Re: [PATCH v3] libbpf: Add some details for BTF parsing failures

[Andrii Nakryiko]

On Thu, Jan 25, 2024 at 3:18 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxx/
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
>
> ---
> v3. Try to address review comments from Andrii Nakryiko.

I did some further simplifications and clean ups while applying.

I dropped an extra faccessat(R_OK) check for /sys/kernel/btf/vmlinux
and instead if F_OK passes, just go ahead and try to parse
/sys/kernel/btf/vmlinux. If we have no access, we should get -EPERM or
-EACCESS (I didn't check which), otherwise we'll either parse or won't
find any BTF, both are errors. If /sys/kernel/btf/vmlinux exists,
there seems to be little point nowadays to try fallback locations,
kernel clearly is modern enough to generate /sys/kernel/btf/vmlinux,
so we just bail out with error.

Please check the landed commit in bpf-next and let me know if it
doesn't cover your use case properly.

> ---
>  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
>