Remember a.out binaries? They were the file format of the Linux kernel till around 1995 when ELF took over. ELF is better. It allows you to load shared libraries anywhere in memory, while a.out binaries need you to register shared library locations. That's fine at small scales, but it gets to be more and more of a headache as you have more and more shared libraries to deal with. But a.out is still supported in the Linux source tree, 25 years after ELF became the standard default format.

Recently, Borislav Petkov recommended deprecating it in the source tree, with the idea of removing it if it turned out there were no remaining users. He posted a patch to implement the deprecation. Alan Cox also remarked that "in the unlikely event that someone actually has an a.out binary they can't live with, they can also just write an a.out loader as an ELF program entirely in userspace."

Richard Weinberger had no problem deprecating a.out and gave his official approval of Borislav's patch.

In fact, there's a reason the issue happens to be coming up now, 25 years after the fact. Linus Torvalds pointed out:

I'd prefer to try to deprecate a.out core dumping first....That's the part that is actually broken, no? In fact, I'd be happy to deprecate a.out entirely, but if somebody _does_ complain, I'd like to be able to bring it back without the core dumping. Because I think the likelihood that anybody cares about a.out core dumps is basically zero. While the likelihood that we have some odd old binary that is still a.out is slightly above zero. So I'd be much happier with this if it was a two-stage thing where we just delete a.out core dumping entirely first, and then deprecate even running a.out binaries separately. Because I think all the known *bugs* we had were with the core dumping code, weren't they? Removing it looks trivial. Untested patch attached. Then I'd be much happier with your "let's deprecate a.out entirely" as a second patch, because I think it's an unrelated issue and much more likely to have somebody pipe up and say "hey, I have this sequence that generates executables dynamically, and I use a.out because it's much simpler than ELF, and now it's broken". Or something.

Jann Horn looked over Linus' patch and suggested additional elements of a.out that would no longer be used by anything, if core dumping was coming out. He suggested those things also could be removed with the same git commit, without risking anyone complaining.

Borislav was a little doubtful about Linus' approach—as he put it, "who knows what else has bitrotten out there through the years". But, he wasn't so doubtful as to suggest an alternative. Instead, he said to Linus, "the easiest would be if you apply your patch directly now and add the a.out phase-out strategy we're going for in its commit message so that people are aware of what we're doing." Then, he added, the architecture maintainers could each remove a.out core dump support from their architectures on a case by case basis, and then Borislav could continue to deprecate a.out in its entirety later on.

Linus said he'd be fine with that, but he also said he'd be happy to apply Borislav's a.out deprecation patch immediately on top of Linus' core-dump removal patch. He didn't care to have a time delay, so long as the two patches could be reverted independently if anyone squawked about one of them.

At this point, various architecture maintainers started commenting on a.out on their particular architectures.

Geert Uytterhoeven said, "I think it's safe to assume no one still runs a.out binaries on m68k."

And, Matt Turner said, "I'm not aware of a reason to keep a.out support on alpha."

The alpha architecture, however, proved more difficult than Matt initially thought. Linus looked into the port and found a lot of a.out support still remaining. And certain parts of the port, he said, didn't even make sense without a.out support. So there would actually be a lot more gutting to do, in the alpha code, as opposed to a simple amputation.

Måns Rullgård also remarked, "Anyone running an Alpha machine likely also has some old OSF/1 binaries they may wish to use. It would be a shame to remove this feature."

This actually made Linus stop dead in his tracks. He replied to Måns:

If that's the case, then we'd have to keep a.out alive for alpha, since that's the OSF/1 binary format (at least the only one we support - I'm not sure if later versions of OSF/1 ended up getting ELF). Which I guess we could do, but the question is whether people really do have OSF/1 binaries. It was really useful early on as a source of known-good binaries to test with, but I'm not convinced it's still in use. It's not like there were OSF/1 binaries that we didn't have access to natively (well, there _were_ special ones that didn't have open source versions, but most of them required more system-side support than Linux ever implemented, afaik).

And Måns replied, "I can well imagine people keeping an Alpha machine for no other reason than the ability to run some (old) application only available (to them) for OSF/1. Running them on Linux rather than Tru64 brings the advantage of being a modern system in other regards."

Matt said he hadn't been aware of this situation on alpha and agreed that it might be necessary to continue to support a.out on that architecture, just for the remaining users who needed it.

As a practical example, Arnd Bergmann recounted, "The main historic use case I've heard of was running Netscape Navigator on Alpha Linux, before there was an open-source version. Doing this today to connect to the open internet is probably a bit pointless, but there may be other use cases."

He also added that:

Looking at the system call table in the kernel...we seem to support a specific subset that was required for a set of applications, and not much more. Old system calls...are listed but not implemented, and the same is true for most of the later calls...just the ones in the middle are there. This would also indicate that it never really worked as a general-purpose emulation layer but was only there for a specific set of applications.

And in terms of anyone potentially complaining about the loss of a.out support, Arnd also pointed out that "osf1 emulation was broken between linux-4.13 and linux-4.16 without anyone noticing."

Linus replied:

Yeah, it never supported arbitrary binaries, particularly since there's often lots of other issues too with running things like that (ie filesystem layout etc). It worked for normal fairly well behaved stuff, but wasn't ever a full OSF/1 emulation environment. I _suspect_ nobody actually runs any OSF/1 binaries any more, but it would obviously be good to verify that. Your argument that timeval handling was broken _may_ be an indication of that (or may just mean very few apps care).

And based on these reassuring considerations, Linus said, "I think we should try the a.out removal and see if anybody notices."

The discussion continued briefly, but it seems like a.out will finally be removed in the relatively near future.

The thing that fascinates me about this is the insistence on continuing to support ancient features if even a single user is found who still relies on it. If even one person came forward with a valid use case for a.out, Linus would leave it in the kernel. At the same time, if no users step forward, Linus won't assume they may be lurking secretly out in the wild somewhere—he'll kill the feature. It's not enough simply to use an ancient feature, the user needs to be an active part of the community—or at least, active enough to report his or her desire to continue to use the feature. And in that case, Linus probably would invite that user to maintain the feature in question.

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