tools/lib/bpf/btf.c | 35 +++++++++++++++++++++++++++-------- 1 file changed, 27 insertions(+), 8 deletions(-)
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@mail.gmail.com/
Signed-off-by: Ian Rogers <irogers@google.com>
---
v3. Try to address review comments from Andrii Nakryiko.
---
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-googOn Thu, Jan 25, 2024 at 3:18 PM Ian Rogers <irogers@google.com> 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@mail.gmail.com/
> Signed-off-by: Ian Rogers <irogers@google.com>
>
> ---
> 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
>
On Mon, Jan 29, 2024 at 4:43 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Jan 25, 2024 at 3:18 PM Ian Rogers <irogers@google.com> 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@mail.gmail.com/
> > Signed-off-by: Ian Rogers <irogers@google.com>
> >
> > ---
> > 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
> >
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