linux-kernel - Re: [PATCH v2 4/4] perf build: Filter out BTF sources without a .BTF section

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Date:   Wed, 7 Jun 2023 00:11:17 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        James Clark <james.clark@....com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Yang Jihong <yangjihong1@...wei.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v2 4/4] perf build: Filter out BTF sources without a .BTF section

On Mon, Jun 5, 2023 at 4:35 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hi Ian,
>
> On Mon, Jun 5, 2023 at 1:28 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > If generating vmlinux.h, make the code to generate it more tolerant by
> > filtering out paths to kernels that lack a .BTF section.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/Makefile.perf | 23 ++++++++++++++++++++---
> >  1 file changed, 20 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> > index f1840af195c0..c3bb27a912b0 100644
> > --- a/tools/perf/Makefile.perf
> > +++ b/tools/perf/Makefile.perf
> > @@ -193,6 +193,7 @@ FLEX    ?= flex
> >  BISON   ?= bison
> >  STRIP   = strip
> >  AWK     = awk
> > +READELF ?= readelf
> >
> >  # include Makefile.config by default and rule out
> >  # non-config cases
> > @@ -1080,12 +1081,28 @@ $(BPFTOOL): | $(SKEL_TMP_OUT)
> >         $(Q)CFLAGS= $(MAKE) -C ../bpf/bpftool \
> >                 OUTPUT=$(SKEL_TMP_OUT)/ bootstrap
> >
> > -VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                           \
> > +# Paths to search for a kernel to generate vmlinux.h from.
> > +VMLINUX_BTF_ELF_PATHS ?= $(if $(O),$(O)/vmlinux)                       \
> >                      $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)    \
> >                      ../../vmlinux                                      \
> > -                    /sys/kernel/btf/vmlinux                            \
> >                      /boot/vmlinux-$(shell uname -r)
> > -VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
> > +
> > +# Paths to BTF information.
> > +VMLINUX_BTF_BTF_PATHS ?= /sys/kernel/btf/vmlinux
> > +
> > +# Filter out kernels that don't exist or without a BTF section.
> > +VMLINUX_BTF_ELF_ABSPATHS ?= $(abspath $(wildcard $(VMLINUX_BTF_ELF_PATHS)))
> > +VMLINUX_BTF_PATHS ?= $(shell for file in $(VMLINUX_BTF_ELF_ABSPATHS); \
> > +                       do \
> > +                               if [ -f $$file ] && ($(READELF) -t "$$file" | grep .BTF); \
>
> Wouldn't it be `readelf -S` instead?  Also I think grep needs -q to
> suppress output.

Makes sense, I can change it in v3.

> > +                               then \
> > +                                       echo "$$file"; \
> > +                               fi; \
> > +                       done) \
> > +                       $(wildcard $(VMLINUX_BTF_BTF_PATHS))
>
> This changes the order of processing the sysfs file.
> But I'm not sure it matters much as both /boot/vmlinux and sysfs
> should refer to the running kernel.

Agreed. readelf fails for /sys/kernel/btf/vmlinux as it isn't an elf
file and I'm not sure it is worth worrying too much about the order
here.

Thanks,
Ian

> Thanks,
> Namhyung
>
>
> > +
> > +# Select the first as the source of vmlinux.h.
> > +VMLINUX_BTF ?= $(firstword $(VMLINUX_BTF_PATHS))
> >
> >  $(SKEL_OUT)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
> >  ifeq ($(VMLINUX_H),)
> > --
> > 2.41.0.rc0.172.g3f132b7071-goog
> >