[PATCH v2 05/11] ovl: lookup redirect by file handle

[Amir Goldstein <amir73il@gmail.com>]

When overlay.fh xattr is found in a directory inode, instead of lookup
of the dentry in next lower layer by name, first try to get it by calling
exportfs_decode_fh().

On failure to lookup by file handle to lower layer, fall back to lookup
by name with or without path redirect.

For now we only support following by file handle from upper if there is a
single lower layer, because fallback from lookup by file hande to lookup
by path in mid layers is not yet implemented.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/namei.c     | 185 +++++++++++++++++++++++++++++++++++++++++++----
 fs/overlayfs/overlayfs.h |   1 +
 fs/overlayfs/util.c      |  14 ++++
 3 files changed, 186 insertions(+), 14 deletions(-)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index d660177..0d1cc8f 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -9,9 +9,11 @@
 
 #include <linux/fs.h>
 #include <linux/cred.h>
+#include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/xattr.h>
 #include <linux/ratelimit.h>
+#include <linux/exportfs.h>
 #include "overlayfs.h"
 #include "ovl_entry.h"
 
@@ -21,7 +23,10 @@ struct ovl_lookup_data {
 	bool opaque;
 	bool stop;
 	bool last;
-	char *redirect;
+	bool by_path;		/* redirect by path */
+	bool by_fh;		/* redirect by file handle */
+	char *redirect;		/* path to follow */
+	struct ovl_fh *fh;	/* file handle to follow */
 };
 
 static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
@@ -81,6 +86,42 @@ static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
 	goto err_free;
 }
 
+static int ovl_check_redirect_fh(struct dentry *dentry,
+				 struct ovl_lookup_data *d)
+{
+	int res;
+	void *buf = NULL;
+
+	res = vfs_getxattr(dentry, OVL_XATTR_FH, NULL, 0);
+	if (res < 0) {
+		if (res == -ENODATA || res == -EOPNOTSUPP)
+			return 0;
+		goto fail;
+	}
+	buf = kzalloc(res, GFP_TEMPORARY);
+	if (!buf)
+		return -ENOMEM;
+
+	if (res == 0)
+		goto fail;
+
+	res = vfs_getxattr(dentry, OVL_XATTR_FH, buf, res);
+	if (res < 0 || !ovl_redirect_fh_ok(buf, res))
+		goto fail;
+
+	kfree(d->fh);
+	d->fh = buf;
+
+	return 0;
+
+err_free:
+	kfree(buf);
+	return 0;
+fail:
+	pr_warn_ratelimited("overlayfs: failed to get file handle (%i)\n", res);
+	goto err_free;
+}
+
 static bool ovl_is_opaquedir(struct dentry *dentry)
 {
 	int res;
@@ -96,22 +137,81 @@ static bool ovl_is_opaquedir(struct dentry *dentry)
 	return false;
 }
 
+/* Check if p1 is connected with a chain of hashed dentries to p2 */
+static bool ovl_is_lookable(struct dentry *p1, struct dentry *p2)
+{
+	struct dentry *p;
+
+	for (p = p2; !IS_ROOT(p); p = p->d_parent) {
+		if (d_unhashed(p))
+			return false;
+		if (p->d_parent == p1)
+			return true;
+	}
+	return false;
+}
+
+/* Check if dentry is reachable from mnt via path lookup */
+static int ovl_dentry_under_mnt(void *ctx, struct dentry *dentry)
+{
+	struct vfsmount *mnt = ctx;
+
+	return ovl_is_lookable(mnt->mnt_root, dentry);
+}
+
+static struct dentry *ovl_lookup_fh(struct vfsmount *mnt,
+				    const struct ovl_fh *fh)
+{
+	int bytes = (fh->len - offsetof(struct ovl_fh, fid));
+
+	/*
+	 * When redirect_fh is disabled, 'invalid' file handles are stored
+	 * to indicate that this entry has been copied up.
+	 */
+	if (!bytes || (int)fh->type == FILEID_INVALID)
+		return ERR_PTR(-ESTALE);
+
+	/*
+	 * Several layers can be on the same fs and decoded dentry may be in
+	 * either one of those layers. We are looking for a match of dentry
+	 * and mnt to find out to which layer the decoded dentry belongs to.
+	 */
+	return exportfs_decode_fh(mnt, (struct fid *)fh->fid,
+				  bytes >> 2, (int)fh->type,
+				  ovl_dentry_under_mnt, mnt);
+}
+
 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 vfsmount *mnt, struct dentry **ret)
 {
 	struct dentry *this;
 	int err;
 
-	this = lookup_one_len_unlocked(name, base, namelen);
+	/*
+	 * Lookup of upper is with null d->fh.
+	 * Lookup of lower is either by_fh with non-null d->fh
+	 * or by_path with null d->fh.
+	 */
+	if (d->fh)
+		this = ovl_lookup_fh(mnt, d->fh);
+	else
+		this = lookup_one_len_unlocked(name, base, namelen);
 	if (IS_ERR(this)) {
 		err = PTR_ERR(this);
 		this = NULL;
 		if (err == -ENOENT || err == -ENAMETOOLONG)
 			goto out;
+		if (d->fh && err == -ESTALE)
+			goto out;
 		goto out_err;
 	}
+
+	/* If found by file handle - don't follow that handle again */
+	kfree(d->fh);
+	d->fh = NULL;
+
 	if (!this->d_inode)
 		goto put_and_out;
 
@@ -135,9 +235,18 @@ 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;
+	if (d->last)
+		goto out;
+	if (d->by_path) {
+		err = ovl_check_redirect(this, d, prelen, post);
+		if (err)
+			goto out_err;
+	}
+	if (d->by_fh) {
+		err = ovl_check_redirect_fh(this, d);
+		if (err)
+			goto out_err;
+	}
 out:
 	*ret = this;
 	return 0;
@@ -152,6 +261,12 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 	return err;
 }
 
+static int ovl_lookup_layer_fh(struct path *path, struct ovl_lookup_data *d,
+			       struct dentry **ret)
+{
+	return ovl_lookup_single(path->dentry, d, "", 0, 0, "", path->mnt, ret);
+}
+
 static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 			    struct dentry **ret)
 {
@@ -162,7 +277,7 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 
 	if (d->name.name[0] != '/')
 		return ovl_lookup_single(base, d, d->name.name, d->name.len,
-					 0, "", ret);
+					 0, "", NULL, ret);
 
 	while (!IS_ERR_OR_NULL(base) && d_can_lookup(base)) {
 		const char *s = d->name.name + d->name.len - rem;
@@ -175,7 +290,7 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 			return -EIO;
 
 		err = ovl_lookup_single(base, d, s, thislen,
-					d->name.len - rem, next, &base);
+					d->name.len - rem, next, NULL, &base);
 		dput(dentry);
 		if (err)
 			return err;
@@ -220,6 +335,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	const struct cred *old_cred;
 	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
 	struct ovl_entry *poe = dentry->d_parent->d_fsdata;
+	struct ovl_entry *roe = dentry->d_sb->s_root->d_fsdata;
 	struct path *stack = NULL;
 	struct dentry *upperdir, *upperdentry = NULL;
 	unsigned int ctr = 0;
@@ -235,7 +351,10 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		.opaque = false,
 		.stop = false,
 		.last = !poe->numlower,
+		.by_path = true,
 		.redirect = NULL,
+		.by_fh = true,
+		.fh = NULL,
 	};
 
 	if (dentry->d_name.len > ofs->namelen)
@@ -259,13 +378,23 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			if (!upperredirect)
 				goto out_put_upper;
 			if (d.redirect[0] == '/')
-				poe = dentry->d_sb->s_root->d_fsdata;
+				poe = roe;
 		}
 		if (d.opaque)
 			type |= __OVL_PATH_OPAQUE;
 	}
 
-	if (!d.stop && poe->numlower) {
+	/*
+	 * For now we only support lower by fh in single layer, because
+	 * fallback from lookup by fh to lookup by path in mid layers for
+	 * merge directory is not yet implemented.
+	 */
+	if (!ofs->redirect_fh || ofs->numlower > 1) {
+		kfree(d.fh);
+		d.fh = NULL;
+	}
+
+	if (!d.stop && (poe->numlower || d.fh)) {
 		err = -ENOMEM;
 		stack = kcalloc(ofs->numlower, sizeof(struct path),
 				GFP_TEMPORARY);
@@ -273,6 +402,35 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			goto out_put_upper;
 	}
 
+	/* Try to lookup lower layers by file handle */
+	d.by_path = false;
+	for (i = 0; !d.stop && d.fh && i < roe->numlower; i++) {
+		struct path lowerpath = poe->lowerstack[i];
+
+		d.last = i == poe->numlower - 1;
+		err = ovl_lookup_layer_fh(&lowerpath, &d, &this);
+		if (err)
+			goto out_put;
+
+		if (!this)
+			continue;
+
+		stack[ctr].dentry = this;
+		stack[ctr].mnt = lowerpath.mnt;
+		ctr++;
+		/*
+		 * Found by fh - won't lookup by path.
+		 * TODO: set d.redirect to dentry_path(this),
+		 *       so lookup can continue by path.
+		 */
+		d.stop = true;
+	}
+
+	/* Fallback to lookup lower layers by path */
+	d.by_path = true;
+	d.by_fh = false;
+	kfree(d.fh);
+	d.fh = NULL;
 	for (i = 0; !d.stop && i < poe->numlower; i++) {
 		struct path lowerpath = poe->lowerstack[i];
 
@@ -291,10 +449,8 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		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;
+		if (d.redirect && d.redirect[0] == '/' && poe != roe) {
+			poe = roe;
 
 			/* Find the current layer on the root dentry */
 			for (i = 0; i < poe->numlower; i++)
@@ -354,6 +510,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	dput(upperdentry);
 	kfree(upperredirect);
 out:
+	kfree(d.fh);
 	kfree(d.redirect);
 	revert_creds(old_cred);
 	return ERR_PTR(err);
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index c3cfbc5..08002ce 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -190,6 +190,7 @@ const char *ovl_dentry_get_redirect(struct dentry *dentry);
 void ovl_dentry_set_redirect(struct dentry *dentry, const char *redirect);
 bool ovl_redirect_fh(struct super_block *sb);
 void ovl_clear_redirect_fh(struct super_block *sb);
+bool ovl_redirect_fh_ok(const char *redirect, size_t size);
 void ovl_dentry_update(struct dentry *dentry, struct dentry *upperdentry);
 void ovl_inode_init(struct inode *inode, struct inode *realinode,
 		    bool is_upper);
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index b3bc117..dba9753 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -254,6 +254,20 @@ void ovl_clear_redirect_fh(struct super_block *sb)
 	ofs->redirect_fh = false;
 }
 
+bool ovl_redirect_fh_ok(const char *redirect, size_t size)
+{
+	struct ovl_fh *fh = (void *)redirect;
+
+	if (size < sizeof(struct ovl_fh) || size < fh->len)
+		return false;
+
+	if (fh->version > OVL_FH_VERSION ||
+	    fh->magic != OVL_FH_MAGIC)
+		return false;
+
+	return true;
+}
+
 void ovl_dentry_update(struct dentry *dentry, struct dentry *upperdentry)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
-- 
2.7.4