From: Rico Pajarola <rp@servium.ch>
To: Clem Cole <clemc@ccc.com>
Cc: The Eunuchs Hysterical Society <tuhs@tuhs.org>
Subject: Re: [TUHS] History of symbol preemption
Date: Mon, 13 Jan 2020 13:40:35 -0800 [thread overview]
Message-ID: <CACwAiQnGghCWDENNou7+8qndDZC1UfiTHadWKDyt7HcbdH_nYg@mail.gmail.com> (raw)
In-Reply-To: <CAC20D2PsgKMtANSSQ2be4of8VDKCZZ5j7jmQYeGa6N5U532mfA@mail.gmail.com>
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On Mon, Jan 13, 2020 at 1:04 PM Clem Cole <clemc@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@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@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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next prev parent reply other threads:[~2020-01-13 21:41 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-13 19:58 Paul Winalski
2020-01-13 20:46 ` Rico Pajarola
2020-01-13 21:04 ` Clem Cole
2020-01-13 21:40 ` Rico Pajarola [this message]
2020-01-13 21:44 ` Paul Winalski
2020-01-13 21:45 ` Rico Pajarola
2020-01-13 22:20 ` Larry McVoy
2020-01-13 21:42 ` Paul Winalski
2020-01-13 22:53 ` Henry Bent
2020-01-14 0:31 ` Clem Cole
2020-01-14 2:53 ` Rob Gingell
2020-01-14 19:21 ` [TUHS] two AIX items [was " Charles H Sauer
2020-01-14 20:31 ` Clem Cole
2020-01-14 23:22 ` Kevin Bowling
2020-01-15 16:41 ` Paul Winalski
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