[smbios] Mangle UUIDs for SMBIOS version 2.6 and newer

iPXE treats UUIDs as being in network byte order (big-endian).  The
SMBIOS specification version 2.6 states that UUIDs are stored with
little-endian values in the first three fields; earlier versions did
not specify an endianness.  This results in some inconsistency between
the BIOS, vendor PXE, iPXE, and operating system interpretations of
the SMBIOS UUID.

dmidecode assumes that the byte order is little-endian if and only if
the SMBIOS version is 2.6 or higher.  Choose to match this behaviour.

Reported-by: Matthew Helton <mwhelton@gmail.com>
Reported-by: Alexandru Bordei <alexandru.bordei@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/smbios/smbios_settings.c

 #include <stdint.h>
 #include <string.h>
 #include <errno.h>
+#include <byteswap.h>
 #include <ipxe/settings.h>
 #include <ipxe/init.h>
 #include <ipxe/uuid.h>
 
 	{
 		uint8_t buf[structure.header.len];
+		const void *raw;
+		union uuid uuid;
 
 		/* Read SMBIOS structure */
 		if ( ( rc = read_smbios_structure ( &structure, buf,
 						    sizeof ( buf ) ) ) != 0 )
 			return rc;
 
+		/* A tag length of zero indicates a string */
 		if ( tag_len == 0 ) {
-			/* String */
 			if ( ( rc = read_smbios_string ( &structure,
 							 buf[tag_offset],
 							 data, len ) ) < 0 ) {
 			if ( ! setting->type )
 				setting->type = &setting_type_string;
 			return rc;
-		} else {
-			/* Raw data */
-			if ( len > tag_len )
-				len = tag_len;
-			memcpy ( data, &buf[tag_offset], len );
-			if ( ! setting->type )
-				setting->type = &setting_type_hex;
-			return tag_len;
 		}
+
+		/* Mangle UUIDs if necessary.  iPXE treats UUIDs as
+		 * being in network byte order (big-endian).  SMBIOS
+		 * specification version 2.6 states that UUIDs are
+		 * stored with little-endian values in the first three
+		 * fields; earlier versions did not specify an
+		 * endianness.  dmidecode assumes that the byte order
+		 * is little-endian if and only if the SMBIOS version
+		 * is 2.6 or higher; we match this behaviour.
+		 */
+		raw = &buf[tag_offset];
+		if ( ( setting->type == &setting_type_uuid ) &&
+		     ( tag_len == sizeof ( uuid ) ) &&
+		     ( smbios_version() >= SMBIOS_VERSION ( 2, 6 ) ) ) {
+			DBG ( "SMBIOS detected mangled UUID\n" );
+			memcpy ( &uuid, &buf[tag_offset], sizeof ( uuid ) );
+			__bswap_32s ( &uuid.canonical.a );
+			__bswap_16s ( &uuid.canonical.b );
+			__bswap_16s ( &uuid.canonical.c );
+			raw = &uuid;
+		}
+
+		/* Return data */
+		if ( len > tag_len )
+			len = tag_len;
+		memcpy ( data, raw, len );
+		if ( ! setting->type )
+			setting->type = &setting_type_hex;
+		return tag_len;
 	}
 }