Re: [PATCH v5 1/2] fork: extend clone3() to support CLONE_SET_TID

[Christian Brauner]

On Mon, Aug 12, 2019 at 06:57:34PM +0200, Oleg Nesterov wrote:
> On 08/12, Christian Brauner wrote:
> > On Mon, Aug 12, 2019 at 06:37:10PM +0200, Oleg Nesterov wrote:
> > > On 08/11, Adrian Reber wrote:
> > > >
> > > >  include/linux/pid.h        |  2 +-
> > > >  include/linux/sched/task.h |  1 +
> > > >  include/uapi/linux/sched.h |  1 +
> > > >  kernel/fork.c              | 22 ++++++++++++++++++++--
> > > >  kernel/pid.c               | 36 +++++++++++++++++++++++++++++-------
> > > >  5 files changed, 52 insertions(+), 10 deletions(-)
> > > 
> > > Looks good to me...
> > > 
> > > A couple of nits below, but I won't insist, feel free to ignore.
> > > 
> > > > +/*
> > > > + * Different sizes of struct clone_args
> > > > + */
> > > > +#define CLONE3_ARGS_SIZE_V0 64
> > > 
> > > I don't really understand why do we want the "size < CLONE3_ARGS_SIZE_V0"
> > > check in copy_clone_args_from_user(), but I won't argue.
> > 
> > To make sure a user can't give us a garbage sized struct that is smaller
> > than the initial version of the struct.
> 
> But why do we want to detect this case?

I have to admit I don't understand that question. Because it's a garbage
sized struct that we can't do anything with. Why shouldn't we detect
this case?

> 
> And why CLONE3_ARGS_SIZE_V0 is special?

Hm? Because that is the first-well known struct size. In fact this
pattern is also used for perf and sched:

	if (!access_ok(uattr, PERF_ATTR_SIZE_VER0))
		return -EFAULT;