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

[Jakub Sitnicki <jakub@cloudflare.com>]

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])
 	$(call msg,MOD,,$@)