perf build: Fix check for btf__load_from_kernel_by_id() in libbpf

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: 218e7b7 ("perf bpf: Provide a weak btf__load_from_kernel_by_id() for older libbpf versions")
Reported-by: Avi Kivity <[email protected]>
Link: https://lore.kernel.org/linux-perf-users/[email protected]
Cc: Adrian Hunter <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/linux-perf-users/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: acmel

tools/build/Makefile.feature

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

tools/build/feature/Makefile

@@ -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)
 

tools/build/feature/test-libbpf-btf__load_from_kernel_by_id.c

@@ -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);
+}

tools/perf/Makefile.config

@@ -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
 

tools/perf/util/bpf-event.c

@@ -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,