Syslinux - Jun 2025 - [PATCH] isohybrid: Make GPT GUIDs reproducible with --uefi and --id

From: Chris Riches <chris.riches at nutanix.com>

We already have the --id option to make the MBR ID reproducible,
however the GPT GUIDs are always random, making it impossible to
hybridise reproducibly with the --uefi option.

Switch from v4 to v5 UUIDs when --id is set, computing them from the MBR
ID via arbitrary fixed namespaces + SHA1 hashing.

Signed-off-by: Chris Riches <chris.riches at nutanix.com>

---
 ?utils/isohybrid.c | 28 +++++++++++++++++++++++++---
 ?1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/utils/isohybrid.c b/utils/isohybrid.c
index 1a20321..54a9545 100644
--- a/utils/isohybrid.c
+++ b/utils/isohybrid.c
@@ -48,6 +48,18 @@ unsigned int padding = 0;

 ?uuid_t disk_uuid, part_uuid, iso_uuid;

+UUID_DEFINE(disk_uuid_ns,
+??? 0x13, 0x40, 0x46, 0xef, 0x45, 0x39, 0x4c, 0xd6,
+??? 0x83, 0x23, 0x27, 0xa9, 0x23, 0x77, 0x78, 0x68);
+
+UUID_DEFINE(part_uuid_ns,
+??? 0x24, 0x08, 0x39, 0x02, 0x5d, 0x03, 0x42, 0x49,
+??? 0x83, 0xb1, 0x99, 0x76, 0xfb, 0xe7, 0x38, 0x5c);
+
+UUID_DEFINE(iso_uuid_ns,
+??? 0x50, 0x3a, 0xbc, 0xed, 0xc7, 0xa4, 0x40, 0x1c,
+??? 0xa1, 0xe3, 0x2c, 0xf6, 0x85, 0x3f, 0xeb, 0x81);
+
 ?uint8_t mode = 0;
 ?enum { VERBOSE = 1 , EFI = 2 , MAC = 4};

@@ -775,6 +787,16 @@ uint32_t chksum_crc32 (unsigned char *block, unsigned int
length)
 ??????? return (crc ^ 0xFFFFFFFF);
 ?}

+void
+gen_uuid(uuid_t out, const uuid_t ns)
+{
+??? if (id) {
+??????? uuid_generate_sha1(out, ns, (char*)&id, sizeof(id));
+??? } else {
+??????? uuid_generate(out);
+??? }
+}
+
 ?void
 ?reverse_uuid(uuid_t uuid)
 ?{
@@ -815,7 +837,7 @@ initialise_gpt(uint8_t *gpt, uint32_t current, uint32_t
alternate, int primary)
 ???? }

 ???? if (primary) {
-?????? uuid_generate(disk_uuid);
+?????? gen_uuid(disk_uuid, disk_uuid_ns);
 ??????? reverse_uuid(disk_uuid);
 ???? }

@@ -842,8 +864,8 @@ initialise_gpt(uint8_t *gpt, uint32_t current, uint32_t
alternate, int primary)

 ???? part = (struct gpt_part_header *)gpt;
 ???? if (primary) {
-?????? uuid_generate(part_uuid);
-?????? uuid_generate(iso_uuid);
+?????? gen_uuid(part_uuid, part_uuid_ns);
+?????? gen_uuid(iso_uuid, iso_uuid_ns);
 ??????? reverse_uuid(part_uuid);
 ??????? reverse_uuid(iso_uuid);
 ???? }
-- 
2.39.3