[PATCH v2] libbpf: Add some details for BTF parsing failures

[Ian Rogers <irogers@google.com>]

As CONFIG_DEBUG_INFO_BTF is default off the existing "failed to find
valid kernel BTF" message makes diagnosing the kernel build issue some
what cryptic. Add a little more detail with the hope of helping users.

Before:
```
libbpf: failed to find valid kernel BTF
libbpf: Error loading vmlinux BTF: -3
libbpf: failed to load object 'lock_contention_bpf'
libbpf: failed to load BPF skeleton 'lock_contention_bpf': -3
```

After no access /sys/kernel/btf/vmlinux:
```
libbpf: Unable to access canonical vmlinux BTF from /sys/kernel/btf/vmlinux
libbpf: Error loading vmlinux BTF: -3
libbpf: failed to load object 'lock_contention_bpf'
libbpf: failed to load BPF skeleton 'lock_contention_bpf': -3
```

After no BTF /sys/kernel/btf/vmlinux:
```
libbpf: Failed to load vmlinux BTF from /sys/kernel/btf/vmlinux, was CONFIG_DEBUG_INFO_BTF enabled?
libbpf: Error loading vmlinux BTF: -3
libbpf: failed to load object 'lock_contention_bpf'
libbpf: failed to load BPF skeleton 'lock_contention_bpf': -3
```

Closes: https://lore.kernel.org/bpf/CAP-5=fU+DN_+Y=Y4gtELUsJxKNDDCOvJzPHvjUVaUoeFAzNnig@mail.gmail.com/
Signed-off-by: Ian Rogers <irogers@google.com>

---
v2. Try to address review comments from Andrii Nakryiko.
---
 tools/lib/bpf/btf.c | 49 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 14 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index ee95fd379d4d..d8a05dda0836 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4920,16 +4920,25 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
 	return 0;
 }
 
+static struct btf *btf__load_vmlinux_btf_path(const char *path)
+{
+	struct btf *btf;
+	int err;
+
+	btf = btf__parse(path, NULL);
+	err = libbpf_get_error(btf);
+	pr_debug("loading kernel BTF '%s': %d\n", path, err);
+	return err ? NULL : btf;
+}
+
 /*
  * Probe few well-known locations for vmlinux kernel image and try to load BTF
  * data out of it to use for target BTF.
  */
 struct btf *btf__load_vmlinux_btf(void)
 {
+	/* fall back locations, trying to find vmlinux on disk */
 	const char *locations[] = {
-		/* try canonical vmlinux BTF through sysfs first */
-		"/sys/kernel/btf/vmlinux",
-		/* fall back to trying to find vmlinux on disk otherwise */
 		"/boot/vmlinux-%1$s",
 		"/lib/modules/%1$s/vmlinux-%1$s",
 		"/lib/modules/%1$s/build/vmlinux",
@@ -4938,29 +4947,41 @@ struct btf *btf__load_vmlinux_btf(void)
 		"/usr/lib/debug/boot/vmlinux-%1$s.debug",
 		"/usr/lib/debug/lib/modules/%1$s/vmlinux",
 	};
-	char path[PATH_MAX + 1];
+	const char *location;
 	struct utsname buf;
 	struct btf *btf;
-	int i, err;
+	int i;
 
-	uname(&buf);
+	/* try canonical vmlinux BTF through sysfs first */
+	location = "/sys/kernel/btf/vmlinux";
+	if (faccessat(AT_FDCWD, location, R_OK, AT_EACCESS) == 0) {
+		btf = btf__load_vmlinux_btf_path(location);
+		if (btf)
+			return btf;
+
+		pr_warn("Failed to load vmlinux BTF from %s, was CONFIG_DEBUG_INFO_BTF enabled?\n",
+			location);
+	} else
+		pr_warn("Unable to access canonical vmlinux BTF from %s\n", location);
 
+	uname(&buf);
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
-		snprintf(path, PATH_MAX, locations[i], buf.release);
+		char path[PATH_MAX + 1];
+
+		snprintf(path, sizeof(path), locations[i], buf.release);
 
+		btf = btf__load_vmlinux_btf_path(path);
 		if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
 			continue;
 
-		btf = btf__parse(path, NULL);
-		err = libbpf_get_error(btf);
-		pr_debug("loading kernel BTF '%s': %d\n", path, err);
-		if (err)
-			continue;
+		btf = btf__load_vmlinux_btf_path(location);
+		if (btf)
+			return btf;
 
-		return btf;
+		pr_warn("Failed to load vmlinux BTF from %s, was CONFIG_DEBUG_INFO_BTF enabled?\n",
+			path);
 	}
 
-	pr_warn("failed to find valid kernel BTF\n");
 	return libbpf_err_ptr(-ESRCH);
 }
 
-- 
2.43.0.429.g432eaa2c6b-goog