[TUHS] History of symbol preemption

[Rico Pajarola]

On Mon, Jan 13, 2020 at 1:04 PM Clem Cole <clemc at ccc.com> wrote:

> @ Rico I'm failing sure ELF came from AT&T Summit, not Sun.
>
yes, but unless my memory is playing tricks, SunOS a.out had this feature.


> @ Steve Johnson were you the manager when was created or were you folks
> still using COFF?
>
> Anyway... There were issues with COFF WRT being
> architecture-independent and supporting dynamic loading well.  Steve Rago
> would also be a good person to ask if you want some of the details.  At one
> point there was a COFF2 document, but it may have been System Vx licenses
> only.   Also, one of the issues was that AT&T had officially tied up COFF
> as a proprietary format -- all part of the 'consider it standard' trying to
> force their lunch down all the other UNIX systems throat which was not
> having it.   As a result, CMU's MachO was about to become the default
> format (OSF and Apple were already using it for that reason), and Unix
> International stepped in and convinced AT&T to released the ELF documents
> (I was on the UI technical board at that point).  I'm not sure how/why OSF
> decided to back off, maybe because after ELF became public it got supported
> by GCC.
>
> Now my memory is a little hazy... I think OSF/1-386 used MachO originally,
> but I've forgotten.   Switching the kernel to use ELF was one of the
> differences between OSF1 and Tru64 IIRC.
>
> On Mon, Jan 13, 2020 at 3:47 PM Rico Pajarola <rp at servium.ch> wrote:
>
>> This seems to have originated with SunOS 4. I believe a good proxy for
>> finding anything that inherited from or was inspired by this is a linker
>> that recognizes LD_PRELOAD. I wonder if there are other independent
>> implementations in the Unix space.
>>
>>
>> On Mon, Jan 13, 2020 at 11:59 AM Paul Winalski <paul.winalski at gmail.com>
>> wrote:
>>
>>> The Executable and Linkable Format (ELF) is the modern standard for
>>> object files in Unix and Unix-like OSes (e.g., Linux), and even for
>>> OpenVMS.  LInux, AIX and probably other implementations of ELF have a
>>> feature in the runtime loader called symbol preemption.  When loading
>>> a shared library, the runtime loader examines the library's symbol
>>> table.  If there is a global symbol with default visibility, and a
>>> value for that symbol has already been loaded, all references to the
>>> symbol in the library being loaded are rebound to the existing
>>> definition.  The existing value thus preempts the definition in the
>>> library.
>>>
>>> I'm curious about the history of symbol preemption.  It does not exist
>>> in other implementations of shared libraries, such as IBM OS/370 and
>>> its descendants, OpenVMS, and Microsoft Windows NT.  ELF apparently
>>> was designed in the mid-1990s.  I have found a copy of the System V
>>> Application Binary Interface from April 2001 that describes symbol
>>> preemption in the section on the ELF symbol table.
>>>
>>> When was symbol preemption when loading shared objects first
>>> implemented in Unix?  Are there versions of Unix that don't do symbol
>>> preemption?
>>>
>>> -Paul W.
>>>
>>
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