[PATCH v3] 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
```

After not accessible:
```
libbpf: access to canonical vmlinux (/sys/kernel/btf/vmlinux) to load BTF failed: No such file or directory
libbpf: was CONFIG_DEBUG_INFO_BTF enabled?
libbpf: failed to find valid kernel BTF
libbpf: Error loading vmlinux BTF: -3
```

After not readable:
```
libbpf: unable to read canonical vmlinux (/sys/kernel/btf/vmlinux): Permission denied
libbpf: failed to find valid kernel BTF
libbpf: Error loading vmlinux BTF: -3
```

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

---
v3. Try to address review comments from Andrii Nakryiko.
---
 tools/lib/bpf/btf.c | 35 +++++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index ec92b87cae01..45983f42aba9 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4932,10 +4932,9 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
  */
 struct btf *btf__load_vmlinux_btf(void)
 {
+	const char *canonical_vmlinux = "/sys/kernel/btf/vmlinux";
+	/* 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",
@@ -4946,14 +4945,34 @@ struct btf *btf__load_vmlinux_btf(void)
 	};
 	char path[PATH_MAX + 1];
 	struct utsname buf;
-	struct btf *btf;
+	struct btf *btf = NULL;
 	int i, err;
 
-	uname(&buf);
+	/* is canonical sysfs location accessible? */
+	err = faccessat(AT_FDCWD, canonical_vmlinux, F_OK, AT_EACCESS);
+	if (err) {
+		pr_warn("access to canonical vmlinux (%s) to load BTF failed: %s\n",
+			canonical_vmlinux, strerror(errno));
+		pr_warn("was CONFIG_DEBUG_INFO_BTF enabled?\n");
+	} else {
+		err = faccessat(AT_FDCWD, canonical_vmlinux, R_OK, AT_EACCESS);
+		if (err) {
+			pr_warn("unable to read canonical vmlinux (%s): %s\n",
+				canonical_vmlinux, strerror(errno));
+		}
+	}
+	if (!err) {
+		/* load canonical and return any parsing failures */
+		btf = btf__parse(canonical_vmlinux, NULL);
+		err = libbpf_get_error(btf);
+		pr_debug("loading kernel BTF '%s': %d\n", canonical_vmlinux, err);
+		return btf;
+	}
 
+	/* try fallback locations */
+	uname(&buf);
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
 		snprintf(path, PATH_MAX, locations[i], buf.release);
-
 		if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
 			continue;
 
@@ -4965,9 +4984,9 @@ struct btf *btf__load_vmlinux_btf(void)
 
 		return btf;
 	}
-
 	pr_warn("failed to find valid kernel BTF\n");
-	return libbpf_err_ptr(-ESRCH);
+	/* return the last error or ESRCH if no fallback locations were found */
+	return btf ?: libbpf_err_ptr(-ESRCH);
 }
 
 struct btf *libbpf_find_kernel_btf(void) __attribute__((alias("btf__load_vmlinux_btf")));
-- 
2.43.0.429.g432eaa2c6b-goog