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 most cases
the vmlinux BTF sysfs path should work.

This patch simplifies dumping by ID usage by attempting to
use vmlinux BTF from sysfs, if the first try of loading BTF by ID
fails with certain conditions.

Signed-off-by: Larysa Zaremba <[email protected]>
Reviewed-by: Alexander Lobakin <[email protected]>

Author: walking-machine

tools/bpf/bpftool/btf.c

@@ -459,6 +459,22 @@ static int dump_btf_c(const struct btf *btf,
 	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;
+}
+
 static int do_dump(int argc, char **argv)
 {
 	struct btf *btf = NULL, *base = NULL;
@@ -536,18 +552,11 @@ static int do_dump(int argc, char **argv)
 		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))
+			base = get_vmlinux_btf_from_sysfs();
 
 		btf = btf__parse_split(*argv, base ?: base_btf);
 		err = libbpf_get_error(btf);
@@ -593,6 +602,14 @@ static int do_dump(int argc, char **argv)
 	if (!btf) {
 		btf = btf__load_from_kernel_by_id_split(btf_id, base_btf);
 		err = libbpf_get_error(btf);
+		if (err == -EINVAL && !base_btf) {
+			btf__free(base);
+			base = get_vmlinux_btf_from_sysfs();
+			p_info("Warning: valid base BTF was not specified with -B option, falling back on standard base BTF (sysfs vmlinux)");
+			btf = btf__load_from_kernel_by_id_split(btf_id, base);
+			err = libbpf_get_error(btf);
+		}
+
 		if (err) {
 			p_err("get btf by id (%u): %s", btf_id, strerror(err));
 			goto done;