Re: Underlinked libs and overlinked applications - an issue?

[Panu Matilainen <pmatilai@redhat.com>]

On 05/19/2016 09:17 PM, Thomas Monjalon wrote:
> 2016-05-19 17:38, Christian Ehrhardt:
>> Hi,
>> I was working on the new 16.04 build system to adapt deb packaging to it.
>> I remember somewhen back in the DPDK 2.2 and shared+combined library days I
>> had some issues with over/underlinking - but it seems those are still
>> existent or came back.
>
> I would say it has always been like that.
> Thanks for raising the issue.
>
>> After a build in almost default config (just disabled the kernel modules)
>> and set RTE_MACHINE to default I find the following.
>>
>> #1
>> The libraries are all only linked against external things - even clearly
>> using internal structures:
>> ldd usr/lib/x86_64-linux-gnu/librte_lpm.so.2
>>        linux-vdso.so.1 =>  (0x00007fff7e7a5000)
>>        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f175d4dd000)
>>        /lib64/ld-linux-x86-64.so.2 (0x0000558d3afbf000)
>
> The DT_NEEDED entries are created only for external dependencies.
> Probably we should create ones for internal dependencies based on the
> variable DEPDIRS-y.

Yes, DT_NEEDED for internal dependencies are needed too, see eg recent 
commits c6417ce61f83 and 8180554d82b3. That was part of Sergios' build 
system improvement patch series last spring or so (but since then 
abandoned), I picked up the work and been doing it in smaller pieces. 
Phase 1 was the external dependencies, phase 2 is internal dependencies 
which I'm working on. Unfortunately health issues have stalled my work a 
lot recently :-/

>> #2
>> The Application then seem to try to make up for that by realizing all that
>> is missing.
>> But looking at the app alone it seems overlinked by that - it is not using
>> all of these on its own.
>> ldd usr/bin/cmdline_test
>>        linux-vdso.so.1 =>  (0x00007ffeec9ea000)
>>        librte_distributor.so.1 => not found
>>        librte_reorder.so.1 => not found
>> [...]
>>        librte_jobstats.so.1 => not found
>> [...]
>> And for example none of the librte_jobstats.so.1 symbols are used
>> "directly" in there.
>
> Yes every libraries are put for every apps in rte.app.mk.
> Probably that we could better use DEPDIRS-y for apps.
>

Another trick from Sergios' original patch series is to utilize 
AS_NEEDED() in the linker script, so it ends up looking something like 
this for shared library config (static would remain as it is now):

INPUT ( librte_eal librte_mempool librte_ring
         AS_NEEDED ( librte_acl librte_timer librte_vhost <...> )
)

...and then use the linker script for linking the apps, which should 
simplify rte.app.mk considerably. Or so the theory goes. Anyway, the 
above requires DT_NEEDED for the internal libraries to be present first, 
but once all these pieces are in place, dynamically linked apps will 
only link to the libraries they actually need.

	- Panu -

	- Panu -