bpf: Support removing kernfs entries

When a bpf object has been exposed in kernfs, there should be a way
to remove it. Kernfs doesn't implement unlink, therefore one can not
remove the entry in a normal way. To remove the file, we can allow
writing a special command to the new entry, which can trigger a
remove_self() for removal.

So far there are two ways to remove an entry that is created by pinning
bpf objects in kernfs:

 1. unpin the object from bpffs.
 2. write a special command to the kernfs entry.

Signed-off-by: Hao Luo <haoluo@google.com>

kernel/bpf/kernfs_node.c

@@ -9,6 +9,9 @@
 
 /* file_operations for kernfs file system */
 
+/* Command for removing a kernfs entry */
+#define REMOVE_CMD "rm"
+
 /* Handler when the watched inode is freed. */
 static void kn_watch_free_inode(void *obj, enum bpf_type type, void *kn)
 {
@@ -22,8 +25,27 @@ static const struct notify_ops notify_ops = {
 	.free_inode = kn_watch_free_inode,
 };
 
+static ssize_t bpf_generic_write(struct kernfs_open_file *of, char *buf,
+				 size_t bytes, loff_t off)
+{
+	if (sysfs_streq(buf, REMOVE_CMD)) {
+		kernfs_remove_self(of->kn);
+		return bytes;
+	}
+
+	return -EINVAL;
+}
+
+static ssize_t bpf_generic_read(struct kernfs_open_file *of, char *buf,
+				size_t bytes, loff_t off)
+{
+	return -EIO;
+}
+
 /* Kernfs file operations for bpf created files. */
 static const struct kernfs_ops bpf_generic_ops = {
+	.write          = bpf_generic_write,
+	.read           = bpf_generic_read,
 };
 
 /* Test whether a given dentry is a kernfs entry. */