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

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Message-ID: <87a64g7wks.fsf@oldenburg.str.redhat.com>
Date:   Thu, 24 Nov 2022 06:25:39 +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 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.  It's more common with pointers, which
are now 64 bits unconditionally: struct futex_waitv, struct rseq_cs and
struct rseq.

If the length or pointer is a system call argument, widening it to 64
bits is not necessary because zero-extension to the full register
eliminates the need for a compat system call.  But if you pass the
address to a size or pointer, you'll need compat syscalls if you don't
make the passed data __u64.

Thanks,
Florian