| Message ID | 20220225155552.30636-1-graf@amazon.com |
|---|---|
| State | New |
| Headers | show |
| Series | ACPI: bus: Match first 9 bytes of device IDs | expand |
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;
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(-)