ovl: lookup non-dir copy-up-origin by file handle

If overlay.origin xattr is found on a non-dir upper inode try to get lower
dentry by calling exportfs_decode_fh().

On failure to lookup by file handle to lower layer, do not lookup the copy
up origin by name, because the lower found by name could be another file in
case the upper file was renamed.

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

fs/overlayfs/namei.c

@@ -12,6 +12,8 @@
 #include <linux/namei.h>
 #include <linux/xattr.h>
 #include <linux/ratelimit.h>
+#include <linux/mount.h>
+#include <linux/exportfs.h>
 #include "overlayfs.h"
 #include "ovl_entry.h"
 
@@ -81,6 +83,90 @@ invalid:
 	goto err_free;
 }
 
+static int ovl_acceptable(void *ctx, struct dentry *dentry)
+{
+	return 1;
+}
+
+static struct dentry *ovl_get_origin(struct dentry *dentry,
+				     struct vfsmount *mnt)
+{
+	int res;
+	struct ovl_fh *fh = NULL;
+	struct dentry *origin = NULL;
+	int bytes;
+
+	res = vfs_getxattr(dentry, OVL_XATTR_ORIGIN, NULL, 0);
+	if (res < 0) {
+		if (res == -ENODATA || res == -EOPNOTSUPP)
+			return NULL;
+		goto fail;
+	}
+	/* Zero size value means "copied up but origin unknown" */
+	if (res == 0)
+		return NULL;
+
+	fh  = kzalloc(res, GFP_TEMPORARY);
+	if (!fh)
+		return ERR_PTR(-ENOMEM);
+
+	res = vfs_getxattr(dentry, OVL_XATTR_ORIGIN, fh, res);
+	if (res < 0)
+		goto fail;
+
+	if (res < sizeof(struct ovl_fh) || res < fh->len)
+		goto invalid;
+
+	if (fh->magic != OVL_FH_MAGIC)
+		goto invalid;
+
+	/* Treat larger version and unknown flags as "origin unknown" */
+	if (fh->version > OVL_FH_VERSION || fh->flags & ~OVL_FH_FLAG_ALL)
+		goto out;
+
+	/* Treat endianness mismatch as "origin unknown" */
+	if (!(fh->flags & OVL_FH_FLAG_ANY_ENDIAN) &&
+	    (fh->flags & OVL_FH_FLAG_BIG_ENDIAN) != OVL_FH_FLAG_CPU_ENDIAN)
+		goto out;
+
+	bytes = (fh->len - offsetof(struct ovl_fh, fid));
+
+	/*
+	 * Make sure that the stored uuid matches the uuid of the lower
+	 * layer where file handle will be decoded.
+	 */
+	if (uuid_be_cmp(fh->uuid, *(uuid_be *) &mnt->mnt_sb->s_uuid))
+		goto out;
+
+	origin = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
+				    bytes >> 2, (int)fh->type,
+				    ovl_acceptable, NULL);
+	if (IS_ERR(origin)) {
+		/* Treat stale file handle as "origin unknown" */
+		if (origin == ERR_PTR(-ESTALE))
+			origin = NULL;
+		goto out;
+	}
+
+	if (ovl_dentry_weird(origin) ||
+	    ((d_inode(origin)->i_mode ^ d_inode(dentry)->i_mode) & S_IFMT)) {
+		dput(origin);
+		origin = NULL;
+		goto invalid;
+	}
+
+out:
+	kfree(fh);
+	return origin;
+
+fail:
+	pr_warn_ratelimited("overlayfs: failed to get origin (%i)\n", res);
+	goto out;
+invalid:
+	pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n", res, fh);
+	goto out;
+}
+
 static bool ovl_is_opaquedir(struct dentry *dentry)
 {
 	int res;
@@ -192,6 +278,45 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 	return 0;
 }
 
+
+static int ovl_check_origin(struct dentry *dentry, struct dentry *upperdentry,
+			    struct path **stackp, unsigned int *ctrp)
+{
+	struct super_block *same_sb = ovl_same_sb(dentry->d_sb);
+	struct ovl_entry *roe = dentry->d_sb->s_root->d_fsdata;
+	struct vfsmount *mnt;
+	struct dentry *origin;
+
+	if (!same_sb || !roe->numlower)
+		return 0;
+
+       /*
+	* Since all layers are on the same fs, we use the first layer for
+	* decoding the file handle.  We may get a disconnected dentry,
+	* which is fine, because we only need to hold the origin inode in
+	* cache and use its inode number.  We may even get a connected dentry,
+	* that is not under the first layer's root.  That is also fine for
+	* using it's inode number - it's the same as if we held a reference
+	* to a dentry in first layer that was moved under us.
+	*/
+	mnt = roe->lowerstack[0].mnt;
+
+	origin = ovl_get_origin(upperdentry, mnt);
+	if (IS_ERR_OR_NULL(origin))
+		return PTR_ERR(origin);
+
+	BUG_ON(*stackp || *ctrp);
+	*stackp = kmalloc(sizeof(struct path), GFP_TEMPORARY);
+	if (!*stackp) {
+		dput(origin);
+		return -ENOMEM;
+	}
+	**stackp = (struct path) { .dentry = origin, .mnt = mnt };
+	*ctrp = 1;
+
+	return 0;
+}
+
 /*
  * Returns next layer in stack starting from top.
  * Returns -1 if this is the last layer.
@@ -254,6 +379,13 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			err = -EREMOTE;
 			goto out;
 		}
+		if (upperdentry && !d.is_dir) {
+			BUG_ON(!d.stop || d.redirect);
+			err = ovl_check_origin(dentry, upperdentry,
+					       &stack, &ctr);
+			if (err)
+				goto out;
+		}
 
 		if (d.redirect) {
 			upperredirect = kstrdup(d.redirect, GFP_KERNEL);