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

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Message-ID: <Y3+cJLBas5hRh7GU@zx2c4.com>
Date:   Thu, 24 Nov 2022 17:30:28 +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>, linux-api@...r.kernel.org
Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall

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.
> 
> The other direction would be making this a u32

I think `unsigned int` is actually a sensible size for what these values
should be. That eliminates the problem and potential bikeshed too. So
I'll go with that for v+1.

Jason