bpf: BPF token support for BPF_BTF_GET_FD_BY_ID

Currently BPF_BTF_GET_FD_BY_ID requires CAP_SYS_ADMIN, which does not
allow running it from user namespace. This creates a problem when
freplace program running from user namespace needs to query target
program BTF.
This patch relaxes capable check from CAP_SYS_ADMIN to CAP_BPF and adds
support for BPF token that can be passed in attributes to syscall.

Signed-off-by: Mykyta Yatsenko <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/uapi/linux/bpf.h

···
 		};
 		__u32		next_id;
 		__u32		open_flags;
+		__s32		fd_by_id_token_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_GET_INFO_BY_FD */

kernel/bpf/syscall.c

···
 	return btf_new_fd(attr, uattr, uattr_size);
 }
 
-#define BPF_BTF_GET_FD_BY_ID_LAST_FIELD btf_id
+#define BPF_BTF_GET_FD_BY_ID_LAST_FIELD fd_by_id_token_fd
 
 static int bpf_btf_get_fd_by_id(const union bpf_attr *attr)
 {
+	struct bpf_token *token = NULL;
+
 	if (CHECK_ATTR(BPF_BTF_GET_FD_BY_ID))
 		return -EINVAL;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (attr->open_flags & ~BPF_F_TOKEN_FD)
+		return -EINVAL;
+
+	if (attr->open_flags & BPF_F_TOKEN_FD) {
+		token = bpf_token_get_from_fd(attr->fd_by_id_token_fd);
+		if (IS_ERR(token))
+			return PTR_ERR(token);
+		if (!bpf_token_allow_cmd(token, BPF_BTF_GET_FD_BY_ID)) {
+			bpf_token_put(token);
+			token = NULL;
+		}
+	}
+
+	if (!bpf_token_capable(token, CAP_SYS_ADMIN)) {
+		bpf_token_put(token);
 		return -EPERM;
+	}
+
+	bpf_token_put(token);
 
 	return btf_get_fd_by_id(attr->btf_id);
 }

tools/include/uapi/linux/bpf.h

···
 		};
 		__u32		next_id;
 		__u32		open_flags;
+		__s32		fd_by_id_token_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_GET_INFO_BY_FD */