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 >
