Re: [PATCH 00/11] Introduce kernel_clone(), kill _do_fork()

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Aug 19, 2020 at 09:43:40AM +0200, peterz@infradead.org wrote:
> On Tue, Aug 18, 2020 at 06:44:47PM +0100, Matthew Wilcox wrote:
> > On Tue, Aug 18, 2020 at 07:34:00PM +0200, Christian Brauner wrote:
> > > The only remaining function callable outside of kernel/fork.c is
> > > _do_fork(). It doesn't really follow the naming of kernel-internal
> > > syscall helpers as Christoph righly pointed out. Switch all callers and
> > > references to kernel_clone() and remove _do_fork() once and for all.
> > 
> > My only concern is around return type.  long, int, pid_t ... can we
> > choose one and stick to it?  pid_t is probably the right return type
> > within the kernel, despite the return type of clone3().  It'll save us
> > some work if we ever go through the hassle of growing pid_t beyond 31-bit.
> 
> We have at least the futex ABI restricting PID space to 30 bits.

Ok, looking into kernel/futex.c I see 

pid_t pid = uval & FUTEX_TID_MASK;

which is probably what this referes to and /proc/sys/kernel/threads-max
is restricted to FUTEX_TID_MASK.

Afaict, that doesn't block switching kernel_clone() to return pid_t. It
can't create anything > FUTEX_TID_MASK anyway without yelling EAGAIN at
userspace. But it means that _if_ we were to change the size of pid_t
we'd likely need a new futex API. 

Christian