[PATCH 0/2] bpftool: improve error handing for missing .BTF section

[PATCH 0/2] bpftool: improve error handing for missing .BTF section

[Changbin Du]

Changbin Du (2):
  libbpf: show more info about missing ".BTF" section
  makefiles: do not generate empty vmlinux.h

 samples/bpf/Makefile                 |  2 +-
 tools/bpf/bpftool/Makefile           |  2 +-
 tools/bpf/runqslower/Makefile        |  2 +-
 tools/lib/bpf/btf.c                  | 12 ++++++++++++
 tools/perf/Makefile.perf             |  2 +-
 tools/testing/selftests/bpf/Makefile |  2 +-
 6 files changed, 17 insertions(+), 5 deletions(-)

-- 
2.37.2

[PATCH 1/2] libbpf: show more info about missing ".BTF" section

[Changbin Du]

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

[PATCH 2/2] makefiles: do not generate empty vmlinux.h

[Changbin Du]

Remove the empty vmlinux.h if bpftool failed to dump btf info.
The emptry vmlinux.h can hide real error when reading output
of make.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
---
 samples/bpf/Makefile                 | 2 +-
 tools/bpf/bpftool/Makefile           | 2 +-
 tools/bpf/runqslower/Makefile        | 2 +-
 tools/perf/Makefile.perf             | 2 +-
 tools/testing/selftests/bpf/Makefile | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 727da3c5879b..ab4788b4883e 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -362,7 +362,7 @@ ifeq ($(VMLINUX_BTF),)
 	$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)",\
 		build the kernel or set VMLINUX_BTF or VMLINUX_H variable)
 endif
-	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
+	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ || { rm $@; exit 1; }
 else
 	$(Q)cp "$(VMLINUX_H)" $@
 endif
diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 4a95c017ad4c..d9d6f890884c 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -177,7 +177,7 @@ BUILD_BPF_SKELS := 1
 
 $(OUTPUT)vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL_BOOTSTRAP)
 ifeq ($(VMLINUX_H),)
-	$(QUIET_GEN)$(BPFTOOL_BOOTSTRAP) btf dump file $< format c > $@
+	$(QUIET_GEN)$(BPFTOOL_BOOTSTRAP) btf dump file $< format c > $@ || { rm $@; exit 1; }
 else
 	$(Q)cp "$(VMLINUX_H)" $@
 endif
diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
index 8b3d87b82b7a..2d7911f4666b 100644
--- a/tools/bpf/runqslower/Makefile
+++ b/tools/bpf/runqslower/Makefile
@@ -77,7 +77,7 @@ ifeq ($(VMLINUX_H),)
 			"specify its location." >&2;			       \
 		exit 1;\
 	fi
-	$(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
+	$(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@ || { rm $@; exit 1; }
 else
 	$(Q)cp "$(VMLINUX_H)" $@
 endif
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index a432e59afc42..0546d408aa4e 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -1064,7 +1064,7 @@ VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 
 $(SKEL_OUT)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
 ifeq ($(VMLINUX_H),)
-	$(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@
+	$(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@ || { rm $@; exit 1; }
 else
 	$(Q)cp "$(VMLINUX_H)" $@
 endif
diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index e6cf21fad69f..9aa2475b4ac6 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -284,7 +284,7 @@ endif
 $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR)
 ifeq ($(VMLINUX_H),)
 	$(call msg,GEN,,$@)
-	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
+	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ || { rm $@; exit 1; }
 else
 	$(call msg,CP,,$@)
 	$(Q)cp "$(VMLINUX_H)" $@
-- 
2.37.2

Re: [PATCH 1/2] libbpf: show more info about missing ".BTF" section

[Andrii Nakryiko]

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
>

Re: [PATCH 2/2] makefiles: do not generate empty vmlinux.h

[Andrii Nakryiko]

On Sat, Nov 26, 2022 at 3:13 AM Changbin Du <changbin.du@gmail.com> wrote:
>
> Remove the empty vmlinux.h if bpftool failed to dump btf info.
> The emptry vmlinux.h can hide real error when reading output
> of make.

wouldn't this be better handled by adding .DELETE_ON_ERROR: to
bpftool's Makefile?

>
> Signed-off-by: Changbin Du <changbin.du@gmail.com>
> ---
>  samples/bpf/Makefile                 | 2 +-
>  tools/bpf/bpftool/Makefile           | 2 +-
>  tools/bpf/runqslower/Makefile        | 2 +-
>  tools/perf/Makefile.perf             | 2 +-
>  tools/testing/selftests/bpf/Makefile | 2 +-
>  5 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 727da3c5879b..ab4788b4883e 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -362,7 +362,7 @@ ifeq ($(VMLINUX_BTF),)
>         $(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)",\
>                 build the kernel or set VMLINUX_BTF or VMLINUX_H variable)
>  endif
> -       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
> +       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ || { rm $@; exit 1; }
>  else
>         $(Q)cp "$(VMLINUX_H)" $@
>  endif
> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
> index 4a95c017ad4c..d9d6f890884c 100644
> --- a/tools/bpf/bpftool/Makefile
> +++ b/tools/bpf/bpftool/Makefile
> @@ -177,7 +177,7 @@ BUILD_BPF_SKELS := 1
>
>  $(OUTPUT)vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL_BOOTSTRAP)
>  ifeq ($(VMLINUX_H),)
> -       $(QUIET_GEN)$(BPFTOOL_BOOTSTRAP) btf dump file $< format c > $@
> +       $(QUIET_GEN)$(BPFTOOL_BOOTSTRAP) btf dump file $< format c > $@ || { rm $@; exit 1; }
>  else
>         $(Q)cp "$(VMLINUX_H)" $@
>  endif
> diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
> index 8b3d87b82b7a..2d7911f4666b 100644
> --- a/tools/bpf/runqslower/Makefile
> +++ b/tools/bpf/runqslower/Makefile
> @@ -77,7 +77,7 @@ ifeq ($(VMLINUX_H),)
>                         "specify its location." >&2;                           \
>                 exit 1;\
>         fi
> -       $(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
> +       $(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@ || { rm $@; exit 1; }
>  else
>         $(Q)cp "$(VMLINUX_H)" $@
>  endif
> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index a432e59afc42..0546d408aa4e 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -1064,7 +1064,7 @@ VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
>
>  $(SKEL_OUT)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
>  ifeq ($(VMLINUX_H),)
> -       $(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@
> +       $(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@ || { rm $@; exit 1; }
>  else
>         $(Q)cp "$(VMLINUX_H)" $@
>  endif
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index e6cf21fad69f..9aa2475b4ac6 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -284,7 +284,7 @@ endif
>  $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR)
>  ifeq ($(VMLINUX_H),)
>         $(call msg,GEN,,$@)
> -       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
> +       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ || { rm $@; exit 1; }
>  else
>         $(call msg,CP,,$@)
>         $(Q)cp "$(VMLINUX_H)" $@
> --
> 2.37.2
>

Re: [PATCH 2/2] makefiles: do not generate empty vmlinux.h

[Changbin Du]

On Mon, Nov 28, 2022 at 10:00:14PM -0800, Andrii Nakryiko wrote:
> On Sat, Nov 26, 2022 at 3:13 AM Changbin Du <changbin.du@gmail.com> wrote:
> >
> > Remove the empty vmlinux.h if bpftool failed to dump btf info.
> > The emptry vmlinux.h can hide real error when reading output
> > of make.
> 
> wouldn't this be better handled by adding .DELETE_ON_ERROR: to
> bpftool's Makefile?
>
yes, let me replace it.

-- 
Cheers,
Changbin Du

Re: [PATCH 1/2] libbpf: show more info about missing ".BTF" section

[Changbin Du]

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