bpf, inode: add support for symlinks and fix mtime/ctime

borkmann · davem330 · commit 0f98621bef5d · 2016-10-31T15:28:11.000-04:00
While commit bb35a6e ("bpf, inode: allow for rename and link ops") added support for hard links that can be used for prog and map nodes, this work adds simple symlink support, which can be used f.e. for directories also when unpriviledged and works with cmdline tooling that understands S_IFLNK anyway. Since the switch in e27f4a9 ("bpf: Use mount_nodev not mount_ns to mount the bpf filesystem"), there can be various mount instances with mount_nodev() and thus hierarchy can be flattened to facilitate object sharing. Thus, we can keep bpf tooling also working by repointing paths. Most of the functionality can be used from vfs library operations. The symlink is stored in the inode itself, that is in i_link, which is sufficient in our case as opposed to storing it in the page cache. While at it, I noticed that bpf_mkdir() and bpf_mkobj() don't update the directories mtime and ctime, so add a common helper for it called bpf_dentry_finalize() that takes care of it for all cases now. Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
@@ -87,6 +87,7 @@ static struct inode *bpf_get_inode(struct super_block *sb,
 	switch (mode & S_IFMT) {
 	case S_IFDIR:
 	case S_IFREG:
+	case S_IFLNK:
 		break;
 	default:
 		return ERR_PTR(-EINVAL);
@@ -119,6 +120,16 @@ static int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
 	return 0;
 }
 
+static void bpf_dentry_finalize(struct dentry *dentry, struct inode *inode,
+				struct inode *dir)
+{
+	d_instantiate(dentry, inode);
+	dget(dentry);
+
+	dir->i_mtime = current_time(dir);
+	dir->i_ctime = dir->i_mtime;
+}
+
 static int bpf_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	struct inode *inode;
@@ -133,9 +144,7 @@ static int bpf_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 	inc_nlink(inode);
 	inc_nlink(dir);
 
-	d_instantiate(dentry, inode);
-	dget(dentry);
-
+	bpf_dentry_finalize(dentry, inode, dir);
 	return 0;
 }
 
@@ -151,9 +160,7 @@ static int bpf_mkobj_ops(struct inode *dir, struct dentry *dentry,
 	inode->i_op = iops;
 	inode->i_private = dentry->d_fsdata;
 
-	d_instantiate(dentry, inode);
-	dget(dentry);
-
+	bpf_dentry_finalize(dentry, inode, dir);
 	return 0;
 }
 
@@ -181,13 +188,37 @@ bpf_lookup(struct inode *dir, struct dentry *dentry, unsigned flags)
 {
 	if (strchr(dentry->d_name.name, '.'))
 		return ERR_PTR(-EPERM);
+
 	return simple_lookup(dir, dentry, flags);
 }
 
+static int bpf_symlink(struct inode *dir, struct dentry *dentry,
+		       const char *target)
+{
+	char *link = kstrdup(target, GFP_USER | __GFP_NOWARN);
+	struct inode *inode;
+
+	if (!link)
+		return -ENOMEM;
+
+	inode = bpf_get_inode(dir->i_sb, dir, S_IRWXUGO | S_IFLNK);
+	if (IS_ERR(inode)) {
+		kfree(link);
+		return PTR_ERR(inode);
+	}
+
+	inode->i_op = &simple_symlink_inode_operations;
+	inode->i_link = link;
+
+	bpf_dentry_finalize(dentry, inode, dir);
+	return 0;
+}
+
 static const struct inode_operations bpf_dir_iops = {
 	.lookup		= bpf_lookup,
 	.mknod		= bpf_mkobj,
 	.mkdir		= bpf_mkdir,
+	.symlink	= bpf_symlink,
 	.rmdir		= simple_rmdir,
 	.rename		= simple_rename,
 	.link		= simple_link,
@@ -324,6 +355,8 @@ static void bpf_evict_inode(struct inode *inode)
 	truncate_inode_pages_final(&inode->i_data);
 	clear_inode(inode);
 
+	if (S_ISLNK(inode->i_mode))
+		kfree(inode->i_link);
 	if (!bpf_inode_type(inode, &type))
 		bpf_any_put(inode->i_private, type);
 }
