Re: [dpdk-dev] [PATCH 1/2] mk: prevent overlinking in applications

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 6/9/2016 11:10 AM, Thomas Monjalon wrote:
> Hi Ferruh,
> 
> 2016-05-27 17:48, Ferruh Yigit:
>> Replace --no-as-needed linker flag with --as-needed flag, which will
>> only link libraries directly called by application. This requires inter
>> library dependencies resolved correctly.
>>
>> Not linking all libraries cause a compile error for lpcap and possible
>> to have other similar compiler errors, so increasing the scope of
>> --start-group argument.
> 
> What is the error?
> 
This is with -as-needed flag, and static library compilation:

.../dpdk/build/lib/librte_pmd_pcap.a(rte_eth_pcap.o): In function
`eth_dev_stop':
rte_eth_pcap.c:(.text+0xcc1): undefined reference to `pcap_dump_close'
rte_eth_pcap.c:(.text+0xcd6): undefined reference to `pcap_close'
rte_eth_pcap.c:(.text+0xd19): undefined reference to `pcap_close'
rte_eth_pcap.c:(.text+0xd58): undefined reference to `pcap_close'
/root/development/dpdk/build/lib/librte_pmd_pcap.a(rte_eth_pcap.o): In
function `open_tx_pcap':
rte_eth_pcap.c:(.text+0x190b): undefined reference to `pcap_open_dead'
rte_eth_pcap.c:(.text+0x191b): undefined reference to `pcap_dump_open'
...

pcap pmd has libpcap calls, but while linking final application
(testpmd), linker is not able to find objects that has these.

The command line for compilation is:
gcc ... -o test ... -Wl,--whole-archive <dpdk libs> -Wl,-lpcap
-Wl,--start-group <more dpdk libs> -Wl,-lrte_pmd_pcap <more dpdk libs>
-Wl,--end-group -Wl,--no-whole-archive

-lpcap is provided, but since before -lrte_pmd_pcap, references not
resolved.

Previously, with "-no-as-needed" flag, all shared libraries linked
always, which was preventing compile error.


>> cmdline_test application causes compile error because of cyclic
>> dependency between librte_eal <-> librte_mempool. A workaround added to
>> cmdline_test for compile error.
>>
>> Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
> 
>> --- a/app/cmdline_test/Makefile
>> +++ b/app/cmdline_test/Makefile
>> @@ -46,6 +46,7 @@ SRCS-y += commands.c
>>  
>>  CFLAGS += -O3
>>  CFLAGS += $(WERROR_FLAGS)
> 
> A comment is required here to explain the workaround.
> 

Sure, I will add a comment and send a new version of the patch.

>> +LDFLAGS += -no-as-needed
> 
> The option should be --no-as-needed.
Right

> 
>> --- a/mk/exec-env/linuxapp/rte.vars.mk
>> +++ b/mk/exec-env/linuxapp/rte.vars.mk
>> @@ -46,7 +46,7 @@ EXECENV_CFLAGS  = -pthread
>>  endif
>>  
>>  # Workaround lack of DT_NEEDED entry
> 
> This comment is obsolete now.
Will remove.

> 
>> -EXECENV_LDFLAGS = --no-as-needed
>> +EXECENV_LDFLAGS = --as-needed
> 
> Why put this option for Linux only?
When this option not defined at all, different compilers behave
different, some has --as-needed as default and cause compilation issues,
I guess that is why for Linux --no-as-needed is forces. I assume BSD
compilers by default use --no-as-needed, so we don't have any
compilation problem.
And it looks like there is no specific reason to not force BSD to
--as-needed, I will test it.

> Shouldn't we restrict this option to app.mk only?
Having --no-needed flag only for app should be OK. Let me test first,
and as a result should we move this to app.mk?

> 
>> --- a/mk/rte.app.mk
>> +++ b/mk/rte.app.mk
>> @@ -58,6 +58,7 @@ _LDLIBS-y += -L$(RTE_SDK_BIN)/lib
>>  #
>>  
>>  _LDLIBS-y += --whole-archive
>> +_LDLIBS-y += --start-group
> 
> --start-group must be used only to solve circular dependencies.
> Ideally we should not use it. I think it's needed only because
> of EAL logs using mempool (must be removed).
No, this is not the reason, please check above lpcap compile error, that
is the reason of this update.

> Why extending it?
> I'm afraid we are masking some issues.
But if we have a convention to add external libraries after dpdk
libraries, that also should solve this issue, but that is more update
than just updating --start-group.

> 
>>  _LDLIBS-$(CONFIG_RTE_LIBRTE_DISTRIBUTOR)    += -lrte_distributor
>>  _LDLIBS-$(CONFIG_RTE_LIBRTE_REORDER)        += -lrte_reorder
>> @@ -111,8 +112,6 @@ _LDLIBS-y                                   += -lcrypto
>>  endif
>>  endif # !CONFIG_RTE_BUILD_SHARED_LIBS
>>  
>> -_LDLIBS-y += --start-group
>> -
>>  _LDLIBS-$(CONFIG_RTE_LIBRTE_KVARGS)         += -lrte_kvargs
>>  _LDLIBS-$(CONFIG_RTE_LIBRTE_MBUF)           += -lrte_mbuf
>>  _LDLIBS-$(CONFIG_RTE_LIBRTE_IP_FRAG)        += -lrte_ip_frag
>