I'm looking for a reliable, unobtrusive runtime check a process can make for whether it's running on Linux without mmu. By unobtrusive, I mean having minimal or no side effects on process state. For example, getting EINVAL from fork would be one indication, but would create a child process if the test failed. Attempting to cause a fault and catch a signal is out of the question since it involves changing global signal dispositions. Anything involving /proc or /sys would be unreliable since they may not be mounted/visible (e.g. in chroot or mount namespace).
Failure of mprotect with ENOSYS seems to be reliable, and can be done without any side effects beyond the need to map a test page to attempt it on. But I'm not sure if it's safe to rely on this.
Are there any better/recommended ways to go about this?
Before anyone tries to challenge the premise and answer that this is known statically at compile time, no, it's not. Assuming you build a position-independent executable for an ISA level supported by both mmu-ful and mmu-less variants of the architecture, it can run on either. (I'm the author of the kernel commit that made this work.)
