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.
> 
> >  	return 0;
> >  }
> >  
> > @@ -2631,6 +2645,10 @@ SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size)
> >  	int err;
> >  
> >  	struct kernel_clone_args kargs;
> > +	pid_t set_tid[MAX_PID_NS_LEVEL];
> > +
> > +	memset(set_tid, 0, sizeof(set_tid));
> > +	kargs.set_tid = set_tid;
> 
> Hm, isn't it a bit much to add two cachelines (and dirtying them via the
> memset) to the stack footprint of clone3, considering that almost nobody
> (relatively speaking) will use this?
> 
> So how about copy_clone_args_from_user() does
> 
> if (args.set_tid) {
>   set_tid = memdup_user(u64_to_user_ptr(), ...)
>   if (IS_ERR(set_tid))
>     return PTR_ERR(set_tid);
>   kargs.set_tid = set_tid;
> }
> 
> Then somebody needs to free that, but this is probably not the last
> clone extension that might need extra data, so one could do
> 
> s/long _do_fork/static long __do_fork/
> 
> and then create a _do_fork that always cleans up the passed-in kargs, i.e.
> 
> long _do_fork(struct kargs *args)
> {
>   long ret = __do_fork(args);
>   kfree(args->set_tid);
>   return ret;
> }

Thanks for your review. Did you had a look at what Christian suggested?
That should solve most of the points you mentioned. I will also remove
the memset() as it is not necessary at all.

		Adrian