[RFC][v3][PATCH 7/7] Define clone_with_pids syscall

[Oren Laadan]

Sukadev Bhattiprolu wrote:
> | Call clone_with_pids as follows:
> | 
> | 	pid_t pids[] = { 0, 77, 99 };
> | 	struct target_pid_set pid_set;
> | 
> | 	pid_set.num_pids = sizeof(pids) / sizeof(int);
> | 	pid_set.target_pids = &pids;
> | 
> | 	syscall(__NR_clone_with_pids, flags, stack, NULL, NULL, NULL, &pid_set);
> 
> 
> Now that we need to do a copy_from_user() anyway, is it worth collapsing
> one or more parameters into a structure ?

Definitely so.

> 
> 	struct clone_with_pid_args {
> 		u64 flags;
> 		void *stack;
> 		void *tls;
> 		struct target_pid_set *pid_set;
> 	};
> 
> 	struct clone_tid_info {
> 		int *parent_tidptr;
> 		int *child_tidptr;
> 	}
> 
> 	struct clone_with_pid_args cwp_args;
> 	struct clone_tid_info ctid_inf;
> 
> 	...
> 
> 	syscall(__NR_clone_with_pids, &cwp_args, &ctid_info);
> 
> One advantage would be that it would make room for 64 bit clone flags, so we
> won't need a new flavor of clone_with_pids() when clone_flags is extended.

Because clone_with_pids() is a super-set of clone(), it is possible
that it (itself) becomes the successor that allows 64-bit clone flags.
(instead of, say, adding a clone64, clone2, clone_next_generation etc).

To allow this, we need to be careful to not copy_from_user for the
when fast-path is needed for common thread/task creation, if possible.

Oren.