[TUHS] 2.11BSD cross compiling update

Nick Downing downing.nick at gmail.com
Sat Jan 14 03:57:38 AEST 2017

So I got a fair bit further advanced on my 2.11BSD cross compiler
project, at the moment it can make a respectable unix tree (/bin /usr
etc) with a unix kernel and most software in it. I unpacked the
resulting tarball and chrooted into it on a running SIMH system and it
worked OK, was a bit rough around the edges (missing a few necessary
files in /usr/share and that kind of thing) but did not crash. I
haven't tested the kernel recently but last time I tested it, it
booted, and the checksys output is OK.

I then ended up doing a fair bit of re-engineering, how this came
about was that I had to port the timezone compiler (zic) to run on the
Linux cross compilation host, since the goal is eventually to build a
SIMH-bootable disk (filesystem) with everything on it. This was a bit
involved, it crashed initially and it turned out it was doing
localtime() on really small and large values to try to figure out the
range of years the system could handle. On the Linux system this
returns NULL for unreasonable time_t values which POSIX allows it to
do. Hence the crash. It wasn't too obvious how to port this code. (But
whatever I did, it had to produce the exact same timezone files as a
native build).

So what I ended up doing was to port a tiny subset of 2.11BSD libc to
Linux, including its types. I copied the ctime.c module and prefixed
everything with "cross_" so there was "cross_time_t" and so forth, and
"#include <time.h>" became "#include <cross/time.h>", in turn this
depends on "#include <cross/sys/types.h>" and so on. That way, the
original logic worked unchanged.

I decided to also redo the cross compilation tools (as, cc, ld, nm,
ranlib and so on) using the same approach, since it was conceptually
elegant. This involved making e.g. "cross_off_t" and "struct
cross_exec" available by "#include <cross/a.out.h>", and obviously the
scheme extends to whatever libc functions we want to use. In
particular we use floating point, and I plan to make a "cross_atof()"
for the C compiler's PDP-11-formatted floating-point constant
handling, etc. (This side of things, like the cross tools, was
working, but was not terribly elegant before).

So then I got to thinking, actually this is an incredibly powerful
approach. Instead of just going at it piecemeal, would it not be
easier just to port the entire thing across? To give an example what I
mean, the linker contains code like this:
  if (nund==0)
  printf("%.*s\n", NNAMESIZE, sp->n_name);
It is printing n_name from a fixed-size char array, so to save the
cost of doing a strncpy they have used that "%.*s" syntax which tells
printf not to go past the end of the char array. But this isn't
supported in Linux. I keep noticing little problems like this
(actually I switched off "-Wformat" which was possibly a bad idea). So
with my latest plan this will actually run the 2.11BSD printf()
instead of the Linux printf(), and the 2.11BSD stdio (fixing various
other breakage that occured because BUFSIZ isn't 512 on the Linux
system), and so on. What I will do is, provide a low level syscalls
module like cross_open(), cross_read(), cross_write() and so on, which
just redirect the request into the normal Linux system calls, while
adjusting for the fact that size_t is 16 bits and so forth. This will
be really great.

In case it sounds like this is over-engineering, well bear in mind
that one knotty problem I hadn't yet tackled is the standalone
utilities, for instance the 2.11BSD tape distribution contains a
standalone "restor" program which is essentially a subset of the
kernel, including its device drivers, packaged with the normal
"restor" utility into one executable that can be loaded off the tape.
It was quite important to me that I get this ported over to Linux, so
that I can produce filesystems, etc, at the Linux level, all ready to
boot when I attach them to SIMH. But it was going to be hugely
challenging, since compiling any program that includes more than the
most basic kernel headers would have caused loads of conflicts with
Linux's kernel headers and system calls. So the new approach
completely fixes all that.

I did some experiments the last few days with a program that I created
called "xify". What it does is to read a C file, and to every
identifier it finds, including macro names, C-level identifiers,
include file names, etc, it prepends the sequence "x_". The logic is a
bit convoluted since it has to leave keywords alone and it has to
translate types so that "unsigned int" becomes "x_unsigned_int" which
I can define with a typedef, and so on. Ancient C constructs like
"register i;" were rather problematic, but I have got a satisfactory
prototype system now.

I also decided to focus on 4.3BSD rather than 2.11BSD, since by this
stage I know the internals and the build system extremely intimately,
and I'm aware of quite a lot of inconsistencies which will be a lot of
work to tidy up, basically things that had been hurriedly ported from
4.3BSD while trying not to change the corresponding 2.8~2.10BSD code
too much. Also in the build system there are quite a few different
ways of implementing "make depend" for example, and this annoys me, I
did have some ambitious projects to tidy it all up but it's too
difficult. So a fresh start is good, and I am satisfied with the
2.11BSD project up to this moment.

So what will happen next is basically once I have "-lx_c" (the "cross"
version of the 4.3BSD C library) including the "xified" versions of
the kernel headers, then I will try to get the 4.3BSD kernel running
on top of Linux, it will be a bit like User-Mode Linux. It will use
simulated network devices like libpcap, or basically just whatever
SIMH uses, since I can easily drop in the relevant SIMH code and then
connect it up using the 4.3BSD kernel's devtab. The standalone
utilities like "restor" should then "just work". The cross toolchain
should also "just work" apart from the floating point issue, since it
was previously targeting the VAX which is little-endian, and the
wordsize issues and the library issues are taken care of by "xifying".
Very nice.

The "xifying" stuff is in a new repository 43bsd.git at my bitbucket
(user nick_d2).

cheers, Nick

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