[XFS] Fix superblock features2 field alignment problem

Due to the xfs_dsb_t structure not being 64 bit aligned, the last field of
the on-disk superblock can vary in location This causes problems when the
filesystem gets moved to a different platform, or there is a 32 bit
userspace and 64 bit kernel.

This patch detects the defect at mount time, logs a warning such as:

XFS: correcting sb_features alignment problem

in dmesg and corrects the problem so that everything is OK. it also
blacklists the bad field in the superblock so it does not get used for
something else later on.

SGI-PV: 977636
SGI-Modid: xfs-linux-melb:xfs-kern:30539a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

fs/xfs/xfs_mount.c

@@ -44,7 +44,7 @@
 #include "xfs_quota.h"
 #include "xfs_fsops.h"
 
-STATIC void	xfs_mount_log_sbunit(xfs_mount_t *, __int64_t);
+STATIC void	xfs_mount_log_sb(xfs_mount_t *, __int64_t);
 STATIC int	xfs_uuid_mount(xfs_mount_t *);
 STATIC void	xfs_uuid_unmount(xfs_mount_t *mp);
 STATIC void	xfs_unmountfs_wait(xfs_mount_t *);
@@ -119,6 +119,7 @@ static const struct {
     { offsetof(xfs_sb_t, sb_logsectsize),0 },
     { offsetof(xfs_sb_t, sb_logsunit),	 0 },
     { offsetof(xfs_sb_t, sb_features2),	 0 },
+    { offsetof(xfs_sb_t, sb_bad_features2), 0 },
     { sizeof(xfs_sb_t),			 0 }
 };
 
@@ -449,6 +450,7 @@ xfs_sb_from_disk(
 	to->sb_logsectsize = be16_to_cpu(from->sb_logsectsize);
 	to->sb_logsunit = be32_to_cpu(from->sb_logsunit);
 	to->sb_features2 = be32_to_cpu(from->sb_features2);
+	to->sb_bad_features2 = be32_to_cpu(from->sb_bad_features2);
 }
 
 /*
@@ -969,6 +971,26 @@ xfs_mountfs(
 	}
 	xfs_mount_common(mp, sbp);
 
+	/*
+	 * Check for a bad features2 field alignment. This happened on
+	 * some platforms due to xfs_sb_t not being 64bit size aligned
+	 * when sb_features was added and hence the compiler put it in
+	 * the wrong place.
+	 *
+	 * If we detect a bad field, we or the set bits into the existing
+	 * features2 field in case it has already been modified and we
+	 * don't want to lose any features. Zero the bad one and mark
+	 * the two fields as needing updates once the transaction subsystem
+	 * is online.
+	 */
+	if (xfs_sb_has_bad_features2(sbp)) {
+		cmn_err(CE_WARN,
+			"XFS: correcting sb_features alignment problem");
+		sbp->sb_features2 |= sbp->sb_bad_features2;
+		sbp->sb_bad_features2 = 0;
+		update_flags |= XFS_SB_FEATURES2 | XFS_SB_BAD_FEATURES2;
+	}
+
 	/*
 	 * Check if sb_agblocks is aligned at stripe boundary
 	 * If sb_agblocks is NOT aligned turn off m_dalign since
@@ -1159,11 +1181,10 @@ xfs_mountfs(
 	}
 
 	/*
-	 * If fs is not mounted readonly, then update the superblock
-	 * unit and width changes.
+	 * If fs is not mounted readonly, then update the superblock changes.
 	 */
 	if (update_flags && !(mp->m_flags & XFS_MOUNT_RDONLY))
-		xfs_mount_log_sbunit(mp, update_flags);
+		xfs_mount_log_sb(mp, update_flags);
 
 	/*
 	 * Initialise the XFS quota management subsystem for this mount
@@ -1878,13 +1899,14 @@ xfs_uuid_unmount(
  * be altered by the mount options. Only the first superblock is updated.
  */
 STATIC void
-xfs_mount_log_sbunit(
+xfs_mount_log_sb(
 	xfs_mount_t	*mp,
 	__int64_t	fields)
 {
 	xfs_trans_t	*tp;
 
-	ASSERT(fields & (XFS_SB_UNIT|XFS_SB_WIDTH|XFS_SB_UUID));
+	ASSERT(fields & (XFS_SB_UNIT | XFS_SB_WIDTH | XFS_SB_UUID |
+			 XFS_SB_FEATURES2 | XFS_SB_BAD_FEATURES2));
 
 	tp = xfs_trans_alloc(mp, XFS_TRANS_SB_UNIT);
 	if (xfs_trans_reserve(tp, 0, mp->m_sb.sb_sectsize + 128, 0, 0,

fs/xfs/xfs_sb.h

@@ -89,6 +89,7 @@ struct xfs_mount;
 
 /*
  * Superblock - in core version.  Must match the ondisk version below.
+ * Must be padded to 64 bit alignment.
  */
 typedef struct xfs_sb {
 	__uint32_t	sb_magicnum;	/* magic number == XFS_SB_MAGIC */
@@ -145,10 +146,21 @@ typedef struct xfs_sb {
 	__uint16_t	sb_logsectsize;	/* sector size for the log, bytes */
 	__uint32_t	sb_logsunit;	/* stripe unit size for the log */
 	__uint32_t	sb_features2;	/* additional feature bits */
+
+	/*
+	 * bad features2 field as a result of failing to pad the sb
+	 * structure to 64 bits. Some machines will be using this field
+	 * for features2 bits. Easiest just to mark it bad and not use
+	 * it for anything else.
+	 */
+	__uint32_t	sb_bad_features2;
+
+	/* must be padded to 64 bit alignment */
 } xfs_sb_t;
 
 /*
- * Superblock - on disk version.  Must match the in core version below.
+ * Superblock - on disk version.  Must match the in core version above.
+ * Must be padded to 64 bit alignment.
  */
 typedef struct xfs_dsb {
 	__be32		sb_magicnum;	/* magic number == XFS_SB_MAGIC */
@@ -205,6 +217,15 @@ typedef struct xfs_dsb {
 	__be16		sb_logsectsize;	/* sector size for the log, bytes */
 	__be32		sb_logsunit;	/* stripe unit size for the log */
 	__be32		sb_features2;	/* additional feature bits */
+	/*
+	 * bad features2 field as a result of failing to pad the sb
+	 * structure to 64 bits. Some machines will be using this field
+	 * for features2 bits. Easiest just to mark it bad and not use
+	 * it for anything else.
+	 */
+	__be32	sb_bad_features2;
+
+	/* must be padded to 64 bit alignment */
 } xfs_dsb_t;
 
 /*
@@ -223,7 +244,7 @@ typedef enum {
 	XFS_SBS_GQUOTINO, XFS_SBS_QFLAGS, XFS_SBS_FLAGS, XFS_SBS_SHARED_VN,
 	XFS_SBS_INOALIGNMT, XFS_SBS_UNIT, XFS_SBS_WIDTH, XFS_SBS_DIRBLKLOG,
 	XFS_SBS_LOGSECTLOG, XFS_SBS_LOGSECTSIZE, XFS_SBS_LOGSUNIT,
-	XFS_SBS_FEATURES2,
+	XFS_SBS_FEATURES2, XFS_SBS_BAD_FEATURES2,
 	XFS_SBS_FIELDCOUNT
 } xfs_sb_field_t;
 
@@ -248,13 +269,15 @@ typedef enum {
 #define XFS_SB_IFREE		XFS_SB_MVAL(IFREE)
 #define XFS_SB_FDBLOCKS		XFS_SB_MVAL(FDBLOCKS)
 #define XFS_SB_FEATURES2	XFS_SB_MVAL(FEATURES2)
+#define XFS_SB_BAD_FEATURES2	XFS_SB_MVAL(BAD_FEATURES2)
 #define	XFS_SB_NUM_BITS		((int)XFS_SBS_FIELDCOUNT)
 #define	XFS_SB_ALL_BITS		((1LL << XFS_SB_NUM_BITS) - 1)
 #define	XFS_SB_MOD_BITS		\
 	(XFS_SB_UUID | XFS_SB_ROOTINO | XFS_SB_RBMINO | XFS_SB_RSUMINO | \
 	 XFS_SB_VERSIONNUM | XFS_SB_UQUOTINO | XFS_SB_GQUOTINO | \
 	 XFS_SB_QFLAGS | XFS_SB_SHARED_VN | XFS_SB_UNIT | XFS_SB_WIDTH | \
-	 XFS_SB_ICOUNT | XFS_SB_IFREE | XFS_SB_FDBLOCKS | XFS_SB_FEATURES2)
+	 XFS_SB_ICOUNT | XFS_SB_IFREE | XFS_SB_FDBLOCKS | XFS_SB_FEATURES2 | \
+	 XFS_SB_BAD_FEATURES2)
 
 
 /*
@@ -296,6 +319,14 @@ static inline int xfs_sb_good_version(xfs_sb_t *sbp)
 }
 #endif /* __KERNEL__ */
 
+/*
+ * Detect a bad features2 field
+ */
+static inline int xfs_sb_has_bad_features2(xfs_sb_t *sbp)
+{
+	return (sbp->sb_bad_features2 != 0);
+}
+
 static inline unsigned xfs_sb_version_tonew(unsigned v)
 {
 	return ((((v) == XFS_SB_VERSION_1) ? \