linux-kernel - Re: [PATCH v5 2/3] ACPI: allow longer device IDs

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Date:   Sun, 27 Feb 2022 12:42:03 +0100
From:   Alexander Graf <graf@...zon.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
CC:     <linux-crypto@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Ard Biesheuvel <ardb@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v5 2/3] ACPI: allow longer device IDs


On 26.02.22 23:06, Jason A. Donenfeld wrote:
> From: Alexander Graf <graf@...zon.com>
>
> We create a list of ACPI "PNP" IDs which contains _HID, _CID, and CLS
> entries of the respective devices. However, we squeeze them into struct
> acpi_device_id, which only has 9 bytes space to store the identifier. It
> originally had 16 bytes, but was changed to only have 9 in 6543becf26ff
> ("mod/file2alias: make modalias generation safe for cross compiling"),
> presumably on the theory that it would match the ACPI spec so it didn't
> matter.
>
> Unfortunately, while most people adhere to the ACPI specs, Microsoft
> decided that its VM Generation Counter device [1] should only be
> identifiable by _CID with a value of "VM_Gen_Counter", which is longer
> than 9 characters.
>
> To allow device drivers to match identifiers that exceed the 9 byte
> limit, this simply ups the length to 16, just like it was before the
> aforementioned commit. Empirical testing indicates that this


This is only true for 64bit systems where padding automatically bloated 
to 9 byte array to 16. I still believe the patch is fine as it is, but 
there will be minor .rodata overhead on 32bit targets which you may want 
to quantify in the patch description.


Thanks a lot for sending this out! And let's hope that 16 bytes is 
enough for everyone.

Alex


> doesn't actually increase vmlinux size, because the ulong in the same
> struct caused there to be 7 bytes of padding anyway.
>
> This patch is a prerequisite to add support for VMGenID in Linux, the
> subsequent patch in this series. It has been confirmed to also work on
> the udev/modalias side in userspace.
>
> [1] https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Len Brown <lenb@...nel.org>
> Cc: Rafael J. Wysocki <rafael@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Co-authored-by: Jason A. Donenfeld <Jason@...c4.com>
> Signed-off-by: Alexander Graf <graf@...zon.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Hi Rafael & Len,
>
> This patchset is directed toward you two specifically. Patches 1/3 and
> 3/3 have been through the ringer of review a bit already and do not
> specifically require your attention, but in v4 we wound up getting hung
> up on an ACPI API limitation. This v5 fixes that limitation with this
> 2/3 patch that you see here, with a trivial one line fix, which does
> require your attention.
>
> Patches 1/3 and 3/3 will go through my random.git tree. However, 3/3
> actually depends on this one here, 2/3, in order to compile without
> warnings (and be functional at all). Therefore, it would be nice if you
> would provide an "Acked-by" on it and permit me to /also/ take it
> through my random.git tree (if it looks like a correct patch to you, of
> course). This would make the merge logistics a lot easier. Plus it's a
> small +1/-1 line change.
>
> Please have a look and let me know what you think.
>
> Thanks,
> Jason
>
>   include/linux/mod_devicetable.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> index 4bb71979a8fd..5da5d990ff58 100644
> --- a/include/linux/mod_devicetable.h
> +++ b/include/linux/mod_devicetable.h
> @@ -211,7 +211,7 @@ struct css_device_id {
>          kernel_ulong_t driver_data;
>   };
>
> -#define ACPI_ID_LEN    9
> +#define ACPI_ID_LEN    16
>
>   struct acpi_device_id {
>          __u8 id[ACPI_ID_LEN];
> --
> 2.35.1
>



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