fix(rsdp): fix rsdp revision and rsdp checksum to fit linux kernel impl

[donjuanplatinum]

1. In Linux kernel source in drivers/acpi/acpica/tbutils.c line 242-259 shows that: when revision > 1, use XSDT. and revision = 0 or 1 use RSDT.

	/* Use XSDT if present and not overridden. Otherwise, use RSDT */

	if ((rsdp->revision > 1) &&
	    rsdp->xsdt_physical_address && !acpi_gbl_do_not_use_xsdt) {
		/*
		 * RSDP contains an XSDT (64-bit physical addresses). We must use
		 * the XSDT if the revision is > 1 and the XSDT pointer is present,
		 * as per the ACPI specification.
		 */
		address = (acpi_physical_address)rsdp->xsdt_physical_address;
		table_entry_size = ACPI_XSDT_ENTRY_SIZE;
	} else {
		/* Root table is an RSDT (32-bit physical addresses) */

		address = (acpi_physical_address)rsdp->rsdt_physical_address;
		table_entry_size = ACPI_RSDT_ENTRY_SIZE;
	}

in acpi-rs in acpi/src/lib.rs line 233-248 shows that: only revision ==0 use RSDT, other use XSDT.

 let revision = rsdp_mapping.revision();
        let root_table_mapping = if revision == 0 {
            /*
             * We're running on ACPI Version 1.0. We should use the 32-bit RSDT address.
             */

            read_root_table!(RSDT, rsdt_address)
        } else {
            /*
             * We're running on ACPI Version 2.0+. We should use the 64-bit XSDT address, truncated
             * to 32 bits on x86.
             */

            read_root_table!(XSDT, xsdt_address)
        };

        Ok(Self { mapping: root_table_mapping, revision, handler })
    }

so , when revision = 1, linux will regard it as RSDT, but acpi-rs see it as a XSDT.
2. the checksum len in acpi 2.0 condition.
in linux source drivers/acpi/acpica/tbxfroot.c line 59-89.

the acpi2.0 checksum len is a constant ACPI_RSDP_XCHECKSUM_LENGTH valued 36 and its defined in include/acpi/acconfig.h in line 173:

/* RSDP checksums */
#define ACPI_RSDP_CHECKSUM_LENGTH       20
#define ACPI_RSDP_XCHECKSUM_LENGTH      36

acpi_status acpi_tb_validate_rsdp(struct acpi_table_rsdp *rsdp)
{

	/*
	 * The signature and checksum must both be correct
	 *
	 * Note: Sometimes there exists more than one RSDP in memory; the valid
	 * RSDP has a valid checksum, all others have an invalid checksum.
	 */
	if (!ACPI_VALIDATE_RSDP_SIG(rsdp->signature)) {

		/* Nope, BAD Signature */

		return (AE_BAD_SIGNATURE);
	}

	/* Check the standard checksum */

	if (acpi_ut_checksum((u8 *)rsdp, ACPI_RSDP_CHECKSUM_LENGTH) != 0) {
		return (AE_BAD_CHECKSUM);
	}

	/* Check extended checksum if table version >= 2 */

	if ((rsdp->revision >= 2) &&
	    (acpi_ut_checksum((u8 *)rsdp, ACPI_RSDP_XCHECKSUM_LENGTH) != 0)) {
		return (AE_BAD_CHECKSUM);
	}

	return (AE_OK);
}

but in acpi-rs in acpi/src/rsdp.rs line 119-127 it use self.length

   let length = if self.revision > 0 {
            // For Version 2.0+, include the number of bytes specified by `length`
            self.length as usize
        } else {
            RSDP_V1_LENGTH
        };

        let bytes = unsafe { slice::from_raw_parts(self as *const Rsdp as *const u8, length) };
        let sum = bytes.iter().fold(0u8, |sum, &byte| sum.wrapping_add(byte));

so when the length of RSDP in memory is not 36 and use ACPI2.0, linux will also see it as 36,but acpi-rs not.

and if firmware write the lenght > 36, acpi-rs will read over the range and go into UB.

[IsaacWoods]

Thanks for the report! This is a good point (and another place the spec falls short).

I hope you don't mind, but I wanted to approach this in a slightly different way than your PR, so I've made a couple of commits to main with you as co-author. These implement both of your fixes, but with an additional fix (your version does not correctly infer the RSDT/XSDT entry size from the revision later, so I've moved to inferring it at the start), and adds an AcpiQuirks structure I've been meaning to chuck in for a while.

These do technically break public API, so will need a major version bump. We've got some more changes with the same requirement coming soon, so I won't do this quite yet.