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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: Build Python 3 to statically link with libpython3.8.a for
From:       Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= <berrange () redhat ! com>
Date:       2019-11-15 15:30:54
Message-ID: 20191115153054.GC2529156 () redhat ! com
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On Fri, Nov 15, 2019 at 09:51:45AM -0500, David Malcolm wrote:
> On Fri, 2019-11-15 at 12:31 +0000, Daniel P. Berrangé wrote:
> > On Fri, Nov 15, 2019 at 01:23:09PM +0100, Miroslav Suchý wrote:
> > > Dne 15. 11. 19 v 10:21 Victor Stinner napsal(a):
> > > > I'm not sure if we need a Fedora change just for a compiler flag.
> > > > Again, the only drawback is that we will no longer be able to
> > > > override a symbol using LD_PRELOAD. Honestly, I never did that. I
> > > > don't see any use case for that. But I used LD_PRELOAD on the
> > > > libc multiple times to mock the system clock for example.
> > > > 
> > > > If someone really needs LD_PRELOAD, it's quite easy to build a
> > > > custom Python without -fno-semantic-interposition.
> > > 
> > > Mock's Nosync plugin use LD_PRELOAD:
> > > https://github.com/rpm-software-management/mock/wiki/Feature-nosync
> > 
> > IIUC mock would not be affected by this change.
> > 
> > The LD_PRELOAD limitation described applies to symbols that are in
> > the libpython.so library.
> > 
> > Those docs suggest mock is replacing the fsync() API in glibc with
> > its
> > LD_PRELOAD, so that should continue to work as normal.
> > 
> > Regards,
> > Daniel
> 
> Thinking aloud: does anyone ever use symbol overriding for anything
> other than glibc?
> 
> What would it do to distro-wide performance if
>   -fno-semantic-interposition
> were added to the default rpm build flags, (and glibc added -fsemantic-
> interposition to override this)?
> 
> Basically, change the default distro-wide to libraries opting-in to
> being able to be interposed, rather than opting-out (-fsemantic-
> interposition appears to be on by default, looking at the source for
> gcc).
> 
> Would other workloads get benefit?  How much would break?

It'd break libvirt's entire test suite. We rely on being able to
mock symbols inside libvirt.so, as well as libc, for unit testing.

Regards,
Daniel
-- 
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