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perf build: Fix check for btf__load_from_kernel_by_id() in libbpf

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Avi Kivity reported a problem where the __weak
btf__load_from_kernel_by_id() in tools/perf/util/bpf-event.c was being
used and it called btf__get_from_id() in tools/lib/bpf/btf.c that in
turn called back to btf__load_from_kernel_by_id(), resulting in an
endless loop.

Fix this by adding a feature test to check if
btf__load_from_kernel_by_id() is available when building perf with
LIBBPF_DYNAMIC=1, and if not then provide the fallback to the old
btf__get_from_id(), that doesn't call back to btf__load_from_kernel_by_id()
since at that time it didn't exist at all.

Tested on Fedora 35 where we have libbpf-devel 0.4.0 with LIBBPF_DYNAMIC
where we don't have btf__load_from_kernel_by_id() and thus its feature
test fail, not defining HAVE_LIBBPF_BTF__LOAD_FROM_KERNEL_BY_ID:

  $ cat /tmp/build/perf-urgent/feature/test-libbpf-btf__load_from_kernel_by_id.make.output
  test-libbpf-btf__load_from_kernel_by_id.c: In function ‘main’:
  test-libbpf-btf__load_from_kernel_by_id.c:6:16: error: implicit declaration of function ‘btf__load_from_kernel_by_id’ [-Werror=implicit-function-declaration]
      6 |         return btf__load_from_kernel_by_id(20151128, NULL);
        |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  $

  $ nm /tmp/build/perf-urgent/perf | grep btf__load_from_kernel_by_id
  00000000005ba180 T btf__load_from_kernel_by_id
  $

  $ objdump --disassemble=btf__load_from_kernel_by_id -S /tmp/build/perf-urgent/perf

  /tmp/build/perf-urgent/perf:     file format elf64-x86-64
  <SNIP>
  00000000005ba180 <btf__load_from_kernel_by_id>:
  #include "record.h"
  #include "util/synthetic-events.h"

  #ifndef HAVE_LIBBPF_BTF__LOAD_FROM_KERNEL_BY_ID
  struct btf *btf__load_from_kernel_by_id(__u32 id)
  {
    5ba180:	55                   	push   %rbp
    5ba181:	48 89 e5             	mov    %rsp,%rbp
    5ba184:	48 83 ec 10          	sub    $0x10,%rsp
    5ba188:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    5ba18f:	00 00
    5ba191:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    5ba195:	31 c0                	xor    %eax,%eax
         struct btf *btf;
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
         int err = btf__get_from_id(id, &btf);
    5ba197:	48 8d 75 f0          	lea    -0x10(%rbp),%rsi
    5ba19b:	e8 a0 57 e5 ff       	call   40f940 <btf__get_from_id@plt>
    5ba1a0:	89 c2                	mov    %eax,%edx
  #pragma GCC diagnostic pop

         return err ? ERR_PTR(err) : btf;
    5ba1a2:	48 98                	cltq
    5ba1a4:	85 d2                	test   %edx,%edx
    5ba1a6:	48 0f 44 45 f0       	cmove  -0x10(%rbp),%rax
  }
  <SNIP>

Fixes: 218e7b775d ("perf bpf: Provide a weak btf__load_from_kernel_by_id() for older libbpf versions")
Reported-by: Avi Kivity <avi@scylladb.com>
Link: https://lore.kernel.org/linux-perf-users/f0add43b-3de5-20c5-22c4-70aff4af959f@scylladb.com
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/linux-perf-users/YobjjFOblY4Xvwo7@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Arnaldo Carvalho de Melo 2022-05-19 21:25:12 -03:00
parent 3d7285a335
commit 0ae065a5d2
5 changed files with 22 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
tools/build/Makefile.feature
View File

@ -98,6 +98,7 @@ FEATURE_TESTS_EXTRA := \
llvm-version \
clang \
libbpf \
libbpf-btf__load_from_kernel_by_id \
libpfm4 \
libdebuginfod \
clang-bpf-co-re

4
tools/build/feature/Makefile
View File

@ -57,6 +57,7 @@ FILES= \
test-lzma.bin \
test-bpf.bin \
test-libbpf.bin \
test-libbpf-btf__load_from_kernel_by_id.bin \
test-get_cpuid.bin \
test-sdt.bin \
test-cxx.bin \
@ -287,6 +288,9 @@ $(OUTPUT)test-bpf.bin:
$(OUTPUT)test-libbpf.bin:
$(BUILD) -lbpf
$(OUTPUT)test-libbpf-btf__load_from_kernel_by_id.bin:
$(BUILD) -lbpf
$(OUTPUT)test-sdt.bin:
$(BUILD)

7
tools/build/feature/test-libbpf-btf__load_from_kernel_by_id.c Normal file
View File

@ -0,0 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
#include <bpf/libbpf.h>
int main(void)
{
return btf__load_from_kernel_by_id(20151128, NULL);
}

7
tools/perf/Makefile.config
View File

@ -553,9 +553,16 @@ ifndef NO_LIBELF
ifeq ($(feature-libbpf), 1)
EXTLIBS += -lbpf
$(call detected,CONFIG_LIBBPF_DYNAMIC)
$(call feature_check,libbpf-btf__load_from_kernel_by_id)
ifeq ($(feature-libbpf-btf__load_from_kernel_by_id), 1)
CFLAGS += -DHAVE_LIBBPF_BTF__LOAD_FROM_KERNEL_BY_ID
endif
else
dummy := $(error Error: No libbpf devel library found, please install libbpf-devel);
endif
else
CFLAGS += -DHAVE_LIBBPF_BTF__LOAD_FROM_KERNEL_BY_ID
endif
endif

4
tools/perf/util/bpf-event.c
View File

@ -22,7 +22,8 @@
#include "record.h"
#include "util/synthetic-events.h"
struct btf * __weak btf__load_from_kernel_by_id(__u32 id)
#ifndef HAVE_LIBBPF_BTF__LOAD_FROM_KERNEL_BY_ID
struct btf *btf__load_from_kernel_by_id(__u32 id)
{
struct btf *btf;
#pragma GCC diagnostic push
@ -32,6 +33,7 @@ struct btf * __weak btf__load_from_kernel_by_id(__u32 id)
return err ? ERR_PTR(err) : btf;
}
#endif
int __weak bpf_prog_load(enum bpf_prog_type prog_type,
const char *prog_name __maybe_unused,