Re: -z defs linker flag activated in Fedora rawhide

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On 25/01/18 15:58, Florian Weimer wrote:
On 01/25/2018 04:37 PM, Petr Pisar wrote:
On 2018-01-25, Daniel P  Berrangé <berrange@xxxxxxxxxx> wrote:
Not neccessarily - with perl, the APIs used by extensions are actually
in libperl.so, not /usr/bin/perl, and the extensions link to libperl.so
So perl binary modules ought to still build without undefined symbols,
as IIUC they're not relying on things in /usr/bin/perl

Perl fails too
<https://kojipkgs.fedoraproject.org/work/tasks/5805/24425805/build.log>.

While extensions link to libperl.so that itself links to libpthread.so
that defines pthread_getspecific symbol, the extensions linking fails
because they use dTHX macro, provided by included thread.h via perl.h,
that expands to pthread_getspecific.

In other words it looks like -z defs requires that all used symbols
are provided by directly linked libraries. I.e. we will have to patch
Perl so that extensions add -lptrhread to the linker flags.

Interesting.  This leads to:

$ eu-readelf --symbols=.dynsym /usr/lib64/perl5/auto/POSIX/POSIX.so | grep specific    52: 0000000000000000      0 NOTYPE  GLOBAL DEFAULT    UNDEF pthread_getspecific

So the symbol version is missing, and there is no guarantee for compatibility with future glibc versions. 8-(

This is exactly the kind of problem -z defs intends to catch.

There seems to be a similar issue with perl extensions that use C++ failing to link due to missing libstdc++ symbols, eg:

https://koji.fedoraproject.org/koji/taskinfo?taskID=24412387

I think in that case the issue is that libgdal is linked to libstdc++ but the perl GDAL.so module was wound up with direct references to symbols in libstdc++ but isn't linking to it directly.

So I think that needs a fix in gdal rather than changing the default libraries linked by perl.

In fact I guess the link should really use g++ not gcc? Not sure how easy that is to do in a perl extension build...

Tom

--
Tom Hughes (tom@xxxxxxxxxx)
http://compton.nu/
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