linux-kernel - Re: [PATCH 1/1] selftests: timers: use LDLIBS instead of LDFLAGS to link against libpthread

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Date:   Wed, 28 Jun 2017 12:04:56 -0400
From:   Denys Dmytriyenko <denys@...com>
To:     Fathi Boudra <fathi.boudra@...aro.org>
CC:     Shuah Khan <shuah@...nel.org>, <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH 1/1] selftests: timers: use LDLIBS instead of LDFLAGS
 to link against libpthread

On Wed, Jun 28, 2017 at 06:56:30PM +0300, Fathi Boudra wrote:
> On 28 June 2017 at 18:11, Shuah Khan <shuah@...nel.org> wrote:
> > On 06/28/2017 09:00 AM, Fathi Boudra wrote:
> >> On 28 June 2017 at 17:41, Shuah Khan <shuah@...nel.org> wrote:
> >>> On 06/28/2017 08:37 AM, Fathi Boudra wrote:
> >>>> On 28 June 2017 at 17:20, Shuah Khan <shuah@...nel.org> wrote:
> >>>>> On 06/28/2017 03:18 AM, Fathi Boudra wrote:
> >>>>>> timers test fails to build due to undefined reference:
> >>>>>>
> >>>>>>  aarch64-linaro-linux-gcc --sysroot=/build/tmp-rpb-glibc/sysroots/hikey
> >>>>>>  -O2 -pipe -g -feliminate-unused-debug-types -O3 -Wl,-no-as-needed -Wall
> >>>>>>  -DKTEST  -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -lrt -lpthread
> >>>>>>  posix_timers.c
> >>>>>>  -o /build/tmp-rpb-glibc/work/hikey-linaro-linux/kselftests/4.12-r0/linux-4.12-rc7/tools/testing/selftests/timers/posix_timers
> >>>>>>  /tmp/cc1FTZzT.o: In function `check_timer_create':
> >>>>>>  /usr/src/debug/kselftests/4.12-r0/linux-4.12-rc7/tools/testing/selftests/timers/posix_timers.c:157:
> >>>>>>  undefined reference to `timer_create'
> >>>>>>  /usr/src/debug/kselftests/4.12-r0/linux-4.12-rc7/tools/testing/selftests/timers/posix_timers.c:170:
> >>>>>>  undefined reference to `timer_settime'
> >>>>>>  collect2: error: ld returned 1 exit status
> >>>>>>
> >>>>>> Use LDLIBS instead of LDFLAGS to properly link against libpthread and fix
> >>>>>> this build issue.
> >>>>>>
> >>>>>> Signed-off-by: Denys Dmytriyenko <denys@...com>
> >>>>>> Signed-off-by: Fathi Boudra <fathi.boudra@...aro.org>
> >>>>>
> >>>>> I can't reproduce this on linux-kselftest next on x86_64. Is this arm64 speocific
> >>>>> problem? Please try this on linux-kselftest latest.
> >>>>
> >>>> yes, I tried linux-kselftest next and can reproduce.
> >>>> Maybe related to native build vs cross-build?
> >>>> I don't think it's arm64 specific.
> >>>>
> >>>
> >>> Can you confirm that this change doesn't cause problems in native
> >>> build environments.
> >>
> >> yes, I tested on x86 in native build environment as well. It builds
> >> and run fine.
> >>
> >> I noticed my patchset for LDFLAGS vs LDLIBS changes don't apply
> >> cleanly on linux-kselftest next branch. I'm happy to respin the serie
> >> on top of next branch if you're willing to take them.
> >>
> >
> > Yeah. I knew your series won't apply on top of linux-kselftest next just
> > looking at that patches. Hence the question whether or not these problems
> > have been reproduced on linux-kselftest.
> >
> > Please redo the patches. The net patch needs to be done on top of
> >
> > SeongJae Park's fix
> >
> > selftest/net/Makefile: Specify output with $(OUTPUT)
> >
> > btw - with the the above fix, I don't see any failures on x86_64.
> > I am curious why it fails in cross-build? Could you figure out if
> > your cross-build environment is good.
> 
> I'm using openembedded to cross-build a full distro, not only kselftest.
> I'm confident my cross-build environment is good.
> 
> The issue encountered is the same as
> https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?h=next&id=dc816e5d84cbc28baf776799839a632d6002236f
> 
> Using LDFLAGS vs LDLIBS affects the ordering of the linker flags.
> * with LDFLAGS
> aarch64-linux-gnu-gcc -lrt -lpthread posix_timers.c -o posix_timers
> * with LDLIBS
> aarch64-linux-gnu-gcc posix_timers.c -lrt -lpthread -o posix_timers

It's GNU Make and linker specific, not cross vs. native.

The default Makefile rule looks like:

$(CC) $(CFLAGS) $(LDFLAGS) $@ $^ $(LDLIBS)

When linking is done by gcc itself, no issue, but when it needs to be passed 
to proper ld, only LDLIBS follows and then ld cannot know what libs to link 
with.

More detail:
https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

LDFLAGS
Extra flags to give to compilers when they are supposed to invoke the linker, 
‘ld’, such as -L. Libraries (-lfoo) should be added to the LDLIBS variable 
instead.

LDLIBS
Library flags or names given to compilers when they are supposed to invoke the 
linker, ‘ld’. LOADLIBES is a deprecated (but still supported) alternative to 
LDLIBS. Non-library linker flags, such as -L, should go in the LDFLAGS 
variable.

https://lkml.org/lkml/2010/2/10/362

tools/perf: libraries must come after objects

Link order matters, especially now since distributions are
using -Wl,--as-needed.

-- 
Denys