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

[Christian Brauner <brauner@kernel.org>]

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.

For system calls that take structs as arguments we use u64 in the struct
for proper alignment so we can extend structs without regressing old
kernels. We have a few of those extensible struct system calls.

But we don't really have a lot system calls that pass u64 as a pointer
outside of a structure so far. Neither as register and nor as pointer
iirc. Passing them as a register arg is problematic because of 32bit
arches. But passing as pointer should be fine but it is indeed uncommon.

> > >
> > > 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?

We try to avoid adding new compat-requiring syscalls like the plague
usually. (At least for new syscalls that don't need to inherit behavior
from earlier syscalls they are a revisions of.)

> 
> 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.

I think making this a size_t is fine. We haven't traditionally used u32
for sizes. All syscalls that pass structs versioned by size use size_t.
So I would recommend to stick with that.

Alternatively, you could also introduce a simple struct versioned by
size for this system call similar to mount_setatt() and clone3() and so
on. This way you don't need to worry about future extensibilty. Just a
thought.