linux-kernel - Re: [PATCH v6 1/3] random: add vgetrandom

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Message-ID: <Y39djiBSmgXfgWJv@zx2c4.com>
Date:   Thu, 24 Nov 2022 13:03:26 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        tglx@...utronix.de, linux-crypto@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall

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.

Jason