Re: [PATCH v2 3/6] perf: build: Only link libebl.a for old libdw

[Leo Yan <leo.yan@arm.com>]

On 6/22/24 00:40, Namhyung Kim wrote:
> 
> On Mon, Jun 10, 2024 at 10:54:30AM +0100, Leo Yan wrote:
>> Since libdw version 0.177, elfutils has merged libebl.a into libdw (see
>> the commit "libebl: Don't install libebl.a, libebl.h and remove backends
>> from spec." in the elfutils repository).
>>
>> As a result, libebl.a does not exist on Debian Bullseye and newer
>> releases, causing static perf builds to fail on these distributions.
> 
> What about libebl.so?  I'm curious why it's ok with dynamic build and
> causing a problem with static builds.

For the new Debian / Ubuntu distros, libebl has been merged in libdw so
merged into libdw, so libebl.so doesn't exist.

'-lebl' is only included only for static building (see below code piece,
it detects '-static' in ${LDFLAGS}).  This is why dynamic build has no
issue.

>> This commit checks the libdw version and only links libebl.a if it
>> detects that the libdw version is older than 0.177.
> 
> Have you tested on the older versions too?

IIRC, I did test on the Debian buster (which contains the libdw version
is 0.176).

>> Signed-off-by: Leo Yan <leo.yan@arm.com>
>> ---
>>   tools/build/feature/Makefile | 12 +++++++++++-
>>   tools/perf/Makefile.config   | 12 +++++++++++-
>>   2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
>> index 084f803093c3..b23b3e8ad5e4 100644
>> --- a/tools/build/feature/Makefile
>> +++ b/tools/build/feature/Makefile
>> @@ -171,7 +171,17 @@ $(OUTPUT)test-libopencsd.bin:
>>
>>   DWARFLIBS := -ldw
>>   ifeq ($(findstring -static,${LDFLAGS}),-static)
>> -DWARFLIBS += -lelf -lebl -lz -llzma -lbz2
>> +  DWARFLIBS += -lelf -lz -llzma -lbz2
>> +
>> +  LIBDW_VERSION := $(shell $(PKG_CONFIG) --modversion libdw)
>> +  LIBDW_VERSION_1 := $(word 1, $(subst ., ,$(LIBDW_VERSION)))
>> +  LIBDW_VERSION_2 := $(word 2, $(subst ., ,$(LIBDW_VERSION)))
>> +
>> +  # Elfutils merged libebl.a into libdw.a starting from version 0.177,
>> +  # Link libebl.a only if libdw is older than this version.
>> +  ifeq ($(shell test $(LIBDW_VERSION_2) -lt 177; echo $$?),0)
>> +    DWARFLIBS += -lebl
> 
> I'm not sure if it's ok to change the order as libebl might depend on
> later libraries like libz.

I confirmed that this change works on Debian Buster (Debian 10) and
Bookworm (Debian 12).

Thanks,
Leo