Starting with the very specific: I do not think it was an accident that the xz backdoor's exploit chain started with a modified version of a third party .m4 file to be compiled into xz's configure script.

It's possible to write incomprehensible, underhanded code in any programming language. There's competitions for it, even. But when you have a programming language, or perhaps a mashup of two languages, that everyone *expects* not to be able to understand — no matter how careful the author is — well, then you have what we might call an attractive nuisance. And when blobs of code in that language are passed around in copy-and-paste fashion without much review or testing or version control, that makes it an even easier target.

So, in my capacity as one of the last few people still keeping autoconf limping along, I'm thinking pretty hard about what could be done to replace its implementation language, and concurrently what could be done to improve development practice for both autoconf and its extensions (the macro archive, gnulib, etc.)

On the subject of implementation language, I have one half-baked idea and one castle in the air.

The half-baked idea is: Suppose ./configure continues to be a shell script, but it ceases to be a *generated* shell script. No more M4. Similarly, the Makefile continues to be a Makefile but it ceases to be generated from Makefile.am. Instead, there is a large library of shell functions and a somewhat smaller library of Make rules that you include and then use.

For ./configure I'm fairly confident it would be possible to do this and remain compatible with POSIX.1-2001 "shell and utilities". (Little known fact: for a long time now, autoconf scripts *do* use shell functions! Internally, wrapped in multiple layers of M4 goo, but still — we haven't insisted on backcompat all the way to System V sh in a long, long time.) For Makefiles I believe it would be necessary to insist on GNU Make.

This would definitely be an improvement on the status quo, but would it be *enough* of one? And would it be less work than migration to something else? (It would be a compatibility break and it would *not* be possible to automate the conversion. Lots of work for everyone no matter what.)

M4 always was horrible, and I say that as the original author of GNU M4.

The original Unix M4 was weird, and there weren't really any good explanations for why it was the way it was. Apparently someone at Bell labs needed a preprocessor and wrote M4, sometimes in the '70, for no other greater purpose than to scratch a personally itch.

GNU M4 only exists because RMS wanted GNU to have what Unix had, and while I wanted to do something different and better, RMS convinced me to do M4 first.