[Gábor Gombás] Bug#564566: libkadm5clnt7: SONAME conflict with Heimdal

[Nicolas Williams]

On Mon, Jan 11, 2010 at 12:24:54PM -0500, Tom Yu wrote:
> Luke Howard <lukeh at padl.com> writes:
> > On 11/01/2010, at 5:58 PM, Nicolas Williams wrote:
> >> Provided the libraries have different install locations, programs
> >> properly use rpath, and that the Heimdal and MIT libraries need never be
> >> loaded in the same process at once, then SONAME conflicts are not a
> >> problem.
> >> 
> >> However, with plug-in interfaces galore it's very likely that both will
> >> be loaded in the same process at some point.
> >
> > Right, but RTLD_LOCAL should be used (HDBBridge uses this to have Heimdal and MIT libraries in the KDC).
> 
> How well do various platforms cope with maintaining isolation between
> shared libraries having the same SONAME but different contents?  If
> the libraries with identical SONAMEs are run-time loaded with
> RTLD_LOCAL and RTLD_GROUP?  If they are link-time dependencies of
> intermediate libraries?  (Does Linux support RTLD_GROUP yet?)

I believe the Solaris RTLD indexes a running process' link map by
SONAME.  If two objects have the same SONAME then only one will be
loaded, regardless of -B group/local or RTLD_GROUP/LOCAL use.

For example, if an executable is linked with -lfoo and later it
dlopen()s an object that too is linked with -lfoo, and both, the
executable and the shared object both record the same SONAME for that
dependency, then only one copy of libfoo will be brought in.  Same if
you have two shared objects linked with -lfoo.

After all dependencies are recorded as SONAMEs, not paths to objects.
The closest to a path you'll come is via a search path.  Thus when
loading an object the RTLD will see if its dependencies are already
loaded.  Even with RTLD_GROUP/LOCAL the RTLD will load only a single
instance of any object.  RTLD_GROUP/LOCAL affect symbol visibility
scope, not instance count (one object loaded twice as a local group will
have a single set of global variables).

I believe much the same applies to the Linux RTLD (except that it lacks
RTLD_GROUP, IIRC).

(There is a way to load multiple instances of the same SONAME, but that
involves using dlmopen(3C), and I don't think anyone really tries it.
It means having multiple C runtimes in the same process, and that
requires a lot of care.)

However, just to be safe I've asked the Solaris linker team, and when I
get an answer I send an update.

Nico
--