Your X server may no longer swing both ways by default

As a long-time PowerPC and Power ISA bigot, there's a lot of Power-based hardware in this house — primarily Apple, but some IBM, and of course several Raptor systems. While many CPUs are capable of running big-endian or little-endian, Power ISA is probably the last architecture where there is still notable interest in running it in both modes: AIX, IBM i (a/k/a i5, AS/400), AmigaOS and OpenBSD run it big, FreeBSD primarily runs it big (but work exists to run it little), and most Linux distros run it little. Compare with the ostensibly bi-endian ARM and MIPS, which virtually all run little, and SPARC, which virtually all runs big (versus s390x, which only runs big, and of course x86 and x86_64 only runs little). Little-endian is gradually displacing big-endian even in the Power world (sorry), but it's still important.

When it was more commonplace for a discrepancy to exist, such as between mainframes and desktop X terminals or PCs, a feature was added to the X protocol where a connecting X client would advertise its endianness and if this did not match the server's, the server would byteswap for it. (Note that current Xorg may not allow remote connections without passing -listen tcp either from gdm/your display manager of choice or on the command line. On my Fedora 37 system, I do startx -- -listen tcp to enable incoming connections on my secured wired network. Don't forget anything you need to do with xhost or other authorizations. ssh forwarding is of course an alternative means.) This makes running X clients from my AIX POWER6, which is strictly big, possible on my Fedora 37 Talos II, which Fedora runs little. Here's the old beast now from the "WalMart server rack" next door.

And here's proof of connection in my usual KDE Plasma desktop (running aixterm and xlogo), showing that even the most current Xorg still supports it.
A new change to Xorg will now prohibit automatic byteswapping in the X server by default. A client connecting to a server that advertises a different endianness will be kicked off with an error. If you want this support, you'll either need to pass +byteswappedclients on the command line to the X server, or put "AllowByteSwappedClients" "on" in the Options stanza in your xorg.conf. This is also a change request for Fedora 38 which of this writing is still proposed and not accepted.

This means not only will this usage of a big-endian client to a little-endian server, which I use infrequently but not rarely, not work without changes, but will also fail for anyone running a bleeding-edge version of Xorg on a big-endian host (say, Linux on your Power Mac G5) that wants to run clients like a more current web browser from a little-endian server. The latter case is certainly less common than the former (mostly retrocomputing, whereas there are mainframe apps that people will want to have a local interface for), but I think there's more out there of both than folks suspect. Chesterton's fence and all that.

I will say that I appreciate this being turned into an option rather than outright removed, keeping in mind this is usually a prelude for outright removal later. After all, the code seems to have no test coverage in a codebase poorly covered by testing generally, and has caused documented security problems in the past. To the extent this is a better compromise than talking to the hand I support it. However, it also makes Wayland even less attractive than it already is because the ability to pass an option to Xwayland is compositor-specific (see this bug for, among others, GNOME Mutter), meaning you're at the mercy of what you're running and may not be able to change it easily yourself. Well, we're Xorg unto death around here anyway.


  1. Is there any actual reason for this being removed from defaults?
    or "just because we can"?

    This seems like the inverse of OpenBSDs security philosophy of maintaining multi-endian and multiplatform code in active use to identify bugs as soon as possible. Wouldn't implementing a fuzzer be better than breaking compatibility for an actively used feature?


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