Re: [PATCH bpf-next] libbpf: fix internal USDT address translation logic for shared libraries

Jakub Sitnicki
2022-07-20T18:37:06Z

On Wed, Jul 20, 2022 at 05:32 PM +02, Jakub Sitnicki wrote:

[...]

>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>> index 8ad7a733a505..e08e8e34e793 100644
>> --- a/tools/testing/selftests/bpf/Makefile
>> +++ b/tools/testing/selftests/bpf/Makefile
>> @@ -172,13 +172,15 @@ $(OUTPUT)/%:%.c
>> # do not fail. Static builds leave urandom_read relying on system-wide shared libraries.
>> $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
>> $(call msg,LIB,,$@)
>> - $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) -fPIC -shared -o $@
>> + $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
>> + -fuse-ld=lld -Wl,-znoseparate-code -fPIC -shared -o $@
>>
>> $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
>> $(call msg,BINARY,,$@)
>> - $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
>> - liburandom_read.so $(LDLIBS) \
>> - -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
>> + $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
>> + liburandom_read.so $(LDLIBS) \
>> + -fuse-ld=lld -Wl,-znoseparate-code \
>> + -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
>
> [...]
>
> Not sure if this was considered - adding a dependency on Clang for the
> target platform makes cross-compiling bpf selftests much harder than it
> was.
>
> Maybe we could use $(CLANG) only when not cross-compiling, and execute
> $(CC) like before otherwise?

OTOH, there seems to be nothing Clang specific about the build
recipe. We just want t use LLVM lld. GCC accepts -fuse-ld as well (bfd
vs lld).

Perhaps we could partially revert to something as below? It "fixes" the
cross-compilation build of bpf selftests for arm64 for me.

WDYT?

--8<--
diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 8d59ec7f4c2d..541e2b0de27a 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -170,24 +170,24 @@ $(OUTPUT)/%:%.c

# LLVM's ld.lld doesn't support all the architectures, so use it only on x86
ifeq ($(SRCARCH),x86)
-LLD := lld
+ALT_LD := lld
else
-LLD := ld
+ALT_LD := bfd
endif

# Filter out -static for liburandom_read.so and its dependent targets so that static builds
# do not fail. Static builds leave urandom_read relying on system-wide shared libraries.
$(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
$(call msg,LIB,,$@)
- $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
- -fuse-ld=$(LLD) -Wl,-znoseparate-code -fPIC -shared -o $@
+ $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS) \
+ -fuse-ld=$(ALT_LD) -Wl,-znoseparate-code -fPIC -shared -o $@

$(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
$(call msg,BINARY,,$@)
- $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
- liburandom_read.so $(LDLIBS) \
- -fuse-ld=$(LLD) -Wl,-znoseparate-code \
- -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
+ $(Q)$(CC) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
+ liburandom_read.so $(LDLIBS) \
+ -fuse-ld=$(ALT_LD) -Wl,-znoseparate-code \
+ -Wl,-rpath=. -Wl,--build-id=sha1 -o $@

$(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
$(call msg,MOD,,$@)