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

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Date:   Thu, 24 Nov 2022 13:15:24 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Jason A. Donenfeld" <Jason@...c4.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

* 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.

Thanks,
Florian