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

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Message-ID: <rgcdc7zokwfoars7c2pzredogea3rvolbnzkvko7q6lbgjnvfx@oeyzed5zalpb>
Date: Sat, 1 Feb 2025 01:23:14 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: Yonghong Song <yonghong.song@...ux.dev>
Cc: shuah@...nel.org, andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, 
	nathan@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev, song@...nel.org, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, 
	jolsa@...nel.org, mykolal@...com, ndesaulniers@...gle.com, morbo@...gle.com, 
	justinstitt@...gle.com, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] selftests: bpf: Support dynamic linking LLVM if static
 not available

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@...uu.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.

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

Thanks,
Daniel