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Overview

NetBSD is an extremely flexible operating system that is designed to be portable across various architectures. This feature makes it attractive for embedded developers. In this article, I will demonstrate a process for creating a very small kernel that can boot, either to a shell prompt or to a login screen.

Booting to the shell prompt allows the developer to quickly give life to a system and perform some basic interactions. The shell itself can be a powerful management tool, combined with the right collection of programs. Doing this requires only the kernel and two binaries.

Booting to a login prompt provides a little added security by controlling access to the machine. This would be useful in devices where the console can be accessed by the user, and you need to control what the user has access to. There are a few added files in this setup, and to keep things small, you will need to manually trim some of them. But this is still a very easy process.

The minimum single user system

The first stage of embedded development usually involves getting a skeleton operating system up and running. Assuming you are using one of NetBSD many supported platforms, this is quite easy. I used a PC booting off of a floppy drive. But this method can also be easily adapted to handle diskless clients that boot off a network.

At an absolute minimum, you will need the kernel, the /dev nodes for your system, and a /sbin/init file. You can craft the init program yourself, putting all of your application code into it. This may be a good solution when you only have one application that you would like to run.

Another option is to use the stock init program and a shell to call your application. This is a bit easier than writing your own init program, and it also allows you to add functionality to your system by simply adding more tools from the stock system. The shell itself is a powerful tool that can make future development much easier. This is the method that will be illustrated here.

Building the kernel

The first thing that needs to be done is to build a kernel with a built in ramdisk. The ramdisk will hold the kernel's root filesystem. A good kernel configuration to start with is the INSTALL_TINY configuration. This kernel has all the important stuff in it, without a lot of bloat. Make sure that it has the following lines in it.

in sys/arch/[arch]/conf

$ cp INSTALL_TINY MYTINY
$ vi MYTINY

# Enable the hooks used for initializing the root memory-disk.
options     MEMORY_DISK_HOOKS
options     MEMORY_DISK_IS_ROOT         # force root on memory disk
options     MEMORY_DISK_SERVER=1        # make the ramdisk writeable
#options    MEMORY_DISK_ROOT_SIZE=2880  # 1.44M, same as a floppy
options     MEMORY_DISK_ROOT_SIZE=8192  # 4Meg
options     MEMORY_RBFLAGS=0            # don't force single user

You will also need to check the ramdisk driver to make sure that it supports the MEMORY_RBFLAGS option. In my 1.6.0 system, it did not, so I needed to add the following code.

in sys/dev/md_root.c

 /*
  * This is called during open (i.e. mountroot)
  */

 ##  #ifndef MEMORY_RBFLAGS
 ##  #define MEMORY_RBFLAGS RB_SINGLE
 ##  #endif

 void
 md_open_hook(int unit, struct md_conf *md)
 {

     //            ............................... added code
     if (unit == 0 && (MEMORY_RBFLAGS & RB_SINGLE) ) {

     /* The root ramdisk only works single-user. */
     boothowto |= RB_SINGLE;
     }
 } 
 

Once this is done, build your kernel.

$ config MYTINY
$ cp ../compile/MYTINY
$ make depend; make

Creating the crunched binaries

Although you could copy the binaries from your host into your mini filesystem, a more efficient way (sizewise) to do this is to use a utility called crunchgen. Many programs in the NetBSD system are linked statically. For example, each program that uses the utility library (libutil) has one copy of the library linked to it. Several programs on the system produce redundant copies of the libraries used by the programs. Crunchgen takes the programs' object files and merges them into one uber-program. This crunched binary is then linked to the libraries, so only one copy of the library is needed for the whole system.

"But how do I use the different programs?", you ask. The answer lies in hard links. A hard link is like a filesystem's alias for a program. For example, If I have a program called print_my_error, I can link that to the alias myerr. Then when I invoke myerr, the filesystem simply follows the link and runs the program print_my_error. I can even make the two program names have slightly different behavior. This is done by having print_my_error look at argv[0] that is passed to the main() function. If it is myerr, then I can have it do a special task.

This is how crunchgen works. It takes all of its constituent programs and globs them together. Then in the crunched binary's main routine, there is logic that examines argv[0] and calls the main routine for the appropriate constituent.

To make a crunched binary, you need to know what binaries you want, where their sources are located, and what libraries they use. Once you settle on what binaries to use, simply search the /usr/src folder for the sources. The layout of /usr/src is straightforward. To look for the source for /sbin/init, you would look in /usr/src/sbin/init. To find the /usr/bin/login, go to /usr/src/usr.bin/login. To find out which libraries are used, open up the program's Makefile, and look for the LDADD lines. You should see entries like "-lutil -lcrypt".

Sometimes, programs in the NetBSD distribution are simply hardlinks to another program. For example, mount_mfs is an alias for newfs. You will find no /usr/src/sbin/mount_mfs directory. So how do you find out where the sources are for mount_mfs? Search through the makefiles for "LINKS= ${BINDIR}/newfs ${BINDIR}/mount_mfs". This shows you that the Makefile will link mount_mfs to newfs.

$  find /usr/src -name Makefile -exec grep mount_mfs {} \;
MAN=    newfs.8 mount_mfs.8
LINKS=  ${BINDIR}/newfs ${BINDIR}/mount_mfs
MLINKS= mount_mfs.8 mfs.8

Armed with this information, you can now create your crunchgen configuration file. This is just a list of the above information. There are some additional features, which I will outline below.

srcdirs /usr/src/bin /usr/src/sbin 

progs init sh reboot ls

ln sh -sh

special init objpaths /usr/src/sbin/init/obj/init.smallprog.o


# libraries used by the programs
# ---------------- Minimum single user files
# init : -lutil -lcrypt
# sh : -ll -ledit -ltermcap
# ---------------- Useful utilities
# ls : -
# reboot : -lutil
#
libs -lutil -lcrypt -ll -ledit -ltermcap 

The special line tells crunchgen that init should not be built from source, but rather just use the specified object file. In this case, I built the init program with the SMALLPROG #define, so I don't get the annoying "Enter pathname of shell or RETURN for sh:" prompt when the system starts in single user mode. Instead, it will silently drop to the shell prompt. To do this, I build my special init program this way.

$ cd /usr/src/sbin/init
$ make -D SMALLPROG
$ cp obj/init.o obj/init.smallprog.o

Finally, the last line tells crunchgen which libraries to link to. Again, you can get this information by looking through the Makefiles for the constituent programs and noting the LDADD lines. I've had trouble with library ordering, so if you have errors indicating unresolved externals, and you know you included the library, try moving the "missing" library closer to the front of the list. This worked for me.

Once your crunchgen file is crafted, build it with the following commands. The finished executable will be named after your conf file, but without the conf extension.

$ crunchgen -m Makefile mytiny.conf
$ make -f Makefile objs exe

Continue to page 2: Creating the filesystem image and the minimum multi user system.