bpftool: Use sysfs vmlinux when dumping BTF by ID

Currently, dumping almost all BTFs specified by id requires
using the -B option to pass the base BTF. For kernel module
BTFs the vmlinux BTF sysfs path should work.

This patch simplifies dumping by ID usage by loading
vmlinux BTF from sysfs as base, if base BTF was not specified
and the ID corresponds to a kernel module BTF.

Signed-off-by: Larysa Zaremba <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Reviewed-by: Alexander Lobakin <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/btf.c

···
 	return err;
 }
 
+static const char sysfs_vmlinux[] = "/sys/kernel/btf/vmlinux";
+
+static struct btf *get_vmlinux_btf_from_sysfs(void)
+{
+	struct btf *base;
+
+	base = btf__parse(sysfs_vmlinux, NULL);
+	if (libbpf_get_error(base)) {
+		p_err("failed to parse vmlinux BTF at '%s': %ld\n",
+		      sysfs_vmlinux, libbpf_get_error(base));
+		base = NULL;
+	}
+
+	return base;
+}
+
+#define BTF_NAME_BUFF_LEN 64
+
+static bool btf_is_kernel_module(__u32 btf_id)
+{
+	struct bpf_btf_info btf_info = {};
+	char btf_name[BTF_NAME_BUFF_LEN];
+	int btf_fd;
+	__u32 len;
+	int err;
+
+	btf_fd = bpf_btf_get_fd_by_id(btf_id);
+	if (btf_fd < 0) {
+		p_err("can't get BTF object by id (%u): %s", btf_id, strerror(errno));
+		return false;
+	}
+
+	len = sizeof(btf_info);
+	btf_info.name = ptr_to_u64(btf_name);
+	btf_info.name_len = sizeof(btf_name);
+	err = bpf_obj_get_info_by_fd(btf_fd, &btf_info, &len);
+	close(btf_fd);
+	if (err) {
+		p_err("can't get BTF (ID %u) object info: %s", btf_id, strerror(errno));
+		return false;
+	}
+
+	return btf_info.kernel_btf && strncmp(btf_name, "vmlinux", sizeof(btf_name)) != 0;
+}
+
 static int do_dump(int argc, char **argv)
 {
 	struct btf *btf = NULL, *base = NULL;
···
 		NEXT_ARG();
 	} else if (is_prefix(src, "file")) {
 		const char sysfs_prefix[] = "/sys/kernel/btf/";
-		const char sysfs_vmlinux[] = "/sys/kernel/btf/vmlinux";
 
 		if (!base_btf &&
 		    strncmp(*argv, sysfs_prefix, sizeof(sysfs_prefix) - 1) == 0 &&
-		    strcmp(*argv, sysfs_vmlinux) != 0) {
-			base = btf__parse(sysfs_vmlinux, NULL);
-			if (libbpf_get_error(base)) {
-				p_err("failed to parse vmlinux BTF at '%s': %ld\n",
-				      sysfs_vmlinux, libbpf_get_error(base));
-				base = NULL;
-			}
-		}
+		    strcmp(*argv, sysfs_vmlinux) != 0)
+			base = get_vmlinux_btf_from_sysfs();
 
 		btf = btf__parse_split(*argv, base ?: base_btf);
 		err = libbpf_get_error(btf);
···
 	}
 
 	if (!btf) {
+		if (!base_btf && btf_is_kernel_module(btf_id)) {
+			p_info("Warning: valid base BTF was not specified with -B option, falling back to standard base BTF (%s)",
+			       sysfs_vmlinux);
+			base_btf = get_vmlinux_btf_from_sysfs();
+		}
+
 		btf = btf__load_from_kernel_by_id_split(btf_id, base_btf);
 		err = libbpf_get_error(btf);
 		if (err) {