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

[Ian Rogers]

On Mon, Jan 29, 2024 at 4:43 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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.

It does, thanks Andrii!

Ian

> > ---
> >  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
> >