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Date:   Thu, 24 Nov 2022 13:24:42 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        tglx@linutronix.de, linux-crypto@vger.kernel.org, x86@kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
        Carlos O'Donell <carlos@redhat.com>, linux-api@vger.kernel.org
Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall
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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?

Jason