Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

Hey again,

On Thu, Nov 24, 2022 at 01:24:42PM +0100, Jason A. Donenfeld wrote:
> Hi Florian,
> 
> On Thu, Nov 24, 2022 at 01:15:24PM +0100, Florian Weimer wrote:
> > * Jason A. Donenfeld:
> > 
> > > Hi Florian,
> > >
> > > On Thu, Nov 24, 2022 at 06:25:39AM +0100, Florian Weimer wrote:
> > >> * Jason A. Donenfeld:
> > >> 
> > >> > Hi Florian,
> > >> >
> > >> > On Wed, Nov 23, 2022 at 11:46:58AM +0100, Florian Weimer wrote:
> > >> >> * Jason A. Donenfeld:
> > >> >> 
> > >> >> > + * The vgetrandom() function in userspace requires an opaque state, which this
> > >> >> > + * function provides to userspace, by mapping a certain number of special pages
> > >> >> > + * into the calling process. It takes a hint as to the number of opaque states
> > >> >> > + * desired, and returns the number of opaque states actually allocated, the
> > >> >> > + * size of each one in bytes, and the address of the first state.
> > >> >> > + */
> > >> >> > +SYSCALL_DEFINE3(vgetrandom_alloc, unsigned long __user *, num,
> > >> >> > +                unsigned long __user *, size_per_each, unsigned int, flags)
> > >> >> 
> > >> >> I think you should make this __u64, so that you get a consistent
> > >> >> userspace interface on all architectures, without the need for compat
> > >> >> system calls.
> > >> >
> > >> > That would be quite unconventional. Most syscalls that take lengths do
> > >> > so with the native register size (`unsigned long`, `size_t`), rather
> > >> > than u64. If you can point to a recent trend away from this by
> > >> > indicating some commits that added new syscalls with u64, I'd be happy
> > >> > to be shown otherwise. But AFAIK, that's not the way it's done.
> > >> 
> > >> See clone3 and struct clone_args.
> > >
> > > The struct is one thing. But actually, clone3 takes a `size_t`:
> > >
> > >     SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size)
> > >
> > > I take from this that I too should use `size_t` rather than `unsigned
> > > long.` And it doesn't seem like there's any compat clone3.
> > 
> > But vgetrandom_alloc does not use unsigned long, but unsigned long *.
> > You need to look at the contents for struct clone_args for comparison.
> 
> Ah! I see what you mean; that's a good point. The usual register
> clearing thing isn't going to happen because these are addresses.
> 
> I still am somewhat hesitant, though, because `size_t` is really the
> "proper" type to be used. Maybe the compat syscall thing is just a
> necessary evil?
> 
> The other direction would be making this a u32, since 640k ought to be
> enough for anybody and such, but maybe that'd be a mistake too.
> 
> So I'm not sure. Anybody else on the list with experience adding
> syscalls have an opinion?

Looks like set_mempolicy, get_mempoliy, and migrate_pages pass an
unsigned long pointer and I don't see any compat stuff around it:

    SYSCALL_DEFINE3(set_mempolicy, int, mode, const unsigned long __user *, nmask,
                    unsigned long, maxnode)
    
    SYSCALL_DEFINE5(get_mempolicy, int __user *, policy,
                    unsigned long __user *, nmask, unsigned long, maxnode,
                    unsigned long, addr, unsigned long, flags)

    SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
                    const unsigned long __user *, old_nodes,
                    const unsigned long __user *, new_nodes)
     

In contrast sched_setaffinity and get_robust_list take a unsigned long
pointer and does have a compat wrapper:

    SYSCALL_DEFINE3(sched_setaffinity, pid_t, pid, unsigned int, len,
                    unsigned long __user *, user_mask_ptr)

    SYSCALL_DEFINE3(get_robust_list, int, pid,
                    struct robust_list_head __user * __user *, head_ptr,
                    size_t __user *, len_ptr)

Jason