Re: [PATCH v12 2/3] ptrace,syscall_user_dispatch: checkpoint/restore support for SUD

[Oleg Nesterov <oleg@redhat.com>]

On 02/28, Dmitry Safonov wrote:
>
> On 2/28/23 16:52, Oleg Nesterov wrote:
> > On 02/27, Dmitry Safonov wrote:
> >>
> >>> +int syscall_user_dispatch_set_config(struct task_struct *task, unsigned long size,
> >>> +                                    void __user *data)
> >>> +{
> >>> +       int rc;
> >>> +       struct ptrace_sud_config cfg;
> >>> +
> >>> +       if (size != sizeof(struct ptrace_sud_config))
> >>> +               return -EINVAL;
> >>> +
> >>> +       if (copy_from_user(&cfg, data, sizeof(struct ptrace_sud_config)))
> >>> +               return -EFAULT;
> >>
> >> It seems that the tool you want here would be copy_struct_from_user(),
> >> which is designed for extendable syscalls.
> >
> > Hmm. Why?
> >
> > In this case ksize == usize, so why do we need copy_struct_from_user ?
>
> In case the structure extends in future, that will let newer userspace
> run on an older kernel (as long as it doesn't use [set] any new fields).

Sure, I understand that, but I don't think it's worth the trouble
in this case.

If (unlikely, I think) this structure ever extends we can switch to
copy_struct_from_user() or do something else if check_zeroed_user()
makes no real sense for the new fields.

Right now I think it is more important to ensure that the new users
of this API use the correct size.

Oleg.