From 9d643185022e2b9e1e239c8a9052fdfa2ee7d98d Mon Sep 17 00:00:00 2001
From: Thomas Schmitt <scdbackup@gmx.net>
Date: Sun, 14 Aug 2016 22:24:53 +0200
Subject: [PATCH] Corrected understanding of relation of RFC 4122 and UEFI
 GUID. Thanks Andrei Borzenkov.

---
 libisofs/libisofs.h    | 23 +++++++++++++----------
 libisofs/system_area.c |  7 +++----
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/libisofs/libisofs.h b/libisofs/libisofs.h
index 6fdd05e..efbedb3 100644
--- a/libisofs/libisofs.h
+++ b/libisofs/libisofs.h
@@ -2597,26 +2597,29 @@ int iso_write_opts_set_efi_bootp(IsoWriteOpts *opts, char *image_path,
 
 /**
  * Control whether the emerging GPT gets a pseudo-randomly generated disk GUID
- * or * whether it gets a user supplied GUID.
- * The partition GUIDs will be generated in a reproducible way by exoring a
- * little-endian 32 bit counter with the disk GUID beginning at byte offset 9.
+ * or whether it gets a user supplied GUID.
+ * The partition GUIDs will be generated in a reproducible way by exoring the
+ * little-endian 32 bit partion number with the disk GUID beginning at byte
+ * offset 9.
  *
  * @param opts
  *        The option set to be manipulated.
  * @param guid
- *        16 bytes of user supplied GUID.
- *        The upper 4 bit of guid[6] and guid[7] should bear the value 4 to
- *        express the version 4 in both endiannesses. Bit 7 of byte[8] should
- *        be set to 1 and bit 6 be set to 0, in order to express the RFC 4122
- *        variant of GUID, where version 4 means "random".
+ *        16 bytes of user supplied GUID. Readily byte-swapped as prescribed by
+ *        UEFI specs: 4 byte, 2 byte, 2 byte as little-endian. The rest as
+ *        big-endian.
+ *        The upper 4 bit of guid[7] should bear the value 4 to express the
+ *        RFC 4122 version 4. Bit 7 of byte[8] should  be set to 1 and bit 6
+ *        be set to 0, in order to express the RFC 4122 variant of UUID,
+ *        where version 4 means "pseudo-random uuid".
  * @param mode
  *        0 = ignore parameter guid and produce the GPT disk GUID by a
  *            pseudo-random algorithm. This is the default setting.
  *        1 = use parameter guid as GPT disk GUID
  *        2 = ignore parameter guid and derive the GPT disk GUID from
  *            parameter vol_uuid of iso_write_opts_set_pvd_times().
- *            The 16 bytes of vol_uuid get copied and bytes 6, 7, 8 get their
- *            upper bits changed to comply to RFC 4122.
+ *            The 16 bytes of vol_uuid get copied and bytes 7, 8 get their
+ *            upper bits changed to comply to RFC 4122 and UEFI.
  *            Error ISO_GPT_NO_VOL_UUID will occur if image production begins
  *            before vol_uuid was set.
  *
diff --git a/libisofs/system_area.c b/libisofs/system_area.c
index 8db8cd4..5ef3e21 100644
--- a/libisofs/system_area.c
+++ b/libisofs/system_area.c
@@ -2362,10 +2362,9 @@ uint32_t iso_crc32_gpt(unsigned char *data, int count, int flag)
 
 void iso_mark_guid_version_4(uint8_t *u)
 {
-    /* Mark as UUID version 4. RFC 4122 says u[6], but isohybrid swapping
-       effectively puts the 4 into u[7]. So i mark both. 4 bits wasted.
+    /* Mark as UUID version 4. RFC 4122 says u[6], but UEFI prescribes
+       bytes 6 and 7 to be swapped.
     */
-    u[6] = (u[6] & 0x0f) | 0x40;
     u[7] = (u[7] & 0x0f) | 0x40;
 
     /* Variant is "1 0 x" as described in RFC 4122.
@@ -2389,7 +2388,7 @@ void iso_generate_gpt_guid(uint8_t guid[16])
 #else
 
     uint8_t *u;
-    /* produced by uuid_generate() and byte-swapped to isohybrid.c habits */
+    /* produced by uuid_generate() and byte-swapped to UEFI specs */
     static uint8_t uuid_template[16] = {
         0xee, 0x29, 0x9d, 0xfc, 0x65, 0xcc, 0x7c, 0x40,
         0x92, 0x61, 0x5b, 0xcd, 0x6f, 0xed, 0x08, 0x34