20211002

Spelling Fixes -- Some Advice

 Some Thoughts on Spelling Fixes

I've been looking at a number of FreeBSD pull requests lately. Many of them are spelling fixes. I'd like to offer some suggestions for people submitting them. The same advice applies to typo fixes and grammar corrections.

  1. Keep number of changes small.
  2. Use separate commits per directory
  3. Use descriptive commit messages
  4. Set your email correctly
  5. Don't correct code

Keep number of changes small

When submitting like this, limit the number of changes to 10-20. More than about 15 changes becomes hard to review. Every single change has to be verified for correctness, and having too many will make your pull request more likely to be overlooked.

Use separate commits per directory

When submitting a number of changes, do one change per subdirectory. When subdirectories are nested, it's OK. For example, if you have changes to bin/ls and bin/rm, do two commits. But if you have changes to both usr.sbin/wpa/wpa_priv and usr.sbin/wpa/wpa_cli, then it's OK to do those as one commit.

Use descriptive commit messages

The commit message "fix spelling" is too generic to be useful. If you are fixing just one word, a better commit message would be "Spell interrupt correctly". It also allows the reviewer to make sure that you are changing the right thing. And it also gives enough detail that people skimming the logs don't need to look at the diffs to know what changed. If you are fixing multiple spelling errors, then a generic message is more appropriate.

Set your email correctly

When you push your branch to github, make sure that you've set the email you want in the commit message. It saves a lot of time. If you are using github's editor, make sure that your profile has this information set correctly.

Don't correct code

Don't make spelling corrections in code variables, #defines, etc. These will likely be ignored. The risk from a comment or an error message being corrected is tiny, while code changes could be attempts to subtly change the system or introduce security impacting issues through a supply chain attack. It's better to work with someone in the community to get these corrected than to correct them via a pull request.

20210828

A new path: vm86-based venix emulator

 Venix Emulator Update

It's been a while since I've had time to work on the Venix emulator. When I set it aside over a year ago, I'd taken it as far as I could with the 8088 emulator I'd found online. It had no FP emulation and there were
a number of things misbehaving that I couldn't quite get right. And exec was proving hard to implement. Despite being written in C++, the original emulator resisted my efforts to make multiple instantiations.

So I set it aside last May, thinking I might get back to it when the qemu bsd-user changes FreeBSD has done have been upstreamed.

The first of August I took some time off from work and got the bsd-user changes in shape to upstream. Well, the first 10% of the changes that were the hardest since it was replacing what was there with something that minimally worked. This helped me learn qemu's x86 CPU much better and it got me thinking that qemu's user-mode stuff might be the way to go.

About this time I also found a vm86 test program in the FreeBSD tree. So I got to wondering, could I do a vm86 implementation of Venix?

So, I stole the bulk of my old 86sim-based Venix implementation, installed a i386 VM using bhyve on my FreeBSD/amd64 box and write a quick little test program. The test program worked, so in a fit of "why not give this a try" I ported the pcvenix.cc from 86sim to being driven from SIGSEGV in vm86 mode. Hello world quickly worked.

Enter VM86VENIX

So, I reworked fork and exec and the a.out loader a bit. I was able to get the C compiler going in this new setup. The 'cc' command is just a fancy script that strings together the pre-processor, compiler, optimizer, assembler and linker. Except on Venix it wasn't a shell script I could hack to run natively on FreeBSD. It was this weird binary that did all the forking, execing, redirecting, etc inline. More on that in a minute.

So vm86 mode is a special mode in 32-bit CPUs that lets you execute old 16-bit code in the context of a normal process. It's super easy to setup, but often of limited use.

Thankfully, the i386 ELF designers thought ahead. The starting address for binaries in ELF is this weird 0x00401430, which is just above 4MB. This means that one can map anything into low memory and it will work. FreeBSD has a security stop on mapping anything at location 0, however, but the rest of the first 4MB is available. The old 8086 could only see the first 1MiB of that, but since Venix binaries are at worst 'small-mode' the largest address space for a process was 128kiB. Plenty of room to find a place to map it.

So, I wrote a loader that would load the old Venix a.out binaries into this space. Or rather I hacked the loader I already had to do the mapping. I was able to reuse all the loader code from the 86sim-based emulator I had before.

I then shamelessly stole the setup code from the FreeBSD vm86 testing binary, which was little more than establishing signal handlers and zeroing the context and setting up a stack and IP as well as the segment registers. With that in hand, I was able to use sigreturn() to set the processor flags such that it would jump to where I wanted to go in the Venix binary. I'd been afraid of vm86 mode after reading through doscmd years ago, but there was no need for the fear: all the cruft in doscmd (I reread it after this) was the accumulation of cruft over the years for DOS, BIOS and other weirdness that evolved around the IBM PC, XT, AT and the plethora of clones which had nothing to do with vm86 mode, per se.

Every INT xx instruction would trap to the kernel. The kernel would note down the registers and send the process a SIGSEGV for these accesses. I was able to then look at CS:IP in the mcontext and decode the instruction that faulted. INT xx is encoded as the bytes 0xCD 0xXX for almost all values of X (INT3 has its own opcode 0xCC). In the signal handler, I could decode this opcode. I knew from past work that INT 0xf1 was the system call, so I hooked up the old venix system call handlers to this and I was back to where I was with 86sim. Further in fact because floating point worked.

Signal handlers have an implicit sigreturn with the context passed to the signal handler at the end. I needed to skip over the faulting instruction after performing the system call, and the process would then resume executing in 16-bit mode after the INT 0xf1. This was straight forward to implement.

I decided to implement fork as a real fork. It would copy the address space, all the open FDs, etc. This proved to be an easy way to cheat so I didn't have to create a context object and use threads to simulate processes.

Exec proved to be just a call to my loader that started all this off. The only thing I've not implemented is close on exec, but the rest was easy.

With these implemented, I could run the C compiler's cc command and generate trivial binaries. But if I needed to include anything, it would fail. I created a VENIX_ROOT env variable. For all opens, it would try VENIX_ROOT / name and then just the plain name if the name arg to open started with a '/'. This was enough for the preprocessor to include .h files and for the canonical hello world to build.

There was just one vexing problem: cc -o hello hello.c worked. However cc -O -o hello hello.c didn't.

Tracking down a silly bug

Well, there was another annoying thing: /bin/sh didn't work. I traced that to the fact I've not implemented passing an environment to the processes, and /bin/sh was choking on that. OK. Fine. I'll implement that later. /bin/csh worked, however, so I was happy. My happiness was short lived, alas, because I'd run a  command and I'd get weird output:
% ls
ls: Sig 44
%

 That's weird. So I added tracing. I tried the cc command, which in this version is a simple program that orchestrates all the different parts of the compiler using fork, exec, dup and the strategic close/open pair to setup stdin etc. All the tracing looked good as well, we'd see something like:

123: fork() 124
123: wait()
124: ... lots of stuff
124: exit 0
123: wait pid 124 status 0

 and then 123 would proceed to delete all the temp files and exit. It was like it was getting an error, despite its children exiting without an error. Every time it was like this, but only when I ran the optimizer. When I'd re-run the ls test, I'd get different Sig values as well.

Available 8086 compilers in 1985

So, I'd assumed that the compiler was derived from the V7 compiler. However, a number of hints in names suggested it wasn't. And the Venix manual had way more exceptions for 8086 than for pdp-11 when it described the options and operation for Intel, so I assumed it wasn't V7 derived. So I started hunting around for C compilers.

MIT produced one at the time for the PC. It could run on the VAX and generated a.out binaries that a conversion program would convert to .COM or .EXE files. I thought this might be where the Venix compiler came with. But after playing around with it for a few hours it was clear it wasn't. First, it had a shell script cc, not a program. Second, it had a number of VAX specific instructions sprinkled inline, and that wasn't going to run on the Rainbow :).

I took a look at the portable C compiler. This I think was the real genesis of what was shipped with Venix, but old versions that support 8086 are hard to come by, even in the successor portable C compiler project that's been going for 20-odd years now. I got the cc program from that compiling with Venix. It was a bit easier to fuss around with than the V7 one (but the V7 one would be close enough to hit the bug I found out later). Looking at the old System III sources that one can find on the internet, there's a copy of the portable C compiler there, rather than the C compiler from DMR as you'll find in the 7th edition. I used the cc program from there to try to build things. I hit the jackpot: it failed faster!

So I instrumented the pcc program and discovered that the status printed after wait() in the program didn't match the status that I'd returned from the kernel. Progress!

The wait(2) system call...

I'd implemented the wait(2) system call as part of getting fork/exec working. I did it from the VENIX manual that's available online. I looked at the first part of the manual:
SYNOPSIS
        wait(statusp)
        int *statusp;

which shows wait taking a pointer. So I assumed this was what the kernel received in the first arg that's passed into the kernel (DX). I assumed that DS:DX pointed to an integer where I'd return the status. Most of the time, DX was something that looked like a pointer on the stack, so I just did a copyout. The problem is that's not right.

So, I took another look at the manual. At the end of the man page I saw:

8086      BX=7
              int 0xf1
              AX = pid of process
              DX = status

and then it hit me.  DX isn't a pointer to anything. After int 0xf1 AX is the pid of the process (the normal return value) and DX is the status. Disassembling wait.o confirmed this:if statusp is not 0, dx is copied back to *statusp. Doh! The classic pointer vs value mistake. Fixing my implementation to take out the copyout and replace it with setting DX in the processor context made pcc work. And cc worked. And the silly test programs I wrote in the middle to debug things worked. Woo Hoo!

Once I fixed this, all weird combinations of compilations suddenly worked for me. I could optimize, strip, etc and there were no oddities.

Conclusion

It helps to read the manual carefully!

I need to try to build the system. There's shell scripts to do that that don't depend on environment variables working if run natively, so I'll see if they work and see how much of the system I can generate via this route. Stay tuned.

My TODO list still contains getting env working (I don't think it is hard, but I think I need to filter things because my default env is larger than the stack on these old x86 machines). I also need to look at rebasing my emulator as a *-user qemu emulator (even if they don't take it upstream). Maybe even add PC/IX and Xenix/86 support as well so that other researchers can play around with this.

20210501

On Updating QEMU's bsd-user fork

 QEMU bsd-user

bsd-user is a 'user mode' emulation tool. It emulates FreeBSD's system calls on FreeBSD well, and $OTHER-BSD system calls elsewhere to varying degrees of success. It's primary mission has been to build FreeBSD packages using user-mode emulation to speed the process over using system mode. It speeds things up because the compilers and other huge CPU hogs can be built natively.

Of late, it has languished, A few years ago, I started to rebase it to the then-tip of qemu in the hopes of upstreaming. At the time, we'd forked off of qemu 1.0 or so. During this time the then-current qemu was 4.0. I got things rebased to around 3.1 before running out of steam. Rebasing patch trains of 1000 commits is hard, and trying to selectively squash commits wasn't much better.  So that's where things stalled. All bug fixes to qemu bsd-user had gone on in our own private branch.

Recently, I'd been asked about it again, so I started to dust things off. I got my name listed as the maintainer so I could push patches upstream a little more easily, and then started contributing by doing basic cleanup in the hopes of redoing 'logically' what had been done to split things up. Those efforts too have come to naught.

So, in one final act of desperation, I copied the 3.1-rebased bsd-user directory directly into qemu 6.0 and got it building. There were lots of little changes I needed, but nothing super huge. I've not done extensive testing, but the basics seem to work.

Trouble was, that diff was 35k lines. Too big to upstream in one go. So, I set out to see what could be done.

First, I labeled the 'yeet it up  to current' branch as 'blitz'. It's a fast hack to get something we can move forward on. In the future, releases and such will be cut from there until I can get it into the upstream tree. Blitz is the German word for 'fast' and has connotations of doing something quick and dirty well enough to move on.

Next, I created another branch from 6.0 called 'kaizen'. Kaizen is the Japanese business practice of continuous improvement. Find the most painful or most expensive part of your business, fix that and iterate. This branch I'll be putting 'diff reduction' patches for upstream, as well as start to move things over from blitz, starting with the loader. I've disconnected everything except x86 from this branch. In upstream qemu, bsd-user core dumps right away, so I'm not turning off anything that's working.

So the plan is that I'll focus on keeping x86 buildable, and get it working as quickly as I can and then add all the system calls from the blitz branch. I'll add them one group at a time, and do the reorgs and new file creation as well. I'll get these reviewed and upstreamed. Once all the system calls are in place, I'll start adding additional architectures as well, getting those patches reviewed too. Finally, I'll get the NetBSD and OpenBSD hosting stuff updated, as well as take a stab at updating their system call tables and seeing how well it works. The work that Stacey Son and others did tried to preserve all this, but it's been a long time since any of it was tested.

I have an agreement in principle with the qemu upstream to do all this work. So approximately monthly, I'll be landing a new branch with the latest diff reductions. I'll rebase kaizen and blitz after each drop and before I upstream. For the moment, this work will go into my gitlab fork (since it has all the CI setup on it) and from time to time I'll publish back to the github qemu-bsd-user repo. Be advised: both the blitz and kaizen branches will rebase often, so you may need to do weird things to update. Though, if you are tracking them with changes, please be in touch so we can coordinate work.

With luck, by this time next year, the kaizen and blitz branches will be nothing but a distant memory and we'll be on to keeping things up to date in qemu head, maybe with doing some refactoring with linux-user where it makes sense.

20210416

Customizing Emacs for Git Commit Messages

 Customizing Emacs for Git Commit Messages

I do a lot of commits to the FreeBSD project and elsewhere. It would be nice if I could setup emacs in a custom way for each commit message that I'm editing.

Fortunately, GNU Emacs provides a nice way to do just that. While I likely could do some of these things with git commit hooks, I find this to be a little nicer.

First up, we need to do something when we pull up the commit message to edit. By convention, git uses the file COMMIT_EDITMSG, though the exact location of this file depends on where the git tree you have checked out is. Emacs has a hook that's run when emacs starts editing a file called 'find-file-hook'. So:
(add-hook 'find-file-hook 'imp-git-hook)

will do the job nicely. Next up, we need to define this function to do something useful when run (indeed, failure to define it will result in an error when visiting all files).

(defun imp-git-hook ()
  (when (string= (file-name-base buffer-file-name) "COMMIT_EDITMSG")
    (freebsd-git-setup))) 

buffer-file-name is a local variable that has the full path name to the buffer, if any. file-name-base is like basename(1) in that it returns the name of the file without the extension, rather than its whole path.

But what is 'freebsd-git-setup'? It's a little function I wrote to set the fill column to 72 (I usually have it at 80, but that produces commit messages that are just a bit wide when git adds leading spaces) and adds a sponsorship tag to my commits:

(defun freebsd-git-setup ()
 (save-excursion
  (setq fill-column 72)
  (if (re-search-forward "^Sponsored by:" nil t)
    t
   if (re-search-forward "^\n#" nil t)
    (replace-match "\nSponsored by:\t\tNetflix\n\n#")))))

But it only adds the sponsorship tag if one isn't there yet.

This is a proof-of-concept function. No doubt it will have to evolve over time. The project adds 'Differential Revision' tags as the last tag in a commit message because differential requires (required?) that. And it wouldn't be good to add this for non-FreeBSD commits, so I'll have to find a way to filter for that... But I thought this would be useful, if nothing else than to my future self as a roadmap for how to do things like this.


20210131

EPSON QX-10 20MB Hard disk

 EPSON QX-10 20MB Hard Disk

I've been looking for some DEC Rainbow 3rd party hard drives of late. QCS (Quality Computer Services) made an external hard disk for the DEC Rainbow. There's advertisements in Digital Review and other trade magazines at the time. It uses a SASI interface, and likely had a DEC Rainbow specific add-in card that they rewarmed from other designs...

One recently came up for sale on E-Bay. I thought I'd buy it to check it out. There was no interface card with it, alas. But it was a box with a WD1006 SASI to MFM controller in it that could handle two different drives. The drives were LUN0 and LUN1.

SASI, for those that don't know, pre-dates SCSI-1. It's kinda sorta SCSI-1 compatible, if you turn off parity, don't allow the drive to signal attention and restrict yourself to a subset of commands. It also doesn't have INQUIRY so you kinda have to know the size of the drive before hand. Most SASI controller drivers of the day wrote a label to drive with this information since it was always possible to read LBA0 w/o knowing anything else about the drive. Some controllers had ways to at least return a size, though that varied a lot...

Since SASI is kinda hard to interface to modern SCSI controllers, I used a MFM reader board I got from David Gesswein over at https://www.pdp8.net/mfm/mfm.shtml to read the drive. I had hoped to find that it was from an old Rainbow and I'd complete my collection of drivers for third party drives...

Much to my surprise, I was able to read it without any errors until it hit the manufacturing tracks (480-489). I pulled a full image, then downloaded it to my FreeBSD box for analysis.

hexdump -C told me it was a CP/M disk (I recognized the directory format). It was clear right away it wasn't a DEC Rainbow disk, however.

The first thing I noticed was the "Bi-Tech Multi Drive Support V4.02" string which indicated who made the driver for it. I also noticed strings like the following
PT.COM for EPSON QX-10 PeachText 5000 date changed - 02/03/84

and similar references to the QX-10 or EPSON CP/M.

So, this was from a Epson QX-10 CP/M system. Looks to be a soft-water service company from South Bend Indiana. All their books and correspondence from the mid 1980s was on it, along with some interesting disk support software. There's even some bits of Z80 assembler, but they are too disjointed to know what they were for.

I've not been able to get cpmtools to read the disk in a structured way, however, so it's hard to share just the interesting bits. Still working on it.

If you have one of these machines, or are interested in preserving software from it, please let me and we may be able to work something out.


 

 

20201006

How to Recover From a BIOS Upgrade

Recovering From Firmware Upgrade

Recently, I booted Windows on my laptop for the first time in a while to play Portal 2 with my son. It asked me to upgrade, and I said 'sure, upgrade the BIOS.'

And then I couldn't boot FreeBSD...  The BIOS upgrade deleted all the BootXXXX variables. So it only booted Windows. I'm stuck, right? I have to download a FreeBSD image and boot off the USB drive. Or did I?

Note: Even the update program called updating the firmware updating the BIOS. BIOS is the generic term for the bit of code that runs before the OS. Sadly, it's also the term people use to describe the pre-UEFI boot environment on PCs, so it can be a confusing term to use. Firmware seems a bit better, but it's also ambiguous because different bits of hardware (like wireless cards) also need firmware loaded.

How to Recover

I was in windows. I needed to mount the system partition (EFI). So, I opened the Administrative Console and got a command prompt from there on the 'Tools' tab. This lead to the familiar C: prompt.  I have no W drive. There I was able to copy FreeBSD's boot loader like so:

C:\WINDOWS\system32> mountvol w: /s
C:\WINDOWS\system32>w:
W:\> cd EFI\Microsoft\Boot
W:\EFI\Microsoft\Boot> ren bootmgfw.efi bootmgfw-back.efi
W:\EFI\Microsoft\Boot> copy W:\EFI\FreeBSD\loader.efi bootmgfw.efi
W:\EFI\Micorsoft\Boot> 

I then rebooted from the menu.

I had remembered from my efibootmgr hacking that the boot loader was here. After I booted to FreeBSD, I was able to confirm:

% sudo efibootmgr -v
Boot to FW : false
BootCurrent: 0001
Timeout    : 0 seconds
BootOrder  : 0001, 2001, 2002, 2003
+Boot0001* Windows Boot Manager HD(1,GPT,f859c46d-19ee-4e40-8975-3ad1ab00ac09,0x800,0x82000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)
                                   nvd0p1:/EFI/Microsoft/Boot/bootmgfw.efi /boot/efi//EFI/Microsoft/Boot/bootmgfw.efi
 Boot2001* EFI USB Device 
 Boot2002* EFI DVD/CDROM 
 Boot2003* EFI Network 


Unreferenced Variables:

I was then able to add FreeBSD back with efibootmgr. I mount the ESP on /boot/efi:

sudo efibootmgr --create --loader /boot/efi/EFI/freebsd/loader.efi --kernel /boot/kernel/kernel --activate --verbose --label FreeBSD
Boot to FW : false
BootCurrent: 0001
Timeout    : 0 seconds
BootOrder  : 0000, 0001, 2001, 2002, 2003
 Boot0000* FreeBSD HD(1,GPT,f859c46d-19ee-4e40-8975-3ad1ab00ac09,0x800,0x82000)/File(\EFI\freebsd\loader.efi)
               nvd0p1:/EFI/freebsd/loader.efi /boot/efi//EFI/freebsd/loader.efi
           HD(6,GPT,68f0614d-c322-11e9-857a-b1710dd81c0d,0x7bf1000,0x1577e000)/File(boot\kernel\kernel)
               nvd0p6:boot/kernel/kernel /boot/kernel/kernel
+Boot0001* Windows Boot Manager HD(1,GPT,f859c46d-19ee-4e40-8975-3ad1ab00ac09,0x800,0x82000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)
                                   nvd0p1:/EFI/Microsoft/Boot/bootmgfw.efi /boot/efi//EFI/Microsoft/Boot/bootmgfw.efi
 Boot2001* EFI USB Device 
 Boot2002* EFI DVD/CDROM 
 Boot2003* EFI Network 


Unreferenced Variables:
%

Once this is in place, I needed to undo what I'd done to Windows:

% cd /boot/efi/EFI/Microsoft/Boot
% sudo mv bootmgfw-back.efi bootmgfw.efi

I was then able to reboot to FreeBSD. And fun fact: since the boot order is 0000, 0001, that means I can boot to Windows by just typing 'quit' at the loader prompt. This causes the boot loader to exit with an error, which causes the BIOS to try the next BootXXXX variable, in this case windows.

And there it is: I was able to recover my system without downloading a USB image...


20201001

FreeBSD Subversion to Git Migration: Pt 2 Primer for Users

FreeBSD git Primer for Users

Today's blog is actually a preview of a git primer I'm writing for the FreeBSD project. It covers what a typical user will need, including those relatively rare users that may have some changes to the base. Please let me know what you think, and ways it can be improved. I'm keen on especially clear and useful pointers for the topics as well.

Also note: The cgit-beta mirror mentioned below is currently for testing purposes only.

If you have a lot of suggests, you can make them directly on the original for this on hackmd.

Scope

If you want to download FreeBSD, compile it from sources and generally keep up to date that way, this primer is for you. If you are looking to do more with the tree, contribute back, or commit changes, then you will need to wait for a later blog where I cover that. It covers getting the sources, updating the sources, how to bisect and touches briefly on how to cope with a few local changes. It covers the basics, and tries to give good pointers to more in-depth treatment for when the readers finds the basics insufficient.

Keeping Current With FreeBSD src tree

First step: cloning a tree. This downloads the entire tree. There’s two ways to download. Most people will want to do a deep clone of the repo. However, there are times that you may wish to do a shallow clone.

Branch names

The branch names in the new git repo are similar to the old names. For the stable branches, they are stable/X where X is the major release (like 11 or 12). The main branch in the new repo is ‘main’. The main branch in the old github mirror is ‘master’. Both reflecting the defaults of git at the time they were created. The main/master branch is the default branch if you omit the ‘-b branch’ or ‘–branch branch’ options below.

Repositories

At the moment, there’s two repositories. The hashes are different between them. The old github repo is similar to the new cgit repo. However, there are a large number of mistakes in the github repo that required us to regenerate the export when we migrated to having a git repo be the source of truth for the project.

The github repo is at https://github.com/freebsd/freebsd.git
The new cgit beta repo is at https://cgit-beta.freebsd.org/src.git
These will be $URL in the commands below.

Note: The project doesn’t use submodules as they are a poor fit for our workflows and development model. How we track changes in third-party applications is discussed elsewhere and generally of little concern to the casual user.

Deep Clone

A deep clone pulls in the entire tree, as well as all the history and branches. It’s the easiest to do. It also allows you to use git’s worktree feature to have all your active branches checked out into separate directories but with only one copy of the repository.

git clone $URL -b branch [dir]

is how you make a deep clone. ‘branch’ should be one of the branches listed in the previous section. It is optional if it is the main/master branch. dir is an optional directory to place it in (the default will be the name of the repo you are clone (freebsd or src)).

You’ll want a deep clone if you are interested in the history, plan on making local changes, or plan on working on more than one branch. It’s the easiest to keep up to date as well. If you are interested in the history, but are working with only one branch and are short on space, you can also use --single-branch to only download the one branch (though some merge commits will not reference the merged-from branch which may be important for some users who are interested in detailed versions of history).

Shallow Clone

A shallow clone copies just the most current code, but none or little of the history. This can be useful when you need to build a specific revision of FreeBSD, or when you are just starting out and plan to track the tree more fully. You can also use it to limit history to only so many revisions.

git clone -b branch --depth 1 $URL [dir]

This clones the repository, but only has the most recent version in the respository. The rest of the history is not downloaded. Should you change your mind later, you can do ‘git fetch --unshallow’ to get the old history.

Building

Once you’ve downloaded, building is done as described in the handbook, eg:

% cd src
% make buildworld
% make buildkernel
% make installkernel
% make installworld

so that won’t be coverd in depth here.

Updating

To update both types of trees uses the same commands. This pulls in all the revisions since your last update.

git pull --ff-only

will update the tree. In git, a ‘fast forward’ merge is one that only needs to set a new branch pointer and doesn’t need to re-create the commits. By always doing a ‘fast forward’ merge/pull, you’ll ensure that you have an identical copy of the FreeBSD tree. This will be important if you want to maintain local patches.

See below for how to manage local changes. The simplest is to use --autostash on the ‘git pull’ command, but more sophisticated options are available.

Selecting a Specific Version

In git, the ‘git checkout’ command can not only checkout branches, but it can also checkout a specific version. Git’s versions are the long hashes rather than a sequential number. You saw them above in the conflict when it said it couldn’t apply “646e0f9cda11”.

When you checkout a specific version, just specify the hash you want on the command line (the git log command can help you decide which hash you might want):

git checkout 08b8197a74

and you have that checked out.

However, as with many things git, it’s not so simple. You’ll be greeted with a message similar to the following:

Note: checking out '08b8197a742a96964d2924391bf9fdfeb788865d'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 08b8197a742a hook gpiokeys.4 to the build

where the last line is generated from the hash you are checking out and the first line of the commit message from that revision. Also, a word about hashes: they can be abbreviated. That’s why you’ll see them have different lengths in different commands or their outputs. These super long hashes are often unique after 6 or 10 characters, so git lets you abbreviate and is somewhat inconsistent about how it presents them to users.

Bisecting

Sometimes, things go wrong. The last version worked, but the one you just updated to does not. A developer may ask to bisect the problem to track down which commit caused the regression.

If you’ve read the last section, you may be thinking to yourself “How the heck do I bisect with crazy version numbers like that?” then this section is for you. It’s also for you if you didn’t think that, but also want to bisect.

Fortunately, one uses the ‘git bisect’ command. Here’s a brief outline in how to use it. For more information, I’d suggest https://www.metaltoad.com/blog/beginners-guide-git-bisect-process-elimination or https://git-scm.com/docs/git-bisect for more details. The man page is good at describing what can go wrong, what to do when versions won’t build, when you want to use terms other than ‘good’ and ‘bad’, etc, none of which will be covered here.

‘git bisect start’ will start the bisection process. Next, you need to tell a range to go through. ‘git bisect good XXXXXX’ will tell it the working version and ‘git bisect bad XXXXX’ will tell it the bad version. The bad version will almost always be HEAD (a special tag for what you have checked out). The good version will be the last one you checked out.

A quick aside: if you want to know the last version you checked out, you should use ‘git reflog’:

5ef0bd68b515 (HEAD -> master, origin/master, origin/HEAD) HEAD@{0}: pull --ff-only: Fast-forward
a8163e165c5b (upstream/master) HEAD@{1}: checkout: moving from b6fb97efb682994f59b21fe4efb3fcfc0e5b9eeb to master
...

shows me moving the working tree to the master branch (a816…) and then updating from upstream (to 5ef0…). In this case, bad would be HEAD (or 5rf0bd68) and good would be a8163e165. As you can see from the output, HEAD@{1} also often works, but isn’t foolproof if you’ve done other things to your git tree after updating, but before you discover the need to bisect.

Back to git bisect. Set the ‘good’ version first, then set the bad (though the order doesn’t matter). When you set the bad version, it will give you some statistics on the process:

% git bisect start
% git bisect good a8163e165c5b
% git bisect bad HEAD
Bisecting: 1722 revisions left to test after this (roughly 11 steps)
[c427b3158fd8225f6afc09e7e6f62326f9e4de7e] Fixup r361997 by balancing parens.  Duh.

You’d then build/install that version. If it’s good you’d type ‘git bisect good’ otherwise ‘git bisect bad’. You’ll get a similar message to the above each step. When you are done, report the bad version to the developer (or fix the bug yourself and send a patch). ‘git bisect reset’ will end the process and return you back to where you started (usually tip of main). Again, the git-bisect manual (linked above) is a good resource for when things go wrong or for unusual cases.

Ports Considerations

The ports tree operates the same way. The branch names are different and the repos are in different locations.

The github mirror is at https://github.com/freebsd/freebsd-ports.git
The cgit mirror is https://cgit-beta.freebsd.org/src.git

As with ports, the ‘current’ branches are ‘master’ and ‘main’ respectively. The quarterly branches are named the same as in FreeBSD’s svn repo.

Coping with Local Changes

Here’s a small collections of topics that are more advanced for the user tracking FreeBSD. If you have no local changes, you can stop reading now (it’s the last section and OK to skip).

One item that’s important for all of them: all changes are local until pushed. Unlike svn, git uses a distributed model. For users, for most things, there’s very little difference. However, if you have local changes, you can use the same tool to manage them as you use to pull in changes from FreeBSD. All changes that you’ve not pushed are local and can easily be modified (git rebase, discussed below does this).

Keeping local changes

The simplest way to keep local changes (especially trivial ones) is to use ‘git stash’. In its simples form, you use ‘git stash’ to record the changes (which pushes them onto the stash stack). Most people use this to save changes before updating the tree as described above. They then use ‘git stash apply’ to re-apply them to the tree. The stash is a stack of changes that can be examined with ‘git stash list’. The git-stash man page (https://git-scm.com/docs/git-stash) has all the details.

This method is suitable when you have tiny tweaks to the tree. When you have anything non trivial, you’ll likely be better off keeping a local branch and rebasing. It is also integreated with the ‘git pull’ command: just add ‘–autostash’ to the command line.

Keeping a local branch

It’s much easier to keep a local branch with git than subversion. In subversion you need to merge the commit, and resolve the conflicts. This is managable, but can lead to a convoluted history that’s hard to upstream should that ever be necessary, or hard to replicate if you need to do so. Git also allows one to merge, along with the same problems. That’s one way to mange the branch, but it’s the least flexible.

Git has a concept of ‘rebasing’ which you can use to avoids these issues. The ‘git rebase’ command will basically replay all the commits relative to the parent branch at a newer location on that parent branch. This section will briefly cover how to do this, but will not cover all scenarios.

Create a branch

Let’s say you want to make a hack to FreeBSD’s ls command to never, ever do color. There’s many reasons to do this, but this example will use that as a baseline. The FreeBSD ls command changes from time to time, and you’ll need to cope with those changes. Fortunately, with git rebase it usually is automatic.

% cd src
% git checkout main
% git checkout -b no-color-ls
% cd bin/ls
% vi ls.c     # hack the changes in
% git diff    # check the changes
diff --git a/bin/ls/ls.c b/bin/ls/ls.c
index 7378268867ef..cfc3f4342531 100644
--- a/bin/ls/ls.c
+++ b/bin/ls/ls.c
@@ -66,6 +66,7 @@ __FBSDID("$FreeBSD$");
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#undef COLORLS
 #ifdef COLORLS
 #include <termcap.h>
 #include <signal.h>
% # these look good, make the commit...
% git commit ls.c

The commit will pop you into an editor to describe what you’ve done. Once you enter that, you have your own local branch in the git repo. Build and install it like you normally would, following the directions in the handbook. git differs from other version control systems in that you have to tell it explicitly which files to use. I’ve opted to do it on the commit command line, but you can also do it with ‘git add’ which many of the more in depth tutorials cover.

Time to update

When it’s time to bring in a new version, it’s almost the same as w/o the branches. You would update like you would above, but there’s one extra command before you update, and one after. The following assumes you are starting with an unmodified tree. It’s important to start rebasing operations with a clean tree (git usually requires this).

% git checkout main
% git pull --no-ff
% git rebase -i main no-color-ls

This will bring up an editor that lists all the commits in it. For this example, don’t change it at all. This is typically what you are doing while updating the baseline (though you also use the git rebase command to curate the commits you have in the branch).

Once you’re done with the above, you’ve move the commits to ls.c forward from the old version of FreeBSD to the newer one.

Sometimes there’s merge conflicts. That’s OK. Don’t panic. You’d handle them the same as you would any other merge conflicts. To keep it simple, I’ll just describe a common issue you might see. A pointer to a more complete treatment can be found at the end of this section.

Let’s say the includes changes upstream in a radical shift to terminfo as well as a name change for the option. When you updated, you might see something like this:

Auto-merging bin/ls/ls.c
CONFLICT (content): Merge conflict in bin/ls/ls.c
error: could not apply 646e0f9cda11... no color ls
Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply 646e0f9cda11... no color ls

which looks scary. If you bring up an editor, you’ll see it’s a typical 3-way merge conflict resolution that you may be familiar with from other source code systems (the rest of ls.c has been omitted):

<<<<<<< HEAD
#ifdef COLORLS_NEW
#include <terminfo.h>
=======
#undef COLORLS
#ifdef COLORLS
#include <termcap.h>
>>>>>>> 646e0f9cda11... no color ls

The new code is first, and your code is second. The right fix here is to just add a #undef COLORLS_NEW before #ifdef and then delete the old changes:

#undef COLORLS_NEW
#ifdef COLORLS_NEW
#include <terminfo.h>

save the file. The rebase was interrupted, so you have to complete it:

% git add ls.c
% git rebase --cont

which tells git that ls.c has changed and to continue the rebase operation. Since there was a conflict, you’ll get kicked into the editor to maybe update the commit message.

If you get stuck during the rebase, don’t panic. git rebase --abort will take you back to a clean slate. It’s important, though, to start with an unmodified tree.

For more on this topic, https://www.freecodecamp.org/news/the-ultimate-guide-to-git-merge-and-git-rebase/ provides a rather extensive treatment. It goes into a lot of cases I didn’t cover here for simplicity that are useful to know since they come up from time to time.

Updating to a New FreeBSD Branch

Let’s say you want to main the jump from FreeBSD stable/12 to FreeBSD current. That’s easy to do as well, if you have a deep clone.

% git checkout main
% # build and install here...

and you are done. If you have a local branch, though, there’s one or two caveats. First, rebase will rewrite history, so you’ll likely want to do something to save it. Second, jumping branches tends to encounter more conflicts. If we pretend the example above was relative to stable/12, then to move to main, I’d suggest the following:

% git checkout no-color-ls
% git checkout -b no-color-ls-stable-12   # create another name for this branch
% git rebase -i stable/12 no-color-ls --onto main

What the above does is checkout no-color-ls. Then create a new name for it (no-color-ls-stable-12) in case you need to get back to it. Then you rebase onto the main branch. This will find all the commits to the current no-color-ls branch (back to where it meets up with the stable/12 branch) and then it will replay them onto the main branch creating a new no-color-ls branch there (which is why I had you create a place holder name).