Re: [dpdk-dev] [PATCH] eal: don't use default library path for static binaries

[Thomas Monjalon <thomas@monjalon.net>]

29/06/2020 18:15, Bruce Richardson:
> On Mon, Jun 29, 2020 at 08:41:46AM -0700, Stephen Hemminger wrote:
> > Bruce Richardson <bruce.richardson@intel.com> wrote:
> > > On Mon, Jun 29, 2020 at 10:11:58AM +0100, Bruce Richardson wrote:
> > > > On Fri, Jun 26, 2020 at 10:05:42AM -0700, Stephen Hemminger wrote:  
> > > > > Bruce Richardson <bruce.richardson@intel.com> wrote:
> > > > >   
> > > > > > When using statically linked DPDK binaries, the EAL checks the default PMD
> > > > > > path and tries to load any drivers there, despite the fact that all drivers
> > > > > > are normally linked into the binary.  This behaviour can cause issues if
> > > > > > the PMD path and lib dir is configured to a non-standard location which is
> > > > > > not in the ld.so.conf paths, e.g. a build with prefix set to a home
> > > > > > directory location. In a case such as this, EAL will try and
> > > > > > (unnecessarily) load the .so driver files but that load will fail as their
> > > > > > dependent libraries, such as ethdev, for example, will not be found.
> > > > > > 
> > > > > > Because of this, it is better if statically linked DPDK apps do not load
> > > > > > drivers from the standard paths automatically. The user can always have
> > > > > > this behaviour by explicitly specifying the path using -d flag, if so
> > > > > > desired.
> > > > > > 
> > > > > > Not loading the libraries automatically can also prevent potential issues
> > > > > > with a user building and running a statically-linked DPDK binary based off
> > > > > > a private copy of DPDK, while there exists on the same machine a
> > > > > > system-wide installation of DPDK in the default locations. Without this
> > > > > > change, the system-installed drivers will be loaded to the binary alongside
> > > > > > the statically-linked drivers, which is not what the user would have
> > > > > > intended.
> > > > > > 
> > > > > > To detect whether we are in a statically or dynamically linked binary, we
> > > > > > can have EAL try to get a dlopen handle to its own shared library, by
> > > > > > calling dlopen with the RTLD_NOLOAD flag. This will return NULL if there is
> > > > > > no such shared lib loaded i.e. the code is executing from a static library,
> > > > > > or a handle to the lib if it is loaded.
> > > > > > 
> > > > > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>  
> > > > > 
> > > > > But what if the majority of the DPDK is statically linked but the
> > > > > application wants also load a dynamically linked driver?
> > > > >   
> > > > They use the -d flag as now. The only change here is that we don't
> > > > *automatically* (and silently) attempt to load all drivers from a system
> > > > location when you have a static binary.  
> > > 
> > > I'd also make a couple of additional points:
> > > 1. If you have a static app and you have extra drivers in your EAL_PMD_PATH
> > > directory you have no way of preventing the loading of them.
> > > 
> > > 2. Since all DPDK apps try to load all files in that directory, all one has
> > > to do is put a non-loadable file into the DPDK PMD directory and suddenly
> > > all DPDK apps on the system will fail to run.  [Patchset
> > > http://patches.dpdk.org/project/dpdk/list/?series=10553 will also help
> > > here]
> > > 
> > > 3. Since this is a behaviour change, perhaps it needs to be deferred to
> > > 20.11? Ideally I think we should fix this now, because I think the current
> > > behaviour doesn't make sense and causes more problems than it solves, but
> > > if it needs to be deferred, so be it.
> > 
> > 
> > Thanks, just trying to poke the corners of this change.
> > Please make sure the documentation and web site match the behavior.
> > 
> > Acked-by: Stephen Hemminger <stephen@networkplumber.org>
> 
> This appears to be a gap, since I looked through the docs and couldn't find
> any reference to this behaviour at all. However, I may well have missed
> something so please flag if so.

Yes we are lacking some doc around the basics of EAL (and also ethdev).
This is a behaviour change, but I prefer applying it in 20.08,
so we can get some user feedback before the LTS release.

Applied, thanks