[PATCH bpf-next v2] bpftool: Use sysfs vmlinux when dumping BTF by ID

[Larysa Zaremba <larysa.zaremba@intel.com>]

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 attempting to
use vmlinux BTF from sysfs, if the first try of loading BTF by ID
fails with certain conditions and the ID corresponds to a kernel
module BTF.

Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
 tools/bpf/bpftool/btf.c | 67 +++++++++++++++++++++++++++++++++++------
 1 file changed, 58 insertions(+), 9 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index a2c665beda87..070e0c1595d7 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -459,6 +459,56 @@ 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 struct btf *btf_try_load_with_vmlinux(__u32 btf_id, struct btf **base)
+{
+	struct bpf_btf_info btf_info = {};
+	unsigned int len;
+	int btf_fd;
+	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 ERR_PTR(btf_fd);
+	}
+
+	len = sizeof(btf_info);
+	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 ERR_PTR(err);
+	}
+
+	if (!btf_info.kernel_btf) {
+		p_err("BTF with ID %u is not a kernel module BTF, cannot use vmlinux as base",
+		      btf_id);
+		return ERR_PTR(-EINVAL);
+	}
+
+	*base = get_vmlinux_btf_from_sysfs();
+	return btf__load_from_kernel_by_id_split(btf_id, *base);
+}
+
 static int do_dump(int argc, char **argv)
 {
 	struct btf *btf = NULL, *base = NULL;
@@ -536,18 +586,11 @@ 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 +636,12 @@ 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) {
+			p_info("Warning: valid base BTF was not specified with -B option, falling back on standard base BTF (sysfs vmlinux)");
+			btf = btf_try_load_with_vmlinux(btf_id, &base);
+			err = libbpf_get_error(btf);
+		}
+
 		if (err) {
 			p_err("get btf by id (%u): %s", btf_id, strerror(err));
 			goto done;
-- 
2.35.1