ovl: prepare for lazy lookup of lowerdata inode

Make the code handle the case of numlower > 1 and missing lowerdata
dentry gracefully.

Missing lowerdata dentry is an indication for lazy lookup of lowerdata
and in that case the lowerdata_redirect path is stored in ovl_inode.

Following commits will defer lookup and perform the lazy lookup on
access.
Reviewed-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -115,6 +115,9 @@ static int ovl_real_fdget_meta(const struct file *file, struct fd *real,
 		ovl_path_real(dentry, &realpath);
 	else
 		ovl_path_realdata(dentry, &realpath);
+	/* TODO: lazy lookup of lowerdata */
+	if (!realpath.dentry)
+		return -EIO;
 
 	/* Has it been copied up since we'd opened it? */
 	if (unlikely(file_inode(real->file) != d_inode(realpath.dentry))) {
@@ -158,6 +161,10 @@ static int ovl_open(struct inode *inode, struct file *file)
 	file->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
 
 	ovl_path_realdata(dentry, &realpath);
+	/* TODO: lazy lookup of lowerdata */
+	if (!realpath.dentry)
+		return -EIO;
+
 	realfile = ovl_open_realfile(file, &realpath);
 	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);

fs/overlayfs/inode.c

@@ -240,15 +240,22 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
 			/*
 			 * If lower is not same as lowerdata or if there was
 			 * no origin on upper, we can end up here.
+			 * With lazy lowerdata lookup, guess lowerdata blocks
+			 * from size to avoid lowerdata lookup on stat(2).
 			 */
 			struct kstat lowerdatastat;
 			u32 lowermask = STATX_BLOCKS;
 
 			ovl_path_lowerdata(dentry, &realpath);
-			err = vfs_getattr(&realpath, &lowerdatastat,
-					  lowermask, flags);
-			if (err)
-				goto out;
+			if (realpath.dentry) {
+				err = vfs_getattr(&realpath, &lowerdatastat,
+						  lowermask, flags);
+				if (err)
+					goto out;
+			} else {
+				lowerdatastat.blocks =
+					round_up(stat->size, stat->blksize) >> 9;
+			}
 			stat->blocks = lowerdatastat.blocks;
 		}
 	}
@@ -709,6 +716,9 @@ static int ovl_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	struct inode *realinode = ovl_inode_realdata(inode);
 	const struct cred *old_cred;
 
+	if (!realinode)
+		return -EIO;
+
 	if (!realinode->i_op->fiemap)
 		return -EOPNOTSUPP;
 

fs/overlayfs/super.c

@@ -81,6 +81,14 @@ static struct dentry *ovl_d_real(struct dentry *dentry,
 	if (real && !inode && ovl_has_upperdata(d_inode(dentry)))
 		return real;
 
+	/*
+	 * XXX: We may need lazy lookup of lowerdata for !inode case to return
+	 * the real lowerdata dentry.  The only current caller of d_real() with
+	 * NULL inode is d_real_inode() from trace_uprobe and this caller is
+	 * likely going to be followed reading from the file, before placing
+	 * uprobes on offset within the file, so lowerdata should be available
+	 * when setting the uprobe.
+	 */
 	lower = ovl_dentry_lowerdata(dentry);
 	if (!lower)
 		goto bug;
@@ -103,6 +111,9 @@ static int ovl_revalidate_real(struct dentry *d, unsigned int flags, bool weak)
 {
 	int ret = 1;
 
+	if (!d)
+		return 1;
+
 	if (weak) {
 		if (d->d_flags & DCACHE_OP_WEAK_REVALIDATE)
 			ret =  d->d_op->d_weak_revalidate(d, flags);

fs/overlayfs/util.c

@@ -159,7 +159,7 @@ void ovl_dentry_init_flags(struct dentry *dentry, struct dentry *upperdentry,
 
 	if (upperdentry)
 		flags |= upperdentry->d_flags;
-	for (i = 0; i < ovl_numlower(oe); i++)
+	for (i = 0; i < ovl_numlower(oe) && lowerstack[i].dentry; i++)
 		flags |= lowerstack[i].dentry->d_flags;
 
 	spin_lock(&dentry->d_lock);