linux-kernel - Re: [PATCH tip v8] x86/setup: Use rng seeds from setup

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Date:   Sat, 9 Jul 2022 15:41:19 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH tip v8] x86/setup: Use rng seeds from setup_data

On 7/9/22 14:57, Borislav Petkov wrote:
> On Sat, Jul 09, 2022 at 02:45:24PM -0700, H. Peter Anvin wrote:
>> On 7/9/22 02:49, Borislav Petkov wrote:
>>> On Fri, Jul 08, 2022 at 06:51:16PM -0700, H. Peter Anvin wrote:
>>>> #define SETUP_ENUM_MAX	SETUP_RNG_SEED
>>>> #define SETUP_INDIRECT	(1<<31)
>>>> #define SETUP_TYPE_MAX	(SETUP_ENUM_MAX | SETUP_INDIRECT)
>>>
>>> Wait, if we get to add a new number, SETUP_ENUM_MAX and thus
>>> SETUP_TYPE_MAX will change. And they're uapi too...
>>
>> Talking API here rather than ABI, i.e. the semantics of those symbols.
> 
> Sure but do we worry about some userspace including those headers and
> relying on the SETUP_ENUM_MAX number?
> 
> Or is userspace required to be recompiled against newer uapi headers?
> 

In ABI/API terms, that symbol has the semantic of connecting the API 
version to the underlying ABI version; a piece of code that sees an 
enumeration type > SETUP_ENUM_MAX must by definition treat it as an 
opaque blob. In the future, should it become warranted, we may add flags 
that indicate how unaware code should handle them, but I don't think we 
can engineer that right now.

	-hpa