Frequently Asked Questions for FreeBSD 6.`X`, 7.`X` and 8.`X`
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# Chapter 18 Advanced Topics

18.1. How can I learn more about FreeBSD's internals?
18.2. How can I contribute to FreeBSD?
18.3. What are snapshots and releases?
18.4. How do I make my own custom release?
18.5. Why does `make `world`` clobber my existing installed binaries?
18.6. Why isn't `cvsup.FreeBSD.org` a round robin DNS entry to share the load amongst the various CVSup servers?
18.7. Can I follow -CURRENT with limited Internet access?
18.8. How did you split the distribution into 1392 KB files?
18.9. I have written a kernel extension, who do I send it to?
18.10. How are Plug N Play ISA cards detected and initialized?
18.11. Can you assign a major number for a device driver I have written?
18.12. What about alternative layout policies for directories?
18.13. How can I make the most of the data I see when my kernel panics?
18.14. Why has `dlsym()` stopped working for ELF executables?
18.15. How can I increase or reduce the kernel address space on i386?

18.1. How can I learn more about FreeBSD's internals?

At this time, there is only one book on FreeBSD-specific OS internals, namely “The Design and Implementation of the FreeBSD Operating System” by Marshall Kirk McKusick and George V. Neville-Neil, ISBN 0-201-70245-2, which focuses on version 5.`X` of FreeBSD.

Additionally, much general UNIX® knowledge is directly applicable to FreeBSD.

For a list of relevant books, please check the Handbook's Operating System Internals Bibliography.

18.2. How can I contribute to FreeBSD?

Please see the article on Contributing to FreeBSD for specific advice on how to do this. Assistance is more than welcome!

18.3. What are snapshots and releases?

There are currently three active/semi-active branches in the FreeBSD CVS Repository. (Earlier branches are only changed very rarely, which is why there are only three active branches of development):

- `RELENG_7` AKA 7-STABLE

- `RELENG_8` AKA 8-STABLE

- `HEAD` AKA -CURRENT AKA 9-CURRENT

`HEAD` is not an actual branch tag, like the other two; it is simply a symbolic constant for “the current, non-branched development stream” which we simply refer to as -CURRENT.

Right now, -CURRENT is the 9.`X` development stream; the `7-STABLE` branch, `RELENG_7`, forked off from -CURRENT in February 2008 and the `8-STABLE` branch, `RELENG_8`, forked off from -CURRENT in November 2009.

18.4. How do I make my own custom release?

Please see the Release Engineering article.

18.5. Why does `make `world`` clobber my existing installed binaries?

Yes, this is the general idea; as its name might suggest, `make `world`` rebuilds every system binary from scratch, so you can be certain of having a clean and consistent environment at the end (which is why it takes so long).

If the environment variable `DESTDIR` is defined while running `make `world`` or `make `install``, the newly-created binaries will be deposited in a directory tree identical to the installed one, rooted at `${DESTDIR}`. Some random combination of shared libraries modifications and program rebuilds can cause this to fail in `make `world`` however.

18.6. Why isn't `cvsup.FreeBSD.org` a round robin DNS entry to share the load amongst the various CVSup servers?

While CVSup mirrors update from the master CVSup server hourly, this update might happen at any time during the hour. This means that some servers have newer code than others, even though all servers have code that is less than an hour old. If `cvsup.FreeBSD.org` was a round robin DNS entry that simply redirected users to a random CVSup server, running CVSup twice in a row could download code older than the code already on the system.

18.7. Can I follow -CURRENT with limited Internet access?

Yes, you can do this without downloading the whole source tree by using the CTM facility.

18.8. How did you split the distribution into 1392 KB files?

Newer BSD based systems have a `-b` option to split(1) that allows them to split files on arbitrary byte boundaries.

Here is an example from `/usr/src/release/Makefile`.

    ZIPNSPLIT=              gzip --no-name -9 -c | split -b 1392k -

18.9. I have written a kernel extension, who do I send it to?

Please take a look at the article on Contributing to FreeBSD to learn how to submit code.

And thanks for the thought!

18.10. How are Plug N Play ISA cards detected and initialized?

By: Frank Durda IV `<uhclem@nemesis.lonestar.org>`

In a nutshell, there a few I/O ports that all of the PnP boards respond to when the host asks if anyone is out there. So when the PnP probe routine starts, it asks if there are any PnP boards present, and all the PnP boards respond with their model # to a I/O read of the same port, so the probe routine gets a wired-OR “yes” to that question. At least one bit will be on in that reply. Then the probe code is able to cause boards with board model IDs (assigned by Microsoft®/Intel®) lower than `X` to go “off-line”. It then looks to see if any boards are still responding to the query. If the answer was `0`, then there are no boards with IDs above `X`. Probe will then ask for boards below `X`. Finally, probe requests boards greater than `X - (limit / 4)` to go off-line. It then repeats this query. By repeating this semi-binary search of IDs-in-range enough times, the probing code will eventually identify all PnP boards present in a given machine with a number of iterations that is much lower than what 264 would take.

The IDs are two 32-bit fields (hence 264) + 8-bit checksum. The first 32 bits are a vendor identifier. They never come out and say it, but it appears to be assumed that different types of boards from the same vendor could have different 32-bit vendor IDs. The idea of needing 32 bits just for unique manufacturers is a bit excessive.

The lower 32 bits are a serial #, or something else that makes this one board unique. The vendor must never produce a second board that has the same lower 32 bits unless the upper 32 bits are also different. So you can have multiple boards of the same type in the machine and the full 64 bits will still be unique.

The 32-bit groups can never be all zero. This allows the wired-OR to show non-zero bits during the initial binary search.

Once the system has identified all the board IDs present, it will reactivate each board, one at a time (via the same I/O ports), and find out what resources the given board needs, what interrupt choices are available, etc. A scan is made over all the boards to collect this information.

This info is then combined with info from any ECU files on the hard disk or wired into the MLB BIOS. The ECU and BIOS PnP support for hardware on the MLB is usually synthetic, and the peripherals do not really do genuine PnP. However by examining the BIOS info plus the ECU info, the probe routines can cause the devices that are PnP to avoid those devices the probe code cannot relocate.

Then the PnP devices are visited once more and given their I/O, DMA, IRQ and Memory-map address assignments. The devices will then appear at those locations and remain there until the next reboot, although there is nothing that says you cannot move them around whenever you want.

There is a lot of oversimplification above, but you should get the general idea.

Microsoft took over some of the primary printer status ports to do PnP, on the logic that no boards decoded those addresses for the opposing I/O cycles. I found a genuine IBM printer board that did decode writes of the status port during the early PnP proposal review period, but Microsoft said “tough”. So they do a write to the printer status port for setting addresses, plus that use that address + `0x800`, and a third I/O port for reading that can be located anywhere between `0x200` and `0x3ff`.

18.11. Can you assign a major number for a device driver I have written?

FreeBSD releases after February 2003 has a facility for dynamically and automatically allocating major numbers for device drivers at runtime (see devfs(5)), so there is no need for this.

18.12. What about alternative layout policies for directories?

In answer to the question of alternative layout policies for directories, the scheme that is currently in use is unchanged from what I wrote in 1983. I wrote that policy for the original fast file system, and never revisited it. It works well at keeping cylinder groups from filling up. As several of you have noted, it works poorly for find. Most file systems are created from archives that were created by a depth first search (aka ftw). These directories end up being striped across the cylinder groups thus creating a worst possible scenario for future depth first searches. If one knew the total number of directories to be created, the solution would be to create `(total / fs_ncg)` per cylinder group before moving on. Obviously, one would have to create some heuristic to guess at this number. Even using a small fixed number like say 10 would make an order of magnitude improvement. To differentiate restores from normal operation (when the current algorithm is probably more sensible), you could use the clustering of up to 10 if they were all done within a ten second window. Anyway, my conclusion is that this is an area ripe for experimentation.

Kirk McKusick `<mckusick@FreeBSD.org>`, September 1998

18.13. How can I make the most of the data I see when my kernel panics?

Here is typical kernel panic:

    Fatal trap 12: page fault while in kernel mode
    fault virtual address   = 0x40
    fault code              = supervisor read, page not present
    instruction pointer     = 0x8:0xf014a7e5
    stack pointer           = 0x10:0xf4ed6f24
    frame pointer           = 0x10:0xf4ed6f28
    code segment            = base 0x0, limit 0xfffff, type 0x1b
                            = DPL 0, pres 1, def32 1, gran 1
    processor eflags        = interrupt enabled, resume, IOPL = 0
    current process         = 80 (mount)
    interrupt mask          =
    trap number             = 12
    panic: page fault

When you see a message like this, it is not enough to just reproduce it and send it in. The instruction pointer value is important; unfortunately, it is also configuration dependent. In other words, the value varies depending on the exact kernel image that you are using. If you are using a `GENERIC` kernel image from one of the snapshots, then it is possible for somebody else to track down the offending function, but if you are running a custom kernel then only you can tell us where the fault occurred.

What you should do is this:

1. Write down the instruction pointer value. Note that the `0x8:` part at the beginning is not significant in this case: it is the `0xf0xxxxxx` part that we want.

2. When the system reboots, do the following:

       % nm -n kernel.that.caused.the.panic | grep f0xxxxxx

   where `f0xxxxxx` is the instruction pointer value. The odds are you will not get an exact match since the symbols in the kernel symbol table are for the entry points of functions and the instruction pointer address will be somewhere inside a function, not at the start. If you do not get an exact match, omit the last digit from the instruction pointer value and try again, i.e.:

       % nm -n kernel.that.caused.the.panic | grep f0xxxxx

   If that does not yield any results, chop off another digit. Repeat until you get some sort of output. The result will be a possible list of functions which caused the panic. This is a less than exact mechanism for tracking down the point of failure, but it is better than nothing.

However, the best way to track down the cause of a panic is by capturing a crash dump, then using kgdb(1) to generate a stack trace on the crash dump.

In any case, the method is this:

1. Make sure that the following line is included in your kernel configuration file (`/usr/src/sys/`arch`/conf/`MYKERNEL``):

       makeoptions     DEBUG=-g          # Build kernel with gdb(1) debug symbols

2. Change to the `/usr/src` directory:

       # cd /usr/src

3. Compile the kernel:

       # make buildkernel KERNCONF=MYKERNEL

4. Wait for make(1) to finish compiling.

5.     # make installkernel KERNCONF=MYKERNEL

6. Reboot.

> Note: If you do not use the `KERNCONF` make variable a `GENERIC` kernel will be built and installed.

The make(1) process will have built two kernels. `/usr/obj/usr/src/sys/`MYKERNEL`/kernel` and `/usr/obj/usr/src/sys/`MYKERNEL`/kernel.debug`. `kernel` was installed as `/boot/kernel/kernel`, while `kernel.debug` can be used as the source of debugging symbols for kgdb(1).

To make sure you capture a crash dump, you need edit `/etc/rc.conf` and set `dumpdev` to point to your swap partition (or `AUTO`). This will cause the rc(8) scripts to use the dumpon(8) command to enable crash dumps. You can also run dumpon(8) manually. After a panic, the crash dump can be recovered using savecore(8); if `dumpdev` is set in `/etc/rc.conf`, the rc(8) scripts will run savecore(8) automatically and put the crash dump in `/var/crash`.

> Note: FreeBSD crash dumps are usually the same size as the physical RAM size of your machine. That is, if you have 512 MB of RAM, you will get a 512 MB crash dump. Therefore you must make sure there is enough space in `/var/crash` to hold the dump. Alternatively, you run savecore(8) manually and have it recover the crash dump to another directory where you have more room. It is possible to limit the size of the crash dump by using `options MAXMEM=`N`` where `N` is the size of kernel's memory usage in KBs. For example, if you have 1 GB of RAM, you can limit the kernel's memory usage to 128 MB by this way, so that your crash dump size will be 128 MB instead of 1 GB.

Once you have recovered the crash dump, you can get a stack trace with kgdb(1) as follows:

    % kgdb /usr/obj/usr/src/sys/MYKERNEL/kernel.debug /var/crash/vmcore.0
    (kgdb) backtrace

Note that there may be several screens worth of information; ideally you should use script(1) to capture all of them. Using the unstripped kernel image with all the debug symbols should show the exact line of kernel source code where the panic occurred. Usually you have to read the stack trace from the bottom up in order to trace the exact sequence of events that lead to the crash. You can also use kgdb(1) to print out the contents of various variables or structures in order to examine the system state at the time of the crash.

> Tip: Now, if you are really insane and have a second computer, you can also configure kgdb(1) to do remote debugging such that you can use kgdb(1) on one system to debug the kernel on another system, including setting breakpoints, single-stepping through the kernel code, just like you can do with a normal user-mode program.

> Note: If you have `DDB` enabled and the kernel drops into the debugger, you can force a panic (and a crash dump) just by typing `panic` at the `ddb` prompt. It may stop in the debugger again during the panic phase. If it does, type `continue` and it will finish the crash dump.

18.14. Why has `dlsym()` stopped working for ELF executables?

The ELF toolchain does not, by default, make the symbols defined in an executable visible to the dynamic linker. Consequently `dlsym()` searches on handles obtained from calls to `dlopen(NULL, flags)` will fail to find such symbols.

If you want to search, using `dlsym()`, for symbols present in the main executable of a process, you need to link the executable using the `--export-dynamic` option to the ELF linker (ld(1)).

18.15. How can I increase or reduce the kernel address space on i386?

By default, the kernel address space is 1 GB (2 GB for PAE) for i386. If you run a network-intensive server (e.g. a large FTP or HTTP server), or you want to use ZFS, you might find that is not enough.

Add the following line to your kernel configuration file to increase available space and rebuild your kernel:

    options KVA_PAGES=N

To find the correct value of `N`, divide the desired address space size (in megabytes) by four. (For example, it is `512` for 2 GB.)

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