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

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Message-ID: <Y39qSe30VYa0ftK4@zx2c4.com>
Date:   Thu, 24 Nov 2022 13:57:45 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Florian Weimer <fweimer@...hat.com>, 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,
        Arnd Bergmann <arnd@...d.de>
Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall

Hi Christian,

Thanks a bunch for chiming in.

On Thu, Nov 24, 2022 at 01:49:27PM +0100, Christian Brauner wrote:
> 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.

Briefly considered that, but it seemed a bit heavy for something like
this. I'm not super heavily opposed, but just seemed like a bit much.

> > > >> >> > +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.

Right, the __u64_aligned business seemed to be mostly about
extensibility.

> > > > 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?
>
> 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.

This isn't quite a struct versioned by size. This is:

    void *vgetrandom_alloc([inout] size_t *num, [out] size_t *size_per_each, unsigned int flags);

You give it an input 'num' and some flags (currently flags=0), and it
gives you back an output 'num' size, an output 'size_per_each' size, and
an opaque pointer value mapping as its return value.

I do like the idea of keeping size_t so that the type is "right". But
the other arguments are equally compelling as well, so not sure.

Jason