.set_tid_size; i++) {
> > +		if (copy_from_user(&kargs->set_tid[i],
> > +		    u64_to_user_ptr(args.set_tid + (i * sizeof(args.set_tid))),
> > +		    sizeof(pid_t)))
> > +			return -EFAULT;
> > +	}
> > +
> 
> If I'm reading this (and your test case) right, you expect the user
> pointer to point at an array of u64, and here you're copying the first
> half of each u64 to the pid_t array. That only works on little-endian.
> 
> It seems more obvious (since I don't think there's any disagreement
> anywhere on sizeof(pid_t)) to expect the user pointer to point at an
> array of pid_t and then simply copy_from_user() the whole thing in one go.

Yes, that was wrong. I changed the test case to use an array of pid_t.

		Adrian