David Chinner wrote:
> There is a bug in mkfs.xfs that can result in writing the features2
> field in the superblock to the wrong location. This only occurs
> on some architectures, typically those with 32 bit userspace and
> 64 bit kernels.
>
> This patch detects the defect at mount time, logs a warning
> such as:
> ...
> /*
> + * 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;
> + }
>
I think there's a minor problem here that while this will update the
superblock with the proper features2 values, features2 has already been
checked, so mp->m_flags won't have, for example, the attr2 flags...
So attr2 will show up next time, but not on this mount.
This probably wouldn't normally matter, except in a weird corner case
I think I've found:
x86_64 set attr2 in bad_features2
bad_features2 was found, kernel & userspace pad the same
filesystem created attr2 attributes
hch's sb endianness annotation actually made bad_features2 *not* found
mount after that thinks there is no attr2
another corner case bug corrupts the fs, would be avoided if attr2 were not
lost.
-Eric
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