[acpi] Allow for extraction of a MAC address from the DSDT/SSDT

Some vendors provide a "system MAC address" within the DSDT/SSDT, to
be used to override the MAC address for a USB docking station.

A full implementation would require an ACPI bytecode interpreter,
since at least one OEM allows the MAC address to be constructed by
executable ACPI bytecode (rather than a fixed data structure).

We instead attempt to extract a plausible-looking "_AUXMAC_#.....#"
string that appears shortly after an "AMAC" or "MACA" signature.  This
should work for most implementations encountered in practice.

Debugged-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/acpimac.c

@@ -0,0 +1,154 @@
+/*
+ * Copyright (C) 2021 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <string.h>
+#include <errno.h>
+#include <ipxe/acpi.h>
+#include <ipxe/base16.h>
+#include <ipxe/ethernet.h>
+#include <ipxe/if_ether.h>
+#include <ipxe/acpimac.h>
+
+/** @file
+ *
+ * ACPI MAC address
+ *
+ */
+
+/** Colour for debug messages */
+#define colour FADT_SIGNATURE
+
+/** AMAC signature */
+#define AMAC_SIGNATURE ACPI_SIGNATURE ( 'A', 'M', 'A', 'C' )
+
+/** MACA signature */
+#define MACA_SIGNATURE ACPI_SIGNATURE ( 'M', 'A', 'C', 'A' )
+
+/** Maximum number of bytes to skip after AMAC/MACA signature
+ *
+ * This is entirely empirical.
+ */
+#define AUXMAC_MAX_SKIP 8
+
+/**
+ * Extract MAC address from DSDT/SSDT
+ *
+ * @v zsdt		DSDT or SSDT
+ * @v len		Length of DSDT/SSDT
+ * @v offset		Offset of signature within DSDT/SSDT
+ * @v data		Data buffer
+ * @ret rc		Return status code
+ *
+ * Some vendors provide a "system MAC address" within the DSDT/SSDT,
+ * to be used to override the MAC address for a USB docking station.
+ *
+ * A full implementation would require an ACPI bytecode interpreter,
+ * since at least one OEM allows the MAC address to be constructed by
+ * executable ACPI bytecode (rather than a fixed data structure).
+ *
+ * We instead attempt to extract a plausible-looking "_AUXMAC_#.....#"
+ * string that appears shortly after an "AMAC" or "MACA" signature.
+ * This should work for most implementations encountered in practice.
+ */
+static int acpi_extract_mac ( userptr_t zsdt, size_t len, size_t offset,
+			      void *data ) {
+	static const char prefix[9] = "_AUXMAC_#";
+	uint8_t *hw_addr = data;
+	size_t skip = 0;
+	char auxmac[ sizeof ( prefix ) /* "_AUXMAC_#" */ +
+		     ( ETH_ALEN * 2 ) /* MAC */ + 1 /* "#" */ + 1 /* NUL */ ];
+	char *mac = &auxmac[ sizeof ( prefix ) ];
+	int decoded_len;
+	int rc;
+
+	/* Skip signature and at least one tag byte */
+	offset += ( 4 /* signature */ + 1 /* tag byte */ );
+
+	/* Scan for "_AUXMAC_#.....#" close to signature */
+	for ( skip = 0 ;
+	      ( ( skip < AUXMAC_MAX_SKIP ) &&
+		( offset + skip + sizeof ( auxmac ) ) < len ) ;
+	      skip++ ) {
+
+		/* Read value */
+		copy_from_user ( auxmac, zsdt, ( offset + skip ),
+				 sizeof ( auxmac ) );
+
+		/* Check for expected format */
+		if ( memcmp ( auxmac, prefix, sizeof ( prefix ) ) != 0 )
+			continue;
+		if ( auxmac[ sizeof ( auxmac ) - 2 ] != '#' )
+			continue;
+		if ( auxmac[ sizeof ( auxmac ) - 1 ] != '\0' )
+			continue;
+		DBGC ( colour, "ACPI found MAC string \"%s\"\n", auxmac );
+
+		/* Terminate MAC address string */
+		mac = &auxmac[ sizeof ( prefix ) ];
+		mac[ ETH_ALEN * 2 ] = '\0';
+
+		/* Decode MAC address */
+		decoded_len = base16_decode ( mac, hw_addr, ETH_ALEN );
+		if ( decoded_len < 0 ) {
+			rc = decoded_len;
+			DBGC ( colour, "ACPI could not decode MAC \"%s\": %s\n",
+			       mac, strerror ( rc ) );
+			return rc;
+		}
+
+		/* Check MAC address validity */
+		if ( ! is_valid_ether_addr ( hw_addr ) ) {
+			DBGC ( colour, "ACPI has invalid MAC %s\n",
+			       eth_ntoa ( hw_addr ) );
+			return -EINVAL;
+		}
+
+		return 0;
+	}
+
+	return -ENOENT;
+}
+
+/**
+ * Extract MAC address from DSDT/SSDT
+ *
+ * @v hw_addr		MAC address to fill in
+ * @ret rc		Return status code
+ */
+int acpi_mac ( uint8_t *hw_addr ) {
+	int rc;
+
+	/* Look for an "AMAC" address */
+	if ( ( rc = acpi_extract ( AMAC_SIGNATURE, hw_addr,
+				   acpi_extract_mac ) ) == 0 )
+		return 0;
+
+	/* Look for a "MACA" address */
+	if ( ( rc = acpi_extract ( MACA_SIGNATURE, hw_addr,
+				   acpi_extract_mac ) ) == 0 )
+		return 0;
+
+	return -ENOENT;
+}

src/include/ipxe/acpimac.h

@@ -0,0 +1,14 @@
+#ifndef _IPXE_ACPIMAC_H
+#define _IPXE_ACPIMAC_H
+
+/** @file
+ *
+ * ACPI MAC address
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+extern int acpi_mac ( uint8_t *hw_addr );
+
+#endif /* _IPXE_ACPIMAC_H */

src/include/ipxe/errfile.h

@@ -77,6 +77,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_fdt		       ( ERRFILE_CORE | 0x00250000 )
 #define ERRFILE_dma		       ( ERRFILE_CORE | 0x00260000 )
 #define ERRFILE_cachedhcp	       ( ERRFILE_CORE | 0x00270000 )
+#define ERRFILE_acpimac		       ( ERRFILE_CORE | 0x00280000 )
 
 #define ERRFILE_eisa		     ( ERRFILE_DRIVER | 0x00000000 )
 #define ERRFILE_isa		     ( ERRFILE_DRIVER | 0x00010000 )