ovl: constant st_ino for non-samefs with xino

On 64bit systems, when overlay layers are not all on the same fs, but
all inode numbers of underlying fs are not using the high bits, use the
high bits to partition the overlay st_ino address space.  The high bits
hold the fsid (upper fsid is 0).  This way overlay inode numbers are unique
and all inodes use overlay st_dev.  Inode numbers are also persistent
for a given layer configuration.

Currently, our only indication for available high ino bits is from a
filesystem that supports file handles and uses the default encode_fh()
operation, which encodes a 32bit inode number.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 22 insertions, 4 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7d97d30cad39..d7284444f404 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -17,6 +17,7 @@
 #include <linux/statfs.h>
 #include <linux/seq_file.h>
 #include <linux/posix_acl_xattr.h>
+#include <linux/exportfs.h>
 #include "overlayfs.h"
 
 MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
@@ -701,6 +702,7 @@ static int ovl_check_namelen(struct path *path, struct ovl_fs *ofs,
 static int ovl_lower_dir(const char *name, struct path *path,
 			 struct ovl_fs *ofs, int *stack_depth, bool *remote)
 {
+	int fh_type;
 	int err;
 
 	err = ovl_mount_dir_noesc(name, path);
@@ -720,15 +722,19 @@ static int ovl_lower_dir(const char *name, struct path *path,
 	 * The inodes index feature and NFS export need to encode and decode
 	 * file handles, so they require that all layers support them.
 	 */
+	fh_type = ovl_can_decode_fh(path->dentry->d_sb);
 	if ((ofs->config.nfs_export ||
-	     (ofs->config.index && ofs->config.upperdir)) &&
-	    !ovl_can_decode_fh(path->dentry->d_sb)) {
+	     (ofs->config.index && ofs->config.upperdir)) && !fh_type) {
 		ofs->config.index = false;
 		ofs->config.nfs_export = false;
 		pr_warn("overlayfs: fs on '%s' does not support file handles, falling back to index=off,nfs_export=off.\n",
 			name);
 	}
 
+	/* Check if lower fs has 32bit inode numbers */
+	if (fh_type != FILEID_INO32_GEN)
+		ofs->xino_bits = 0;
+
 	return 0;
 
 out_put:
@@ -952,6 +958,7 @@ static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
 {
 	struct vfsmount *mnt = ofs->upper_mnt;
 	struct dentry *temp;
+	int fh_type;
 	int err;
 
 	err = mnt_want_write(mnt);
@@ -1001,12 +1008,16 @@ static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
 	}
 
 	/* Check if upper/work fs supports file handles */
-	if (ofs->config.index &&
-	    !ovl_can_decode_fh(ofs->workdir->d_sb)) {
+	fh_type = ovl_can_decode_fh(ofs->workdir->d_sb);
+	if (ofs->config.index && !fh_type) {
 		ofs->config.index = false;
 		pr_warn("overlayfs: upper fs does not support file handles, falling back to index=off.\n");
 	}
 
+	/* Check if upper fs has 32bit inode numbers */
+	if (fh_type != FILEID_INO32_GEN)
+		ofs->xino_bits = 0;
+
 	/* NFS export of r/w mount depends on index */
 	if (ofs->config.nfs_export && !ofs->config.index) {
 		pr_warn("overlayfs: NFS export requires \"index=on\", falling back to nfs_export=off.\n");
@@ -1185,6 +1196,11 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 		}
 		ofs->numlower++;
 	}
+
+	/* When all layers on same fs, overlay can use real inode numbers */
+	if (!ofs->numlowerfs || (ofs->numlowerfs == 1 && !ofs->upper_mnt))
+		ofs->xino_bits = 0;
+
 	err = 0;
 out:
 	return err;
@@ -1308,6 +1324,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	sb->s_stack_depth = 0;
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
+	/* Assume underlaying fs uses 32bit inodes unless proven otherwise */
+	ofs->xino_bits = BITS_PER_LONG - 32;
 	if (ofs->config.upperdir) {
 		if (!ofs->config.workdir) {
 			pr_err("overlayfs: missing 'workdir'\n");