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[PATCH 3/3] xfs: sanitise sb_bad_features2 handling

To: xfs@xxxxxxxxxxx
Subject: [PATCH 3/3] xfs: sanitise sb_bad_features2 handling
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 8 Jan 2015 08:51:05 +1100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1420667465-7453-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1420667465-7453-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

We currently have to ensure that every time we update sb_features2
that we update sb_bad_features2. Now that we log and format the
superblock in it's entirety we actually don't have to care because
we can simply update the sb_bad_features2 when we format it into the
buffer. This removes the need for anything but the mount and
superblock formatting code to care about sb_bad_features2, and
hence removes the possibility that we forget to update bad_features2
when necessary in the future.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/libxfs/xfs_format.h | 14 +++++++-------
 fs/xfs/libxfs/xfs_sb.c     |  8 +++++++-
 fs/xfs/xfs_mount.c         | 23 +++++++++++------------
 3 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 4762732..8eb7189 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -151,10 +151,13 @@ typedef struct xfs_sb {
        __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.
+        * 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.
+        *
+        * This is not kept up to date in memory; it is always overwritten by
+        * the value in sb_features2 when formatting the incore superblock to
+        * the disk buffer.
         */
        __uint32_t      sb_bad_features2;
 
@@ -453,13 +456,11 @@ static inline void xfs_sb_version_addattr2(struct xfs_sb 
*sbp)
 {
        sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
        sbp->sb_features2 |= XFS_SB_VERSION2_ATTR2BIT;
-       sbp->sb_bad_features2 |= XFS_SB_VERSION2_ATTR2BIT;
 }
 
 static inline void xfs_sb_version_removeattr2(struct xfs_sb *sbp)
 {
        sbp->sb_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
-       sbp->sb_bad_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
        if (!sbp->sb_features2)
                sbp->sb_versionnum &= ~XFS_SB_VERSION_MOREBITSBIT;
 }
@@ -475,7 +476,6 @@ static inline void xfs_sb_version_addprojid32bit(struct 
xfs_sb *sbp)
 {
        sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
        sbp->sb_features2 |= XFS_SB_VERSION2_PROJID32BIT;
-       sbp->sb_bad_features2 |= XFS_SB_VERSION2_PROJID32BIT;
 }
 
 /*
diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
index 6ee3af6..b440839 100644
--- a/fs/xfs/libxfs/xfs_sb.c
+++ b/fs/xfs/libxfs/xfs_sb.c
@@ -488,7 +488,6 @@ xfs_sb_to_disk(
        to->sb_fdblocks = cpu_to_be64(from->sb_fdblocks);
        to->sb_frextents = cpu_to_be64(from->sb_frextents);
 
-
        to->sb_flags = from->sb_flags;
        to->sb_shared_vn = from->sb_shared_vn;
        to->sb_inoalignmt = cpu_to_be32(from->sb_inoalignmt);
@@ -498,6 +497,13 @@ xfs_sb_to_disk(
        to->sb_logsectlog = from->sb_logsectlog;
        to->sb_logsectsize = cpu_to_be16(from->sb_logsectsize);
        to->sb_logsunit = cpu_to_be32(from->sb_logsunit);
+
+       /*
+        * We need to ensure that bad_features2 always matches features2.
+        * Hence we enforce that here rather than having to remember to do it
+        * everywhere else that updates features2.
+        */
+       from->sb_bad_features2 = from->sb_features2;
        to->sb_features2 = cpu_to_be32(from->sb_features2);
        to->sb_bad_features2 = cpu_to_be32(from->sb_bad_features2);
 
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 5ef9aa2..4fa80e6 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -640,25 +640,24 @@ xfs_mountfs(
        xfs_sb_mount_common(mp, sbp);
 
        /*
-        * Check for a mismatched features2 values.  Older kernels
-        * read & wrote into the wrong sb offset for sb_features2
-        * on some platforms due to xfs_sb_t not being 64bit size aligned
-        * when sb_features2 was added, which made older superblock
-        * reading/writing routines swap it as a 64-bit value.
+        * Check for a mismatched features2 values.  Older kernels read & wrote
+        * into the wrong sb offset for sb_features2 on some platforms due to
+        * xfs_sb_t not being 64bit size aligned when sb_features2 was added,
+        * which made older superblock reading/writing routines swap it as a
+        * 64-bit value.
         *
         * For backwards compatibility, we make both slots equal.
         *
-        * If we detect a mismatched field, we OR the set bits into the
-        * existing features2 field in case it has already been modified; we
-        * don't want to lose any features.  We then update the bad location
-        * with the ORed value so that older kernels will see any features2
-        * flags, and mark the two fields as needing updates once the
-        * transaction subsystem is online.
+        * If we detect a mismatched field, we OR the set bits into the existing
+        * features2 field in case it has already been modified; we don't want
+        * to lose any features.  We then update the bad location with the ORed
+        * value so that older kernels will see any features2 flags. The
+        * superblock writeback code ensures the new sb_features2 is copied to
+        * sb_bad_features2 before it is logged or written to disk.
         */
        if (xfs_sb_has_mismatched_features2(sbp)) {
                xfs_warn(mp, "correcting sb_features alignment problem");
                sbp->sb_features2 |= sbp->sb_bad_features2;
-               sbp->sb_bad_features2 = sbp->sb_features2;
                mp->m_update_sb = true;
 
                /*
-- 
2.0.0

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