On 5/1/20 1:00 AM, Greg Ungerer wrote: >> This sounds correct. My understanding of FLAT shared library support >> is that it's really bad and based on having preassigned slot indices >> for each library on the system, and a global array per-process to give >> to data base address for each library. Libraries are compiled to know >> their own slot numbers so that they just load from fixed_reg[slot_id] >> to get what's effectively their GOT pointer.
fdpic is to elf what binflt is to a.out, and a.out shared libraries were never pretty. Or easy. >> I'm not sure if anybody has actually used this in over a decade. Last >> time I looked the tooling appeared broken, but in this domain lots of >> users have forked private tooling that's not publicly available or at >> least not publicly indexed, so it's hard to say for sure. > > Be at least 12 or 13 years since I last had a working shared library > build for m68knommu. I have not bothered with it since then, not that I > even used it much when it worked. Seemed more pain than it was worth. Shared libraries worked fine with fdpic on sh2 last I checked, it's basically just ELF PIC with the ability to move the 4 segments (text/rodata/bss/data) independently of each other. (4 base pointers, no waiting.) I don't think I've _ever_ used shared binflt libraries. I left myself breadcrumbs back when I was wrestling with that stuff: https://landley.net/notes-2014.html#07-12-2014 But it looks like that last time I touched anything using elf2flt was: https://landley.net/notes-2018.html#08-05-2018 And that was just because arm's fdpic support stayed out of tree for years so I dug up binflt and gave it another go. (It sucked so much I wound up building static pie for cortex-m, taking the efficiency hit, and moving on. Running pie binaries on nommu _works_, it's just incredibly inefficient. Since the writeable and readable segments of the ELF are all relative to the same single base pointer, you can't share the read-only parts of the binaries without address remapping, so if you launch 4 instances of PIE bash on nommu you've loaded 4 instances of the bash text and rodata, and of course none of it can even be demand faulted. In theory shared libraries _do_ help there but I hit some ld.so bug and didn't want to debug a half-assed solution, so big hammer and moved on until arm fdpic got merged and fixed it _properly_...) Rob P.S. The reason for binflt is bare metal hardware engineers who are conceptually uncomfortable with software love them, because it's as close to "objcopy -O binary" as they can get. Meanwhile on j-core we've had an 8k ROM boot loader that loads vmlinux images and does the ELF relocations for 5 years now, and ever since the switch to device tree that's our _only_ way to feed a dtb to the kernel without statically linking it in, so it's ELF all the way down for us.
