Re: [PATCH v8 04/15] x86: Secure Launch Resource Table header file

From: ross.philipson@oracle.com
To: Andrew Cooper <andrew.cooper3@citrix.com>,
	Ard Biesheuvel <ardb@kernel.org>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-integrity@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-crypto@vger.kernel.org, kexec@lists.infradead.org,
	linux-efi@vger.kernel.org, dpsmith@apertussolutions.com,
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, dave.hansen@linux.intel.com, mjg59@srcf.ucam.org,
	James.Bottomley@hansenpartnership.com, peterhuewe@gmx.de,
	jarkko@kernel.org, jgg@ziepe.ca, luto@amacapital.net,
	nivedita@alum.mit.edu, herbert@gondor.apana.org.au,
	davem@davemloft.net, kanth.ghatraju@oracle.com,
	trenchboot-devel@googlegroups.com
Subject: Re: [PATCH v8 04/15] x86: Secure Launch Resource Table header file
Date: Wed, 21 Feb 2024 18:10:43 -0800	[thread overview]
Message-ID: <d250f64a-dd4b-44ca-8fb9-464bbd812c54@oracle.com> (raw)
In-Reply-To: <08ac69b4-4d12-4454-aa16-40db7e54816f@citrix.com>

On 2/21/24 6:03 PM, 'Andrew Cooper' via trenchboot-devel wrote:
> On 15/02/2024 8:08 am, Ard Biesheuvel wrote:
>> On Wed, 14 Feb 2024 at 23:31, Ross Philipson <ross.philipson@oracle.com> wrote:
>>> +/*
>>> + * Primary SLR Table Header
> 
> I know it's just a comment, but SLR ought to be written in longhand here.

Will do, thanks. Ross.

> 
>>> + */
>>> +struct slr_table {
>>> +       u32 magic;
>>> +       u16 revision;
>>> +       u16 architecture;
>>> +       u32 size;
>>> +       u32 max_size;
>>> +       /* entries[] */
>>> +} __packed;
>> Packing this struct has no effect on the layout so better drop the
>> __packed here. If this table is part of a structure that can appear
>> misaligned in memory, better to pack the outer struct or deal with it
>> there in another way.
> 
> As you note, __packed does two things not one.
> 
> The consumer of the random integer that is expected to be a pointer to a
> struct lsr_table doesn't know whether it was invoked by a 16bit
> bootloader or a 32bit bootloader, and this really does make a difference
> for an ABI described only in C.
> 
> Then again, we're holding off on setting the spec in stone until there's
> an agreement in principle, so we could retrofit a statement about the
> expected alignment of this structure in memory.
> 
> The sane choices are either 8b alignment (there are uint64_t's in
> entires[], but I also see there are some misaligned uint64_t's too,
> which is dull), or using the good old x86 fallback or paragraph
> alignment just in case we really want to extend it with a uint128_t in
> future.
> 
> Thoughts?
> 
> ~Andrew
> 