mm/shmem: fix chattr fsflags support in tmpfs

ext[234] have always allowed unimplemented chattr flags to be set, but
other filesystems have tended to be stricter.  Follow the stricter
approach for tmpfs: I don't want to have to explain why csu attributes
don't actually work, and we won't need to update the chattr(1) manpage;
and it's never wrong to start off strict, relaxing later if persuaded. 
Allow only a (append only) i (immutable) A (no atime) and d (no dump).

Although lsattr showed 'A' inherited, the NOATIME behavior was not being
inherited: because nothing sync'ed FS_NOATIME_FL to S_NOATIME.  Add
shmem_set_inode_flags() to sync the flags, using inode_set_flags() to
avoid that instant of lost immutablility during fileattr_set().

But that change switched generic/079 from passing to failing: because
FS_IMMUTABLE_FL and FS_APPEND_FL had been unconventionally included in the
INHERITED fsflags: remove them and generic/079 is back to passing.

Link: https://lkml.kernel.org/r/2961dcb0-ddf3-b9f0-3268-12a4ff996856@google.com
Fixes: e408e695 ("mm/shmem: support FS_IOC_[SG]ETFLAGS in tmpfs")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Radoslaw Burny <rburny@google.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/shmem_fs.h

@@ -29,15 +29,10 @@ struct shmem_inode_info {
 	struct inode		vfs_inode;
 };
 
-#define SHMEM_FL_USER_VISIBLE FS_FL_USER_VISIBLE
-#define SHMEM_FL_USER_MODIFIABLE FS_FL_USER_MODIFIABLE
-#define SHMEM_FL_INHERITED FS_FL_USER_MODIFIABLE
-
-/* Flags that are appropriate for regular files (all but dir-specific ones). */
-#define SHMEM_REG_FLMASK (~(FS_DIRSYNC_FL | FS_TOPDIR_FL))
-
-/* Flags that are appropriate for non-directories/regular files. */
-#define SHMEM_OTHER_FLMASK (FS_NODUMP_FL | FS_NOATIME_FL)
+#define SHMEM_FL_USER_VISIBLE		FS_FL_USER_VISIBLE
+#define SHMEM_FL_USER_MODIFIABLE \
+	(FS_IMMUTABLE_FL | FS_APPEND_FL | FS_NODUMP_FL | FS_NOATIME_FL)
+#define SHMEM_FL_INHERITED		(FS_NODUMP_FL | FS_NOATIME_FL)
 
 struct shmem_sb_info {
 	unsigned long max_blocks;   /* How many blocks are allowed */

mm/shmem.c

@@ -2281,16 +2281,34 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
-/* Mask out flags that are inappropriate for the given type of inode. */
-static unsigned shmem_mask_flags(umode_t mode, __u32 flags)
+#ifdef CONFIG_TMPFS_XATTR
+static int shmem_initxattrs(struct inode *, const struct xattr *, void *);
+
+/*
+ * chattr's fsflags are unrelated to extended attributes,
+ * but tmpfs has chosen to enable them under the same config option.
+ */
+static void shmem_set_inode_flags(struct inode *inode, unsigned int fsflags)
+{
+	unsigned int i_flags = 0;
+
+	if (fsflags & FS_NOATIME_FL)
+		i_flags |= S_NOATIME;
+	if (fsflags & FS_APPEND_FL)
+		i_flags |= S_APPEND;
+	if (fsflags & FS_IMMUTABLE_FL)
+		i_flags |= S_IMMUTABLE;
+	/*
+	 * But FS_NODUMP_FL does not require any action in i_flags.
+	 */
+	inode_set_flags(inode, i_flags, S_NOATIME | S_APPEND | S_IMMUTABLE);
+}
+#else
+static void shmem_set_inode_flags(struct inode *inode, unsigned int fsflags)
 {
-	if (S_ISDIR(mode))
-		return flags;
-	else if (S_ISREG(mode))
-		return flags & SHMEM_REG_FLMASK;
-	else
-		return flags & SHMEM_OTHER_FLMASK;
 }
+#define shmem_initxattrs NULL
+#endif
 
 static struct inode *shmem_get_inode(struct super_block *sb, struct inode *dir,
 				     umode_t mode, dev_t dev, unsigned long flags)
@@ -2319,7 +2337,8 @@ static struct inode *shmem_get_inode(struct super_block *sb, struct inode *dir,
 		info->i_crtime = inode->i_mtime;
 		info->fsflags = (dir == NULL) ? 0 :
 			SHMEM_I(dir)->fsflags & SHMEM_FL_INHERITED;
-		info->fsflags = shmem_mask_flags(mode, info->fsflags);
+		if (info->fsflags)
+			shmem_set_inode_flags(inode, info->fsflags);
 		INIT_LIST_HEAD(&info->shrinklist);
 		INIT_LIST_HEAD(&info->swaplist);
 		simple_xattrs_init(&info->xattrs);
@@ -2468,12 +2487,6 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 static const struct inode_operations shmem_symlink_inode_operations;
 static const struct inode_operations shmem_short_symlink_operations;
 
-#ifdef CONFIG_TMPFS_XATTR
-static int shmem_initxattrs(struct inode *, const struct xattr *, void *);
-#else
-#define shmem_initxattrs NULL
-#endif
-
 static int
 shmem_write_begin(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len,
@@ -3179,18 +3192,13 @@ static int shmem_fileattr_set(struct user_namespace *mnt_userns,
 
 	if (fileattr_has_fsx(fa))
 		return -EOPNOTSUPP;
+	if (fa->flags & ~SHMEM_FL_USER_MODIFIABLE)
+		return -EOPNOTSUPP;
 
 	info->fsflags = (info->fsflags & ~SHMEM_FL_USER_MODIFIABLE) |
 		(fa->flags & SHMEM_FL_USER_MODIFIABLE);
 
-	inode->i_flags &= ~(S_APPEND | S_IMMUTABLE | S_NOATIME);
-	if (info->fsflags & FS_APPEND_FL)
-		inode->i_flags |= S_APPEND;
-	if (info->fsflags & FS_IMMUTABLE_FL)
-		inode->i_flags |= S_IMMUTABLE;
-	if (info->fsflags & FS_NOATIME_FL)
-		inode->i_flags |= S_NOATIME;
-
+	shmem_set_inode_flags(inode, info->fsflags);
 	inode->i_ctime = current_time(inode);
 	return 0;
 }