Proposal to reduce anti-bundling requirements

[Carlos O'Donell]

On 09/14/2015 08:29 AM, Florian Weimer wrote:
> On 09/11/2015 04:34 PM, Carlos O'Donell wrote:
> 
>>> How do you propose to resolve symbol conflicts if both the system
>>> library and the bundled library are loaded into the same process?  The
>>> current ELF linking scheme lacks good support for bundling libraries
>>> whose exported symbols have not been mangled in some way.
> 
>> If you need different versions of the same shared libraries in the same
>> process then you must use dlmopen to load the conflicting set into a
>> distinct load namespace.
>>
>> At present dlmopen is not functional for this purpose in upstream
>> glibc master. It is something that I'm working on fixing [1].
> 
> I looked at the Solaris documentation, and I'm not sure if it's the
> right use-case.  This seems to provide complete isolation, and would
> break things like SQLite (at least older versions without file-private
> locks) which need process-global state.

Correct, with dlmopen you have complete isolation, but I don't understand
the SQLite issue, so if you could expand on the requirement I'm happy
to listen.

> I think the real issue here is the ELF model with backwards/forwards
> linking and symbol interposition.  Ideally, we should load each DSO
> exactly once, resolve object symbols only against explicit DT_NEEDED
> dependencies (not indirect dependencies), and make most symbols
> non-interposable by default.  At least this is my gut feeling.  This is
> a very difficult problem, especially at distribution scale.

I'm not sure how difficult this would be because I'm not sure how many
symbols rely upon indirect dependencies. I think it would be a worthwhile
cleanup to turn on something like you suggest, and attempt to bootstrap
the OS using Fedora Bootstrap [1].

> We currently do not perform proper symbol namespace management in Fedora
> (as we discussed before).  Perhaps we should try to track DSO symbol
> namespaces first, and use that data to guide further evolution of
> dynamic linking.
 
Agreed. We do indeed need some infrastructure in tools to extract all
symbols out of the entire distribution and review them.

Cheers,
Carlos.

[1] http://fedora-bootstrap.osop.rhcloud.com/ (currently down?)