Description
Depending on the version of gcc or linux distribution, gcc may build with the -fPIE (position independent executable) option enabled by default. PIE code is incompatible with Fiwix for several reasons. Therefore, the -fno-pie option must be added to the CFLAGS in the Makefile in order to build Fiwix properly for some builds or versions of gcc.
For me, I built Fiwix on Ubuntu 20.04 and Fiwix would not boot. There was no obvious indication what the problem was and it took me several days of difficult debugging to fully understand the issue and the solution. My version of gcc is "gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0" and it builds with -fPIE enabled by default. This can be verified by building with the gcc options "-Q -v". In contrast, the version of gcc (4.8.5) on Centos 7 does not enable -fPIE by default.
I tried to make Fiwix work with my gcc, as is, but I was unable. First, gcc with -fPIE produces several new ELF sections such as ".data.rel" which are not accounted for in the fiwix.ld linker script. After solving that, I found that gcc's PIE code populates the ebx register with the current instruction pointer and locates data structures relative to that register. However, routines in core386.S such as tlbinfo modify the ebx register without saving it, which is catastrophic. (Not consistently saving registers is a separate issue which is probably worth fixing). After solving that, I found that Fiwix copies the init_trampoline function to a different location, so data structures cannot be found in the same place, relative to the instruction pointer. Fixing that could be done by porting init_trampoline to assembly, but I eventually figured out that simply adding the -fno-pie option would get gcc to build Fiwix the way it was intended.