by Yann Droneaud

[permalink] [raw]

Subject: Re: [PATCH v1 1/2] fork: add clone3

Hi,

Le samedi 01 juin 2019 à 00:08 +0200, Christian Brauner a écrit :
> On Wed, May 29, 2019 at 05:42:14PM +0200, Yann Droneaud wrote:
> > Le mercredi 29 mai 2019 à 17:22 +0200, Christian Brauner a écrit :
> > > This adds the clone3 system call.
> > >
> > >
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index b4cba953040a..6bc3e3d17150 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -2472,7 +2475,96 @@ SYSCALL_DEFINE5(clone, unsigned long,
> > > clone_flags, unsigned long, newsp,
> > > unsigned long, tls)
> > > #endif
> > > {
> > > - return _do_fork(clone_flags, newsp, 0, parent_tidptr,
> > > child_tidptr, tls);
> > > + struct kernel_clone_args args = {
> > > + .flags = clone_flags,
> > > + .stack = newsp,
> > > + .pidfd = parent_tidptr,
> > > + .parent_tidptr = parent_tidptr,
> > > + .tls = tls,
> > > + .child_tidptr = child_tidptr,
> > > + };
> > > +
> > > + /* clone(CLONE_PIDFD) uses parent_tidptr to return a pidfd */
> > > + if ((clone_flags & CLONE_PIDFD) && (clone_flags &
> > > CLONE_PARENT_SETTID))
> > > + return -EINVAL;
> > > +
> > > + return _do_fork(&args);
> > > +}
> > > +
> > > +static bool clone3_flags_valid(u64 flags)
> > > +{
> > > + if (flags & CLONE_DETACHED)
> > > + return false;
> > > +
> > > + if (flags & ~CLONE_MAX)
> > > + return false;
> > > +
> > > + return true;
> > > +}
> > > +
> > > +static int copy_clone_args_from_user(struct kernel_clone_args
> > > *kargs,
> > > + struct clone_args __user *uargs,
> > > + size_t size)
> > > +{
> > > + struct clone_args args;
> > > +
> > > + if (unlikely(size > PAGE_SIZE))
> > > + return -E2BIG;
> > > +
> > > + if (unlikely(size < sizeof(struct clone_args)))
> > > + return -EINVAL;
> > > +
> > > + if (unlikely(!access_ok(uargs, size)))
> > > + return -EFAULT;
> > > +
> > > + if (size > sizeof(struct clone_args)) {
> > > + unsigned char __user *addr;
> > > + unsigned char __user *end;
> > > + unsigned char val;
> > > +
> > > + addr = (void __user *)uargs + sizeof(struct
> > > clone_args);
> > > + end = (void __user *)uargs + size;
> > > +
> > > + for (; addr < end; addr++) {
> > > + if (get_user(val, addr))
> > > + return -EFAULT;
> > > + if (val)
> > > + return -E2BIG;
> >
> > Should be -EINVAL: having something after the structure should be
> > handled just like an invalid flags, while still allowing future
> > userspace program to probe for support for newer feature.
>
> (Traveling until Monday, so sorry for delayed responses.)
>
> This copies what:
>
> kernel/sched/core.c:sched_copy_attr()
> kernel/event/core.c:perf_copy_attr()
>
> are already doing. Consistency might be good here but, I think.
>

I would have prefer all the above to returns -EINVAL for consistency
with the unknown flags check ...

"Designing the API: Planning for Extension" [1] doesn't mandate return
-EINVAL for that case, but does make perf_event_open() and
perf_copy_attr() the example to follow ... so you're right.

[1]
https://www.kernel.org/doc/html/v5.1/process/adding-syscalls.html#designing-the-api-planning-for-extension

Regards.

--
Yann Droneaud
OPTEYA