Re: [PATCH] selftests: bpf: Support dynamic linking LLVM if static not available

[Yonghong Song <yonghong.song@linux.dev>]

On 2/1/25 12:23 AM, Daniel Xu wrote:
> Hi Yonghong,
>
> On Thu, Jan 30, 2025 at 10:28:11PM -0800, Yonghong Song wrote:
>>
>>
>> On 1/30/25 2:33 PM, Daniel Xu wrote:
>>> Since 67ab80a01886 ("selftests/bpf: Prefer static linking for LLVM
>>> libraries"), only statically linking test_progs is supported. However,
>>> some distros only provide a dynamically linkable LLVM.
>>>
>>> This commit adds a fallback for dynamically linking LLVM if static
>>> linking is not available. If both options are available, static linking
>>> is chosen.
>>>
>>> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
>>> ---
>>>    tools/testing/selftests/bpf/Makefile | 11 ++++++++---
>>>    1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>>> index 6722080b2107..da514030a153 100644
>>> --- a/tools/testing/selftests/bpf/Makefile
>>> +++ b/tools/testing/selftests/bpf/Makefile
>>> @@ -184,9 +184,14 @@ ifeq ($(feature-llvm),1)
>>>      LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
>>>      # both llvm-config and lib.mk add -D_GNU_SOURCE, which ends up as conflict
>>>      LLVM_CFLAGS  += $(filter-out -D_GNU_SOURCE,$(shell $(LLVM_CONFIG) --cflags))
>>> -  LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>> -  LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>> -  LLVM_LDLIBS  += -lstdc++
>>> +  # Prefer linking statically if it's available, otherwise fallback to shared
>>> +  ifeq ($(shell $(LLVM_CONFIG) --link-static --libs &> /dev/null && echo static),static)
>>> +    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>> +    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>> +    LLVM_LDLIBS  += -lstdc++
>>> +  else
>>> +    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-shared --libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>> +  endif
>>>      LLVM_LDFLAGS += $(shell $(LLVM_CONFIG) --ldflags)
>>>    endif
>> Although your change looks good, but maybe you can look at bpftool Makefile?
>>
>>    # If LLVM is available, use it for JIT disassembly
>>    CFLAGS  += -DHAVE_LLVM_SUPPORT
>>    LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
>>    # llvm-config always adds -D_GNU_SOURCE, however, it may already be in CFLAGS
>>    # (e.g. when bpftool build is called from selftests build as selftests
>>    # Makefile includes lib.mk which sets -D_GNU_SOURCE) which would cause
>>    # compilation error due to redefinition. Let's filter it out here.
>>    CFLAGS  += $(filter-out -D_GNU_SOURCE,$(shell $(LLVM_CONFIG) --cflags))
>>    LIBS    += $(shell $(LLVM_CONFIG) --libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>    ifeq ($(shell $(LLVM_CONFIG) --shared-mode),static)
>>      LIBS += $(shell $(LLVM_CONFIG) --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
>>      LIBS += -lstdc++
>>    endif
>>    LDFLAGS += $(shell $(LLVM_CONFIG) --ldflags)
>>
>> It would be great if the selftests shared library handling to be the same as bpftool's.
> So bpftool is both an internally consumed (from selftests) dependency as
> well as a tool packaged up by distros. For the latter case, distros
> prefer dynamic linking.

I hacked llvm to have both static and shared libraries installed and indeed
`llvm-config --shared-mode` prefers shared mode.

So yes, your existing change looks good. Thanks.

>
> So unfortunately, I think these probably need to be defined separately.
> The code looks similar but the use cases are different.
>
> Thanks,
> Daniel