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From:   Alexander Graf <graf@amazon.com>
To:     <linux-acpi@vger.kernel.org>
CC:     <linux-kernel@vger.kernel.org>, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        "Greg KH" <gregkh@linuxfoundation.org>, <ardb@kernel.org>,
        <dwmw@amazon.co.uk>
Subject: [PATCH] ACPI: bus: Match first 9 bytes of device IDs
Date:   Fri, 25 Feb 2022 16:55:52 +0100
Message-ID: <20220225155552.30636-1-graf@amazon.com>
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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
based on the ACPI spec:

"""
A _HID object evaluates to either a numeric 32-bit compressed EISA
type ID or a string. If a string, the format must be an alphanumeric
PNP or ACPI ID with no asterisk or other leading characters.
A valid PNP ID must be of the form "AAA####" where A is an uppercase
letter and # is a hex digit.
A valid ACPI ID must be of the form "NNNN####" where N is an uppercase
letter or a digit ('0'-'9') and # is a hex digit. This specification
reserves the string "ACPI" for use only with devices defined herein.
It further reserves all strings representing 4 HEX digits for
exclusive use with PCI-assigned Vendor IDs.
"""

While most people adhere to the ACPI specs, Microsoft decided that its
VM Generation Counter device [1] should only be identifiable by _CID with
an value of "VM_Gen_Counter" - longer than 9 characters.

To still allow device drivers to match identifiers that exceed the 9 byte
limit, without wasting memory for the unlikely case that you have long
identifiers, let's match only the first 9 characters of the identifier.

This patch is a prerequisite to add support for VMGenID in Linux [2].

[1] https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
[2] https://lore.kernel.org/lkml/20220225124848.909093-1-Jason@zx2c4.com/

Signed-off-by: Alexander Graf <graf@amazon.com>

---

Alternatives to the approach above would be:

  1) Always set id[8] = '\0' in acpi_add_id()
  2) Allocate the id in struct acpi_device_id dynamically

I'm happy to explore option 1 instead if people believe it's cleaner.
Option 2 on the other hand seems overkill for the issue at hand. We don't
have a lot of devices that exceed the 8 character threshold, so chance of
collision is quite small. On the other hand, the extra overhead of
maintaining the string allocation dynamically will quickly become a
headache.

---
 drivers/acpi/bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
index 07f604832fd6..aba93171739f 100644
--- a/drivers/acpi/bus.c
+++ b/drivers/acpi/bus.c
@@ -829,7 +829,7 @@ static bool __acpi_match_device(struct acpi_device *device,
 		/* First, check the ACPI/PNP IDs provided by the caller. */
 		if (acpi_ids) {
 			for (id = acpi_ids; id->id[0] || id->cls; id++) {
-				if (id->id[0] && !strcmp((char *)id->id, hwid->id))
+				if (id->id[0] && !strncmp((char *)id->id, hwid->id, ACPI_ID_LEN))
 					goto out_acpi_match;
 				if (id->cls && __acpi_match_device_cls(id, hwid))
 					goto out_acpi_match;
-- 
2.16.4


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