Show more information about why failed instead of just saying "No such file or
directory".

Now will print below info:
libbpf: can not find '.BTF' section
libbpf: is CONFIG_DEBUG_INFO_BTF enabled for kernel?
Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory

Signed-off-by: Changbin Du <changbin.du@gmail.com>
---
 tools/lib/bpf/btf.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index d88647da2c7f..3f661d991808 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -906,6 +906,15 @@ struct btf *btf__new(const void *data, __u32 size)
 	return libbpf_ptr(btf_new(data, size, NULL));
 }
 
+static bool is_vmlinux(const char *path)
+{
+	size_t path_len = strlen(path);
+	size_t suffix_len = strlen("vmlinux");
+
+	return (path_len >= suffix_len) &&
+	       (!memcmp(path + path_len - suffix_len, "vmlinux", suffix_len));
+}
+
 static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 				 struct btf_ext **btf_ext)
 {
@@ -990,6 +999,9 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 	err = 0;
 
 	if (!btf_data) {
+		pr_warn("can not find '%s' section\n", BTF_ELF_SEC);
+		if (is_vmlinux(path))
+			pr_warn("is CONFIG_DEBUG_INFO_BTF enabled for kernel?\n");
 		err = -ENOENT;
 		goto done;
 	}
-- 
2.37.2

On Sat, Nov 26, 2022 at 3:13 AM Changbin Du <changbin.du@gmail.com> wrote:
>
> Show more information about why failed instead of just saying "No such file or
> directory".
>
> Now will print below info:
> libbpf: can not find '.BTF' section
> libbpf: is CONFIG_DEBUG_INFO_BTF enabled for kernel?
> Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory
>
> Signed-off-by: Changbin Du <changbin.du@gmail.com>
> ---
>  tools/lib/bpf/btf.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index d88647da2c7f..3f661d991808 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -906,6 +906,15 @@ struct btf *btf__new(const void *data, __u32 size)
>         return libbpf_ptr(btf_new(data, size, NULL));
>  }
>
> +static bool is_vmlinux(const char *path)
> +{
> +       size_t path_len = strlen(path);
> +       size_t suffix_len = strlen("vmlinux");
> +
> +       return (path_len >= suffix_len) &&
> +              (!memcmp(path + path_len - suffix_len, "vmlinux", suffix_len));
> +}
> +
>  static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
>                                  struct btf_ext **btf_ext)
>  {
> @@ -990,6 +999,9 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
>         err = 0;
>
>         if (!btf_data) {
> +               pr_warn("can not find '%s' section\n", BTF_ELF_SEC);
> +               if (is_vmlinux(path))
> +                       pr_warn("is CONFIG_DEBUG_INFO_BTF enabled for kernel?\n");

this is generic piece of BTF loading code in libbpf, it knows nothing
and should know nothing about vmlinux and CONFIG_DEBUG_INFO_BTF, this
is not the right place to add such suggestions.

Check bpf_object__load_vmlinux_btf(), libbpf emits vmlinux-specific
error there: "Error loading vmlinux BTF". If we need to mention
CONFIG_DEBUG_INFO_BTF anywhere, that would be the place to do that.

>                 err = -ENOENT;
>                 goto done;
>         }
> --
> 2.37.2
>

On Mon, Nov 28, 2022 at 09:59:00PM -0800, Andrii Nakryiko wrote:
> On Sat, Nov 26, 2022 at 3:13 AM Changbin Du <changbin.du@gmail.com> wrote:
> >
> > Show more information about why failed instead of just saying "No such file or
> > directory".
> >
> > Now will print below info:
> > libbpf: can not find '.BTF' section
> > libbpf: is CONFIG_DEBUG_INFO_BTF enabled for kernel?
> > Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory
> >
> > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> > ---
> >  tools/lib/bpf/btf.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index d88647da2c7f..3f661d991808 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -906,6 +906,15 @@ struct btf *btf__new(const void *data, __u32 size)
> >         return libbpf_ptr(btf_new(data, size, NULL));
> >  }
> >
> > +static bool is_vmlinux(const char *path)
> > +{
> > +       size_t path_len = strlen(path);
> > +       size_t suffix_len = strlen("vmlinux");
> > +
> > +       return (path_len >= suffix_len) &&
> > +              (!memcmp(path + path_len - suffix_len, "vmlinux", suffix_len));
> > +}
> > +
> >  static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >                                  struct btf_ext **btf_ext)
> >  {
> > @@ -990,6 +999,9 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >         err = 0;
> >
> >         if (!btf_data) {
> > +               pr_warn("can not find '%s' section\n", BTF_ELF_SEC);
> > +               if (is_vmlinux(path))
> > +                       pr_warn("is CONFIG_DEBUG_INFO_BTF enabled for kernel?\n");
> 
> this is generic piece of BTF loading code in libbpf, it knows nothing
> and should know nothing about vmlinux and CONFIG_DEBUG_INFO_BTF, this
> is not the right place to add such suggestions.
> 
> Check bpf_object__load_vmlinux_btf(), libbpf emits vmlinux-specific
> error there: "Error loading vmlinux BTF". If we need to mention
> CONFIG_DEBUG_INFO_BTF anywhere, that would be the place to do that.
>
Agreed. I think adding "can not find '.BTF' section" is enough to diagnose the problem.
The standalone error msg "No such file or directory" is really weird.

> >                 err = -ENOENT;
> >                 goto done;
> >         }
> > --
> > 2.37.2
> >

-- 
Cheers,
Changbin Du