[PATCH v1] 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:
```
libbpf: failed to find a kernel (typically /sys/kernel/btf/vmlinux) for BTF data
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:
```
libbpf: failed to find BTF in kernel (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>
---
 tools/lib/bpf/btf.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index ee95fd379d4d..505c0fb2d1ed 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4942,6 +4942,7 @@ struct btf *btf__load_vmlinux_btf(void)
 	struct utsname buf;
 	struct btf *btf;
 	int i, err;
+	bool found_path = false;
 
 	uname(&buf);
 
@@ -4951,6 +4952,7 @@ struct btf *btf__load_vmlinux_btf(void)
 		if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
 			continue;
 
+		found_path = true;
 		btf = btf__parse(path, NULL);
 		err = libbpf_get_error(btf);
 		pr_debug("loading kernel BTF '%s': %d\n", path, err);
@@ -4960,7 +4962,11 @@ struct btf *btf__load_vmlinux_btf(void)
 		return btf;
 	}
 
-	pr_warn("failed to find valid kernel BTF\n");
+	if (found_path)
+		pr_warn("failed to find BTF in kernel (was CONFIG_DEBUG_INFO_BTF enabled?)\n");
+	else
+		pr_warn("failed to find a kernel (typically /sys/kernel/btf/vmlinux) for BTF data\n");
+
 	return libbpf_err_ptr(-ESRCH);
 }
 
-- 
2.43.0.429.g432eaa2c6b-goog