[dpdk-dev] [dpdk-stable] [PATCH] app: fix plugin load on static builds

[Bruce Richardson]

On Thu, Nov 26, 2020 at 06:07:32PM +0100, Thomas Monjalon wrote:
> 26/11/2020 17:37, Bruce Richardson:
> > On Thu, Nov 26, 2020 at 05:32:26PM +0100, Olivier Matz wrote:
> > > Hi Bruce,
> > > 
> > > Thanks for the feedback.
> > > 
> > > On Thu, Nov 26, 2020 at 03:28:46PM +0000, Bruce Richardson wrote:
> > > > On Thu, Nov 26, 2020 at 03:20:42PM +0100, Olivier Matz wrote:
> > > > > When dpdk is compiled as static libraries, it is not possible
> > > > > to load a plugin from an application. We get the following error:
> > > > > 
> > > > >   EAL: librte_pmd_xxxx.so: undefined symbol: per_lcore__rte_errno
> > > > > 
> > > > > This happens because the dpdk symbols are not exported. Add them to the
> > > > > dynamic symbol table by using '-Wl,--export-dynamic'. This option was
> > > > > previously present when compiled with Makefiles, it was introduced in
> > > > > commit f9a08f650211 ("eal: add support for shared object drivers")
> > > > > 
> > > > > Fixes: 16ade738fd0d ("app/testpmd: build with meson")
> > > > > Fixes: 89f0711f9ddf ("examples: build some samples with meson")
> > > > > Cc: stable at dpdk.org
> > > > > 
> > > > > Signed-off-by: Olivier Matz <olivier.matz at 6wind.com>
> > > > > ---
> > > > > 
> > > > > Hi,
> > > > > 
> > > > > Maybe the patch can be improved: I suppose that --export-dynamic
> > > > > should only be added in case we are linking in static mode. Any
> > > > > help about how to do that is welcome.
> > > > 
> > > > get_option('default_library') == 'static'
> > > > 
> > > > However, if the flag is harmless in shared linking mode, I don't think we
> > > > need bother with this.
> > > 
> > > ok
> > > 
> > > > > 
> > > > > There is probably a better place to define the default ldflags for
> > > > > all applications (to factorize between app and example).
> > > > > 
> > > > > Also, should this flag be advertised in pkg-config?
> > > > > 
> > > > Perhaps. However, I'm not sure how common it would be for people to static
> > > > link their own apps with DPDK and then want to plug in extra drivers after
> > > > the fact? Are there any possible negative impacts to making that change?
> > > 
> > > I don't know if it is common, but this is something we do :)
> > > 
> > > > If we weren't right before the release deadline I'd definitely suggest
> > > > adding it to the pkg-config files. At this late stage in release, I'm more
> > > > cautious.
> > > 
> > > Yes, it is too late for 20.11. Maybe even for this patch without
> > > updating pkg-config.
> > > 
> > 
> > Well, since this is something clearly broken that is of use to you, I think
> > we should strive to get some fix for it in. Based on the lateness of the
> > hour, I think your patch is pretty close to the least-risky option we could
> > take here. Therefore, despite my previous comment about not needing to
> > limit the additional linking flag to static-builds only, I would suggest -
> > out of an abundance of caution - that we do so, and keep the rest of your
> > patch as-is for 20.11. Thereafter we can look at other possible approaches
> > and simplifications in 21.02 and backport them.
> > 
> > So, for your patch with "if get_option('default_library') ..." checks added
> > appropriately:
> > 
> > Acked-by: Bruce Richardson <bruce.richardson at intel.com>
> 
> The most critical issue is for externally built applications
> statically linked using external plugins.
> This patch fixes only internal apps built with meson.
> 
> I am for keeping the issue as-is for 20.11.0,
> because it is really too late to change a compilation option.
> We have already been bitten by compilers not supporting an option
> or being buggy in specific contexts. I prefer not taking this risk
> for a fix which helps nobody really.
> 
I'm ok with that fairly conservative stance.

In general the workaround for this issue is to set the appropriate ldflags
when building. For those building their own app, I would assume adding the
flag can be fairly easily done, and for anyone building their own DPDK who
needs it, it can be set via "-Dc_link_args" meson flag. Anyone using
pre-built DPDK from packagers will already have a shared library build
anyway.

/Bruce