ovl: lookup redirects

If a directory has the "trusted.overlay.redirect" xattr, it means that the
value of the xattr should be used to find the underlying directory on the
next lower layer.

The redirect may be relative or absolute.  Absolute redirects begin with a
slash.

A relative redirect means: instead of the current dentry's name use the
value of the redirect to find the directory in the next lower
layer. Relative redirects must not contain a slash.

An absolute redirect means: look up the directory relative to the root of
the overlay using the value of the redirect in the next lower layer.

Redirects work on lower layers as well.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/namei.c

@@ -10,6 +10,7 @@
 #include <linux/fs.h>
 #include <linux/namei.h>
 #include <linux/xattr.h>
+#include <linux/ratelimit.h>
 #include "overlayfs.h"
 #include "ovl_entry.h"
 
@@ -19,8 +20,66 @@ struct ovl_lookup_data {
 	bool opaque;
 	bool stop;
 	bool last;
+	char *redirect;
 };
 
+static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
+			      size_t prelen, const char *post)
+{
+	int res;
+	char *s, *next, *buf = NULL;
+
+	res = vfs_getxattr(dentry, OVL_XATTR_REDIRECT, NULL, 0);
+	if (res < 0) {
+		if (res == -ENODATA || res == -EOPNOTSUPP)
+			return 0;
+		goto fail;
+	}
+	buf = kzalloc(prelen + res + strlen(post) + 1, GFP_TEMPORARY);
+	if (!buf)
+		return -ENOMEM;
+
+	if (res == 0)
+		goto invalid;
+
+	res = vfs_getxattr(dentry, OVL_XATTR_REDIRECT, buf, res);
+	if (res < 0)
+		goto fail;
+	if (res == 0)
+		goto invalid;
+	if (buf[0] == '/') {
+		for (s = buf; *s++ == '/'; s = next) {
+			next = strchrnul(s, '/');
+			if (s == next)
+				goto invalid;
+		}
+	} else {
+		if (strchr(buf, '/') != NULL)
+			goto invalid;
+
+		memmove(buf + prelen, buf, res);
+		memcpy(buf, d->name.name, prelen);
+	}
+
+	strcat(buf, post);
+	kfree(d->redirect);
+	d->redirect = buf;
+	d->name.name = d->redirect;
+	d->name.len = strlen(d->redirect);
+
+	return 0;
+
+err_free:
+	kfree(buf);
+	return 0;
+fail:
+	pr_warn_ratelimited("overlayfs: failed to get redirect (%i)\n", res);
+	goto err_free;
+invalid:
+	pr_warn_ratelimited("overlayfs: invalid redirect (%s)\n", buf);
+	goto err_free;
+}
+
 static bool ovl_is_opaquedir(struct dentry *dentry)
 {
 	int res;
@@ -38,6 +97,7 @@ static bool ovl_is_opaquedir(struct dentry *dentry)
 
 static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 			     const char *name, unsigned int namelen,
+			     size_t prelen, const char *post,
 			     struct dentry **ret)
 {
 	struct dentry *this;
@@ -74,6 +134,9 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 		d->stop = d->opaque = true;
 		goto out;
 	}
+	err = ovl_check_redirect(this, d, prelen, post);
+	if (err)
+		goto out_err;
 out:
 	*ret = this;
 	return 0;
@@ -91,7 +154,32 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 			    struct dentry **ret)
 {
-	return ovl_lookup_single(base, d, d->name.name, d->name.len, ret);
+	const char *s = d->name.name;
+	struct dentry *dentry = NULL;
+	int err;
+
+	if (*s != '/')
+		return ovl_lookup_single(base, d, d->name.name, d->name.len,
+					 0, "", ret);
+
+	while (*s++ == '/' && !IS_ERR_OR_NULL(base) && d_can_lookup(base)) {
+		const char *next = strchrnul(s, '/');
+		size_t slen = strlen(s);
+
+		if (WARN_ON(slen > d->name.len) ||
+		    WARN_ON(strcmp(d->name.name + d->name.len - slen, s)))
+			return -EIO;
+
+		err = ovl_lookup_single(base, d, s, next - s,
+					d->name.len - slen, next, &base);
+		dput(dentry);
+		if (err)
+			return err;
+		dentry = base;
+		s = next;
+	}
+	*ret = dentry;
+	return 0;
 }
 
 /*
@@ -127,6 +215,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	unsigned int ctr = 0;
 	struct inode *inode = NULL;
 	bool upperopaque = false;
+	char *upperredirect = NULL;
 	struct dentry *this;
 	unsigned int i;
 	int err;
@@ -136,6 +225,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		.opaque = false,
 		.stop = false,
 		.last = !poe->numlower,
+		.redirect = NULL,
 	};
 
 	if (dentry->d_name.len > ofs->namelen)
@@ -153,12 +243,20 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			err = -EREMOTE;
 			goto out;
 		}
+
+		if (d.redirect) {
+			upperredirect = kstrdup(d.redirect, GFP_KERNEL);
+			if (!upperredirect)
+				goto out_put_upper;
+			if (d.redirect[0] == '/')
+				poe = dentry->d_sb->s_root->d_fsdata;
+		}
 		upperopaque = d.opaque;
 	}
 
 	if (!d.stop && poe->numlower) {
 		err = -ENOMEM;
-		stack = kcalloc(poe->numlower, sizeof(struct path),
+		stack = kcalloc(ofs->numlower, sizeof(struct path),
 				GFP_TEMPORARY);
 		if (!stack)
 			goto out_put_upper;
@@ -178,6 +276,22 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		stack[ctr].dentry = this;
 		stack[ctr].mnt = lowerpath.mnt;
 		ctr++;
+
+		if (d.stop)
+			break;
+
+		if (d.redirect &&
+		    d.redirect[0] == '/' &&
+		    poe != dentry->d_sb->s_root->d_fsdata) {
+			poe = dentry->d_sb->s_root->d_fsdata;
+
+			/* Find the current layer on the root dentry */
+			for (i = 0; i < poe->numlower; i++)
+				if (poe->lowerstack[i].mnt == lowerpath.mnt)
+					break;
+			if (WARN_ON(i == poe->numlower))
+				break;
+		}
 	}
 
 	oe = ovl_alloc_entry(ctr);
@@ -208,9 +322,11 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 
 	revert_creds(old_cred);
 	oe->opaque = upperopaque;
+	oe->redirect = upperredirect;
 	oe->__upperdentry = upperdentry;
 	memcpy(oe->lowerstack, stack, sizeof(struct path) * ctr);
 	kfree(stack);
+	kfree(d.redirect);
 	dentry->d_fsdata = oe;
 	d_add(dentry, inode);
 
@@ -224,7 +340,9 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	kfree(stack);
 out_put_upper:
 	dput(upperdentry);
+	kfree(upperredirect);
 out:
+	kfree(d.redirect);
 	revert_creds(old_cred);
 	return ERR_PTR(err);
 }

fs/overlayfs/overlayfs.h

@@ -19,6 +19,7 @@ enum ovl_path_type {
 
 #define OVL_XATTR_PREFIX XATTR_TRUSTED_PREFIX "overlay."
 #define OVL_XATTR_OPAQUE OVL_XATTR_PREFIX "opaque"
+#define OVL_XATTR_REDIRECT OVL_XATTR_PREFIX "redirect"
 
 #define OVL_ISUPPER_MASK 1UL
 

fs/overlayfs/ovl_entry.h

@@ -35,6 +35,7 @@ struct ovl_entry {
 	union {
 		struct {
 			u64 version;
+			const char *redirect;
 			bool opaque;
 		};
 		struct rcu_head rcu;

fs/overlayfs/super.c

@@ -37,6 +37,7 @@ static void ovl_dentry_release(struct dentry *dentry)
 		unsigned int i;
 
 		dput(oe->__upperdentry);
+		kfree(oe->redirect);
 		for (i = 0; i < oe->numlower; i++)
 			dput(oe->lowerstack[i].dentry);
 		kfree_rcu(oe, rcu);