[smbios] Allow reading an entire SMBIOS data structure as a setting

The general syntax for SMBIOS settings:

  smbios/<instance>.<type>.<offset>.<length>

is currently extended such that a <length> of zero indicates that the
byte at <offset> contains a string index, and an <offset> of zero
indicates that the <length> contains a literal string index.

Since the byte at offset zero can never contain a string index, and a
literal string index can never have a zero value, the combination of
both <length> and <offset> being zero is currently invalid and will
always return "not found".

Extend the syntax such that the combination of both <length> and
<offset> being zero may be used to read the entire data structure.

Signed-off-by: Michael Brown <[email protected]>

src/interface/smbios/smbios_settings.c

@@ -117,8 +117,16 @@ static int smbios_fetch ( struct settings *settings __unused,
 		 * <offset> contains a string index.  An <offset> of
 		 * zero indicates that the <length> contains a literal
 		 * string index.
+		 *
+		 * Since the byte at offset zero can never contain a
+		 * string index, and a literal string index can never
+		 * be zero, the combination of both <length> and
+		 * <offset> being zero indicates that the entire
+		 * structure is to be read.
 		 */
-		if ( ( tag_len == 0 ) || ( tag_offset == 0 ) ) {
+		if ( ( tag_len == 0 ) && ( tag_offset == 0 ) ) {
+			tag_len = sizeof ( buf );
+		} else if ( ( tag_len == 0 ) || ( tag_offset == 0 ) ) {
 			index = ( ( tag_offset == 0 ) ?
 				  tag_len : buf[tag_offset] );
 			if ( ( rc = read_smbios_string ( &structure, index,