bpf: btf: Add BPF_BTF_LOAD command

This patch adds a BPF_BTF_LOAD command which
1) loads and verifies the BTF (implemented in earlier patches)
2) returns a BTF fd to userspace. In the next patch, the
BTF fd can be specified during BPF_MAP_CREATE.

It currently limits to CAP_SYS_ADMIN.

Signed-off-by: Martin KaFai Lau <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

include/linux/btf.h

···
 
 struct btf;
 struct btf_type;
+union bpf_attr;
 
+void btf_put(struct btf *btf);
+int btf_new_fd(const union bpf_attr *attr);
+struct btf *btf_get_by_fd(int fd);
 /* Figure out the size of a type_id.  If type_id is a modifier
  * (e.g. const), it will be resolved to find out the type with size.
  *

include/uapi/linux/bpf.h

···
 	BPF_OBJ_GET_INFO_BY_FD,
 	BPF_PROG_QUERY,
 	BPF_RAW_TRACEPOINT_OPEN,
+	BPF_BTF_LOAD,
 };
 
 enum bpf_map_type {
···
 		__u64 name;
 		__u32 prog_fd;
 	} raw_tracepoint;
+
+	struct { /* anonymous struct for BPF_BTF_LOAD */
+		__aligned_u64	btf;
+		__aligned_u64	btf_log_buf;
+		__u32		btf_size;
+		__u32		btf_log_size;
+		__u32		btf_log_level;
+	};
 } __attribute__((aligned(8)));
 
 /* BPF helper function descriptions:

kernel/bpf/btf.c

···
 #include <linux/compiler.h>
 #include <linux/errno.h>
 #include <linux/slab.h>
+#include <linux/anon_inodes.h>
+#include <linux/file.h>
 #include <linux/uaccess.h>
 #include <linux/kernel.h>
 #include <linux/bpf_verifier.h>
···
 	u32 nr_types;
 	u32 types_size;
 	u32 data_size;
+	refcount_t refcnt;
 };
 
 enum verifier_phase {
···
 	kvfree(btf->resolved_ids);
 	kvfree(btf->data);
 	kfree(btf);
+}
+
+static void btf_get(struct btf *btf)
+{
+	refcount_inc(&btf->refcnt);
+}
+
+void btf_put(struct btf *btf)
+{
+	if (btf && refcount_dec_and_test(&btf->refcnt))
+		btf_free(btf);
 }
 
 static int env_resolve_init(struct btf_verifier_env *env)
···
 
 	if (!err) {
 		btf_verifier_env_free(env);
+		btf_get(btf);
 		return btf;
 	}
 
···
 	const struct btf_type *t = btf_type_by_id(btf, type_id);
 
 	btf_type_ops(t)->seq_show(btf, t, type_id, obj, 0, m);
+}
+
+static int btf_release(struct inode *inode, struct file *filp)
+{
+	btf_put(filp->private_data);
+	return 0;
+}
+
+static const struct file_operations btf_fops = {
+	.release	= btf_release,
+};
+
+int btf_new_fd(const union bpf_attr *attr)
+{
+	struct btf *btf;
+	int fd;
+
+	btf = btf_parse(u64_to_user_ptr(attr->btf),
+			attr->btf_size, attr->btf_log_level,
+			u64_to_user_ptr(attr->btf_log_buf),
+			attr->btf_log_size);
+	if (IS_ERR(btf))
+		return PTR_ERR(btf);
+
+	fd = anon_inode_getfd("btf", &btf_fops, btf,
+			      O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		btf_put(btf);
+
+	return fd;
+}
+
+struct btf *btf_get_by_fd(int fd)
+{
+	struct btf *btf;
+	struct fd f;
+
+	f = fdget(fd);
+
+	if (!f.file)
+		return ERR_PTR(-EBADF);
+
+	if (f.file->f_op != &btf_fops) {
+		fdput(f);
+		return ERR_PTR(-EINVAL);
+	}
+
+	btf = f.file->private_data;
+	btf_get(btf);
+	fdput(f);
+
+	return btf;
 }

kernel/bpf/syscall.c

···
  */
 #include <linux/bpf.h>
 #include <linux/bpf_trace.h>
+#include <linux/btf.h>
 #include <linux/syscalls.h>
 #include <linux/slab.h>
 #include <linux/sched/signal.h>
···
 	return err;
 }
 
+#define BPF_BTF_LOAD_LAST_FIELD btf_log_level
+
+static int bpf_btf_load(const union bpf_attr *attr)
+{
+	if (CHECK_ATTR(BPF_BTF_LOAD))
+		return -EINVAL;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	return btf_new_fd(attr);
+}
+
 SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
 {
 	union bpf_attr attr = {};
···
 		break;
 	case BPF_RAW_TRACEPOINT_OPEN:
 		err = bpf_raw_tracepoint_open(&attr);
+		break;
+	case BPF_BTF_LOAD:
+		err = bpf_btf_load(&attr);
 		break;
 	default:
 		err = -EINVAL;