linux-kernel - Re: [PATCH v17 4/5] random: introduce generic vDSO getrandom() implementation

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Message-ID: <20240618084548.GE31592@noisy.programming.kicks-ass.net>
Date: Tue, 18 Jun 2024 10:45:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Andy Lutomirski <luto@...capital.net>, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev, tglx@...utronix.de,
	linux-crypto@...r.kernel.org, linux-api@...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>,
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Jann Horn <jannh@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>
Subject: Re: [PATCH v17 4/5] random: introduce generic vDSO getrandom()
 implementation

On Tue, Jun 18, 2024 at 02:12:40AM +0200, Jason A. Donenfeld wrote:
> Hi Andy,
> 
> On Mon, Jun 17, 2024 at 05:06:22PM -0700, Andy Lutomirski wrote:
> > On Fri, Jun 14, 2024 at 12:08 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> > >
> > > Provide a generic C vDSO getrandom() implementation, which operates on
> > > an opaque state returned by vgetrandom_alloc() and produces random bytes
> > > the same way as getrandom(). This has a the API signature:
> > >
> > >   ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);
> > 
> > Last time around, I mentioned some potential issues with this function
> > signature, and I didn't see any answer.  My specific objection was to
> > the fact that the caller passes in a pointer but not a length, and
> > this potentially makes reasoning about memory safety awkward,
> > especially if anything like CRIU is involved.
> 
> Oh, I understood this backwards last time - I thought you were
> criticizing the size_t len argument, which didn't make any sense.
> 
> Re-reading now, what you're suggesting is that I add an additional
> argument called `size_t opaque_len`, and then the implementation does
> something like:

Exactly, that's how I read amluto's suggestion as well. Also, I recently
ran into this clang rfc:

  https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854