[PATCH V2] xfs_db: disallow sb UUID write on v5 filesystems

[Eric Sandeen]

Do not allow xfs_db (or the xfs_admin frontend) to change the UUID
of a V5 filesystem; this will cause UUID mismatches across the
filesystem, and we currently have no mechanism to update them all.
Changing only the superblock UUID makes all other metadata look
invalid, and xfs_repair reacts by junking everything.

Addresses-Debian-Bug: 782012
Reported-by: F. Stoyan <fstoyan at swapon.de>
Signed-off-by: Eric Sandeen <sandeen at redhat.com>
Reviewed-by: Brian Foster <bfoster at redhat.com>
---

V2, fix up comment whitespace :/
(and add brian's review for dave's convenience)

diff --git a/db/sb.c b/db/sb.c
index 6cb665d..f31f4a2 100644
--- a/db/sb.c
+++ b/db/sb.c
@@ -363,6 +363,18 @@ uuid_f(
 			return 0;
 		}
 
+		/*
+		 * For now, changing the UUID of V5 superblock filesystems is
+		 * not supported; we do not have the infrastructure to fix all
+		 * other metadata when a new superblock UUID is generated.
+		 */
+		if (xfs_sb_version_hascrc(&mp->m_sb) &&
+		    strcasecmp(argv[1], "rewrite")) {
+			dbprintf(_("%s: only 'rewrite' supported on V5 fs\n"),
+				progname);
+			return 0;
+		}
+
 		if (!strcasecmp(argv[1], "generate")) {
 			platform_uuid_generate(&uu);
 		} else if (!strcasecmp(argv[1], "nil")) {

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