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

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Date: Wed, 21 Feb 2024 18:10:43 -0800
From: ross.philipson@...cle.com
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        Ard Biesheuvel <ardb@...nel.org>
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 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@...cle.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
>