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

To: Eric Sandeen <sandeen@xxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, "F. Stoyan" <fstoyan@xxxxxxxxx>
Subject: [PATCH V2] xfs_db: disallow sb UUID write on v5 filesystems
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Tue, 07 Apr 2015 08:58:06 -0500
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5522A0DB.2010309@xxxxxxxxxx>
References: <5522A0DB.2010309@xxxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>

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")) {
                } else if (!strcasecmp(argv[1], "nil")) {

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