[PATCH v2 09/10] ntfs: persist append-only in the $LXFLAGS EA

[Baolin Liu]

From: Baolin Liu <liubaolin@xxxxxxxxxx>

Extend the persistent fileattr interface to append-only using bit 1
of LXFLAGS. Restore S_APPEND on inode load and update it after successful
EA writes. Include append in the known-bit mask while retaining all other
unknown bits. Reuse the existing VFS capability checks and EA safeguards.

Document append-only alongside immutable. Both flags now survive inode
reclaim and remount without changing the NTFS attribute format.

Signed-off-by: Baolin Liu <liubaolin@xxxxxxxxxx>
---
 Documentation/filesystems/ntfs.rst | 10 +++++-----
 fs/ntfs/ea.c                       |  2 ++
 fs/ntfs/ea.h                       |  4 +++-
 fs/ntfs/file.c                     | 13 ++++++-------
 4 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/ntfs.rst b/Documentation/filesystems/ntfs.rst
index ced164216a5b..f00b618a3911 100644
--- a/Documentation/filesystems/ntfs.rst
+++ b/Documentation/filesystems/ntfs.rst
@@ -38,21 +38,21 @@ The project is available at:
 Linux file attributes
 =====================
 
-The driver supports lsattr(1) and setting immutable with chattr(1).
-Immutable is stored in bit 0 of a private ``$LXFLAGS`` extended attribute
+The driver supports lsattr(1) and setting immutable and append-only with chattr(1).
+Immutable is stored in bit 0 and append-only in bit 1 of a private ``$LXFLAGS`` extended attribute
 within the standard NTFS ``$EA``/``$EA_INFORMATION`` attributes. Its value
 is a 32-bit little-endian bitmask. Unknown bits are preserved; the entry
-is removed when the entire value becomes zero. The setting survives
+is removed when the entire value becomes zero. These settings survive
 inode eviction, unmount and reboot. Direct writes or removal through
 xattr interfaces are rejected; use chattr(1) instead.
 
 System metadata files and files protected by ``sys_immutable`` cannot
 have their immutable protection cleared. Protection derived solely from
-that policy is not stored in the EA. Compression, encryption, append-only
+that policy is not stored in the EA. Compression, encryption
 and mount-wide case folding are reported but cannot be changed through
 chattr(1).
 
-Windows does not enforce the Linux immutable flag stored in this EA.
+Windows does not enforce the Linux immutable and append-only flags stored in this EA.
 
 Supported mount options
 =======================
diff --git a/fs/ntfs/ea.c b/fs/ntfs/ea.c
index 1e9b66e88655..ddac912de78c 100644
--- a/fs/ntfs/ea.c
+++ b/fs/ntfs/ea.c
@@ -500,6 +500,8 @@ int ntfs_ea_get_lxflags(struct inode *inode)
 	ni->lxflags = le32_to_cpu(value);
 	if (ni->lxflags & NTFS_LXFLAGS_IMMUTABLE)
 		inode->i_flags |= S_IMMUTABLE;
+	if (ni->lxflags & NTFS_LXFLAGS_APPEND)
+		inode->i_flags |= S_APPEND;
 	return 0;
 }
 
diff --git a/fs/ntfs/ea.h b/fs/ntfs/ea.h
index 19aea162b909..eac5ab4da44a 100644
--- a/fs/ntfs/ea.h
+++ b/fs/ntfs/ea.h
@@ -9,7 +9,9 @@
 
 /* $LXFLAGS stores these bits in a single little-endian 32-bit value. */
 #define NTFS_LXFLAGS_IMMUTABLE	BIT(0)
-#define NTFS_LXFLAGS_MASK	NTFS_LXFLAGS_IMMUTABLE
+#define NTFS_LXFLAGS_APPEND	BIT(1)
+#define NTFS_LXFLAGS_MASK	(NTFS_LXFLAGS_IMMUTABLE | \
+				 NTFS_LXFLAGS_APPEND)
 
 extern const struct xattr_handler *const ntfs_xattr_handlers[];
 
diff --git a/fs/ntfs/file.c b/fs/ntfs/file.c
index 882d33b67560..5b2a31790275 100644
--- a/fs/ntfs/file.c
+++ b/fs/ntfs/file.c
@@ -190,7 +190,7 @@ int ntfs_fileattr_set(struct mnt_idmap *idmap, struct dentry *dentry,
 {
 	struct inode *vi = d_inode(dentry);
 	struct ntfs_inode *ni = NTFS_I(vi);
-	u32 allowed = FS_IMMUTABLE_FL;
+	u32 allowed = FS_IMMUTABLE_FL | FS_APPEND_FL;
 	u32 readonly = 0;
 	u32 lxflags = ni->lxflags & ~NTFS_LXFLAGS_MASK;
 	unsigned int new_fl = 0;
@@ -203,11 +203,6 @@ int ntfs_fileattr_set(struct mnt_idmap *idmap, struct dentry *dentry,
 	if (fileattr_has_fsx(fa))
 		return -EOPNOTSUPP;
 
-	if ((fa->flags & FS_APPEND_FL) !=
-	    (IS_APPEND(vi) ? FS_APPEND_FL : 0))
-		return -EOPNOTSUPP;
-	allowed |= fa->flags & FS_APPEND_FL;
-
 	/* chattr passes the unchanged read-only flags back to us too. */
 	if (NInoCompressed(ni) || NInoWofCompressed(ni))
 		readonly |= FS_COMPR_FL;
@@ -234,6 +229,10 @@ int ntfs_fileattr_set(struct mnt_idmap *idmap, struct dentry *dentry,
 		    (ni->lxflags & NTFS_LXFLAGS_IMMUTABLE))
 			lxflags |= NTFS_LXFLAGS_IMMUTABLE;
 	}
+	if (fa->flags & FS_APPEND_FL) {
+		new_fl |= S_APPEND;
+		lxflags |= NTFS_LXFLAGS_APPEND;
+	}
 
 	mutex_lock(&ni->mrec_lock);
 	err = ntfs_ea_set_lxflags(vi, lxflags);
@@ -241,7 +240,7 @@ int ntfs_fileattr_set(struct mnt_idmap *idmap, struct dentry *dentry,
 	if (err)
 		return err;
 
-	inode_set_flags(vi, new_fl, S_IMMUTABLE);
+	inode_set_flags(vi, new_fl, S_IMMUTABLE | S_APPEND);
 	inode_set_ctime_current(vi);
 	mark_inode_dirty(vi);
 	return 0;
-- 
2.51.0