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

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Message-ID: <08ac69b4-4d12-4454-aa16-40db7e54816f@citrix.com>
Date: Thu, 22 Feb 2024 02:03:19 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Ard Biesheuvel <ardb@...nel.org>,
 Ross Philipson <ross.philipson@...cle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-crypto@...r.kernel.org, kexec@...ts.infradead.org,
 linux-efi@...r.kernel.org, dpsmith@...rtussolutions.com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, hpa@...or.com, dave.hansen@...ux.intel.com,
 mjg59@...f.ucam.org, James.Bottomley@...senpartnership.com,
 peterhuewe@....de, jarkko@...nel.org, jgg@...pe.ca, luto@...capital.net,
 nivedita@...m.mit.edu, herbert@...dor.apana.org.au, davem@...emloft.net,
 kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v8 04/15] x86: Secure Launch Resource Table header file

On 15/02/2024 8:08 am, Ard Biesheuvel wrote:
> On Wed, 14 Feb 2024 at 23:31, Ross Philipson <ross.philipson@...cle.com> wrote:
>> +/*
>> + * Primary SLR Table Header

I know it's just a comment, but SLR ought to be written in longhand here.

>> + */
>> +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