FWL builds a cross-compiler and then uses it to build a minimal system containing a native compiler, BusyBox and uClibc. Then it runs this minimal system under an emulator (QEMU) to natively build the final system. Finally it packages the resulting system (kernel, initramfs, and root filesystem) into a single file that can boot and run (on x86 by using a modified version of LILO).
Firmware Linux builds in stages:
The first stage builds a cross-compiler, which runs on the host system and produces binaries that run on the target system. (See my Introduction to cross compiling if you're unfamiliar with this.)
We have to cross-compile even if the host and target system are both x86, because the host probably use different C libraries. If the host has glibc and the target uses uClibc, then the (dynamically linked) target binaries we produce won't run on the host. This is what distinguishes cross-compiling from native compiling: different processors are just one reason the binaries might not run. Of course, as long as we've got the cross-compiling support anyway, we might as well support building for x86_64, arm, mips, or ppc targets...
Building a cross-compiler toolchain requires four packages. The bulk of it is binutils, gcc, and uClibc, but building those requires header files from the Linux kernel which describe the target system.
Because cross-compiling is persnickety and difficult, we do as little of it as possible. Instead we use the cross-compiler to generate the smallest possible native build environment for the target, and then run the rest of the build in that environment, under an emulator.
The emulator we use is QEMU. The minimal build environment powerful enough to boot and compile a complete Linux system requires seven packages: the Linux kernel, binutils, gcc, uClibc, BusyBox, make, and bash. It's packaged using the Linux From Scratch /tools directory approach, staying out of the way so the minimal build environment doesn't get mixed into the final system.
Running a native build under QEMU is much slower than cross-compiling, but it's a lot easier and more reliable.
A trick to accelerate the build is to use distcc to call out to the cross-compiler, feeding the results back into the emulator through the virtual network. (This is still a TODO item.)
The actual build run under stange 3 can be a fairly straightforward Linux From Scratch approach, or another source based Linux build system like Gentoo.
The reason for the name Firmware Linux is that the entire operating system (kernel, initramfs, and read-only squashfs root filesystem) are glued together into a single file. A modified version of LILO is included which can boot and run this file on x86.
The Linux From Scratch approach is to build a minimal intermediate system with just enough packages to be able to compile stuff, chroot into that, and build the final system from there. This isolates the host from the target, which means you should be able to build under a wide variety of distributions. It also means the final system is built with a known set of tools, so you get a consistent result.
A minimal build environment consists of a C library, a compiler, and BusyBox. So in theory you just need three packages:
Unfortunately, that doesn't work yet.
Environmental dependencies are things that need to be installed before you can build or run a given package. Lots of packages depend on things like zlib, SDL, texinfo, and all sorts of other strange things. (The GnuCash project stalled years ago after it released a version with so many environmental dependencies it was impossible to build or install. Environmental dependencies have a complexity cost, and are thus something to be minimized.)
A good build system will scan its environment to figure out what it has available, and disable functionality that depends on stuff that isn't available. (This is generally done with autoconf, which is disgusting but suffers from a lack of alternatives.) That way, the complexity cost is optional: you can build a minimal version of the package if that's all you need.
A really good build system can be told that the environment it's building in and the environment the result will run in are different, so just because it finds zlib on the build system doesn't mean that the target system will have zlib installed on it. (And even if it does, it may not be the same version. This is one of the big things that makes cross-compiling such a pain. One big reason for statically linking programs is to eliminate this kind of environmental dependency.)
The Firmware Linux build process is structured the way it is to eliminate as many environmental dependencies as possible. Some are unavoidable (such as C libraries needing kernel headers or gcc needing binutils), but the intermediate system is the minimal fully functional Linux development environment I currently know how to build, and then we switch into that and work our way back up from there by building more packages in the new environment.
To build uClibc you need kernel headers identifying the syscalls and such it can make to the OS. Way back when you could use the kernel headers straight out of the Linux kernel 2.4 tarball and they'd work fine, but sometime during 2.5 the kernel developers decided that exporting a sane API to userspace wasn't the kernel's job, and stopped doing it.
The 0.8x series of Firmware Linux used kernel headers manually cleaned up by Mariusz Mazur, but after the 2.6.12 kernel he had an attack of real life and fell too far behind to catch up again.
The current practice is to use the Linux kernel's "make headers_install" target, created by David Woodhouse. This runs various scripts against the kernel headers to sanitize them for use by userspace. This was merged in 2.6.18-rc1, and was more or less debugged by 2.6.19. So can use the Linux Kernel tarball as a source of headers again.
Another problem is that the busybox shell situation is a mess with four implementations that share little or no code (depending on how they're configured). The first question when trying to fix them is "which of the four do you fix?", and I'm just not going there. So until bbsh goes in we substitute bash.
Finally, most packages expect gcc. The tcc project isn't a drop-in gcc replacement yet, and doesn't include a "make" program. Most importantly, tcc development appears stalled because Fabrice Bellard's other major project (qemu) is taking up all his time these days. In 2004 Fabrice built a modified Linux kernel with tcc, and listed what needed to be upgraded in TCC to build an unmodified kernel, but since then he hardly seems to have touched tcc. Hopefully, someday he'll get back to it and put out a 1.0 release of tcc that's a drop-in gcc replacment. (And if he does, I'll add a make implementation to toybox so we don't need to use any of the gnu toolchain). But in the meantime the only open source compiler that can build a complete Linux system is still the gnu compiler.
The gnu compiler actually consists of three packages (binutils, gcc, and make), which is why it's generally called the gnu "toolchain". (The split between binutils and gcc is for purely historical reasons, and you have to match the right versions with each other or things break.)
This means that to compile a minimal build environment, you need seven packages, and to actually run the result we use an eighth package (QEMU).
This can actually be made to work. The next question is how?
The first problem is that we're cross-compiling. We can't help it. You're cross-compiling any time you create target binaries that won't run on the host system. Even when both the host and target are on the same processor, if they're sufficiently different that one can't run the other's binaries, then you're cross-compiling. In our case, the host is usually running both a different C library and an older kernel version than the target, even when it's the same processor.
The second problem is that we want to avoid requiring root access to build Firmware Linux. If the build can run as a normal user, it's a lot more portable and a lot less likely to muck up the host system if something goes wrong. This means we can't modify the host's / directory (making anything that requires absolute paths problematic). We also can't mknod, chown, chgrp, mount (for --bind, loopback, tmpfs)...
In addition, the gnu toolchain (gcc/binutils) is chock-full of hardwired assumptions, such as what C library it's linking binaries against, where to look for #included headers, where to look for libraries, the absolute path the compiler is installed at... Silliest of all, it assumes that if the host and target use the same processor, you're not cross-compiling (even if they have a different C library and a different kernel, and even if you ./configure it for cross-compiling it switches that back off because it knows better than you do). This makes it very brittle, and it also tends to leak its assumptions into the programs it builds. New versions may someday fix this, but for now we have to hit it on the head repeatedly with a metal bar to get anything remotely useful out of it, and run it in a separate filesystem (chroot environment) so it can't reach out and grab the wrong headers or wrong libraries despite everything we've told it.
The absolute paths problem affects target binaries because all dynamically linked apps expect their shared library loader to live at an absolute path (in this case /lib/ld-uClibc.so.0). This directory is only writeable by root, and even if we could install it there polluting the host like that is just ugly.
The Firmware Linux build has to assume it's cross-compiling because the host is generally running glibc, and the target is running uClibc, so the libraries the target binaries need aren't installed on the host. Even if they're statically linked (which also mitigates the absolute paths problem somewhat), the target often has a newer kernel than the host, so the set of syscalls uClibc makes (thinking it's talking to the new kernel, since that's what the ABI the kernel headers it was built against describe) may not be entirely understood by the old kernel, leading to segfaults. (One of the reasons glibc is larger than uClibc is it checks the kernel to see if it supports things like long filenames or 32-bit device nodes before trying to use them. uClibc should always work on a newer kernel than the one it was built to expect, but not necessarily an older one.)
Cross compiling is a pain. There are a lot of ways to get it to sort of kinda work for certain versions of certain packages built on certain versions of certain distributions. But making it reliable or generally applicable is hard to do.
I wrote an introduction to cross-compiling which explains the terminology, plusses and minuses, and why you might want to do it. Keep in mind that I wrote that for a company that specializes in cross-compiling. Personally, I consider cross-compiling a necessary evil to be minimized, and that's how Firmware Linux is designed. We cross-compile just enough stuff to get a working native build environment for the new platform, which we then run under emulation.
The emulator Firmware Linux 0.8x used was User Mode Linux (here's a UML mini-howto I wrote while getting this to work). Since we already need the linux-kernel source tarball anyway, building User Mode Linux from it was convenient and minimized the number of packages we needed to build the minimal system.
The first stage of the build compiled a UML kernel and ran the rest of the build under that, using UML's hostfs to mount the parent's root filesystem as the root filesystem for the new UML kernel. This solved both the kernel version and the root access problems. The UML kernel was the new version, and supported all the new syscalls and ioctls and such that the uClibc was built to expect, translating them to calls to the host system's C library as necessary. Processes running under User Mode Linux had root access (at least as far as UML was concerned), and although they couldn't write to the hostfs mounted root partition, they could create an ext2 image file, loopback mount it, --bind mount in directories from the hostfs partition to get the apps they needed, and chroot into it. Which is what the build did.
Current Firmware Linux has switched to a different emulator, QEMU, because as long as we're we're cross-compiling anyway we might as well have the ability to cross-compile for non-x86 targets. We still build a new kernel to run the uClibc binaries with the new kernel ABI, we just build a bootable kernel and run it under QEMU.
The main difference with QEMU is a sharper dividing line between the host system and the emulated target. Under UML we could switch to the emulated system early and still run host binaries (via the hostfs mount). This meant we could be much more relaxed about cross compiling, because we had one environment that ran both types of binaries. But this doesn't work if we're building an ARM, PPC, or x86-64 system on an x86 host.
Instead, we need to sequence more carefully. We build a cross-compiler, use that to cross-compile a minimal intermediate system from the seven packages listed earlier, and build a kernel and QEMU. Then we run the kernel under QEMU with the new intermediate system, and have it build the rest natively.
It's possible to use other emulators instead of QEMU, and I have a todo item to look at armulator from uClinux. (I looked at another nommu system simulator at Ottawa Linux Symposium, but after resolving the third unnecessary environmental dependency and still not being able to get it to finish compiling yet, I gave up. Armulator may be a patch against an obsolete version of gdb, but I could at least get it to build.)
The single file packaging combines a linux kernel, initramfs, squashfs partition, and cryptographic signature.
In Linux 2.6, the kernel and initramfs are already combined into a single file. At the start of this file is either the obsolete floppy boot sector (just a stub in 2.6), or an ELF header which has 12 used bytes followed by 8 unused bytes. Either way, we can generally use the 4 bytes starting at offset 12 to store the original length of the kernel image, then append a squashfs root partition to the file, followed by a whole-file cryptographic signature.
Loading an ELF kernel (such as User Mode Linux or a non-x86 ELF kernel) is controlled by the ELF segments, so the appended data is ignored. (Note: don't strip the file or the appended data will be lost.) Loading an x86 bzImage kernel requires a modified boot loader that can be told the original size of the kernel, rather than querying the current file length (which would be too long). Hence the patch to Lilo allowing a "length=xxx" argument in the config file.
Upon boot, the kernel runs the initramfs code which finds the firmware file. In the case of User Mode Linux, the symlink /proc/self/exe points to the path of the file. A bootable kernel needs a command line argument of the form firmware=device:/path/to/file (it can lookup the device in /sys/block and create a temporary device node to mount it with; this is in expectation of dynamic major/minor happening sooner or later). Once the file is found, /dev/loop0 is bound to it with an offset (losetup -o, with a value extracted from the 4 bytes stored at offset 12 in the file), and the resulting squashfs is used as the new root partition.
The cryptographic signature can be verified on boot, but more importantly it can be verified when upgrading the firmware. New firmware images can be installed beside old firmware, and LILO can be updated with boot options for both firmware, with a default pointing to the _old_ firmware. The lilo -R option sets the command line for the next boot only, and that can be used to boot into the new firmware. The new firmware can run whatever self-diagnostic is desired before permanently changing the default. If the new firmware doesn't boot (or fails its diagnostic), power cycle the machine and the old firmware comes up. (Note that grub does not have an equivalent for LILO's -R option; which would mean that if the new firmware doesn't run, you have a brick.)
Firmware Linux's directory hierarchy is a bit idiosyncratic: some redundant directories have been merged, with symlinks from the standard positions pointing to their new positions. On the bright side, this makes it easy to make the root partition read-only.
The set "bin->usr/bin, sbin->usr/sbin, lib->usr/lib" all serve to consolidate all the executables under /usr. This has a bunch of nice effects: making a a read-only run-from-CD filesystem easier to do, allowing du /usr to show the whole system size, allowing everything outside of there to be mounted noexec, and of course having just one place to look for everything. (Normal executables are in /usr/bin. Root only executables are in /usr/sbin. Libraries are in /usr/lib.)
For those of you wondering why /bin and /usr/sbin were split in the first place, the answer is it's because Ken Thompson and Dennis Ritchie ran out of space on the original 2.5 megabyte RK-05 disk pack their root partition lived on in 1971, and leaked the OS into their second RK-05 disk pack where the user home directories lived. When they got more disk space, they created a new direct (/home) and moved all the user home directories there.
The real reason we kept it is tradition. The execuse is that the root partition contains early boot stuff and /usr may get mounted later, but these days we use initial ramdisks (initrd and initramfs) to handle that sort of thing. The version skew issues of actually trying to mix and match different versions of /lib/libc.so.* living on a local hard drive with a /usr/bin/* from the network mount are not pretty.
I.E. The seperation is just a historical relic, and I've consolidated it in the name of simplicity.
On a related note, there's no reason for "/opt". After the original Unix leaked into /usr, Unix shipped out into the world in semi-standardized forms (Version 7, System III, the Berkeley Software Distribution...) and sites that installed these wanted places to add their own packages to the system without mixing their additions in with the base system. So they created "/usr/local" and created a third instance of bin/sbin/lib and so on under there. Then Linux distributors wanted a place to install optional packages, and they had /bin, /usr/bin, and /usr/local/bin to choose from, but the problem with each of those is that they were already in use and thus might be cluttered by who knows what. So a new directory was created, /opt, for "optional" packages like firefox or open office.
It's only a matter of time before somebody suggests /opt/local, and I'm not humoring this. Executables for everybody go in /usr/bin, ones usable only by root go in /usr/sbin. There's no /usr/local or /opt. /bin and /sbin are symlinks to the corresponding /usr directories, but there's no reason to put them in the $PATH.
All the editable stuff has been moved under "var", starting with symlinking tmp->var/tmp. Although /tmp is much less useful these days than it used to be, some things (like X) still love to stick things like named pipes in there. Long ago in the days of little hard drive space and even less ram, people made extensive use of temporary files and they threw them in /tmp because ~home had an ironclad quota. These days, putting anything in /tmp with a predictable filename is a security issue (symlink attacks, you can be made to overwrite any arbitrary file you have access to). Most temporary files for things like the printer or email migrated to /var/spool (where there are persistent subdirectories with known ownership and permissions) or in the user's home directory under something like "~/.kde".
The theoretical difference between /tmp and /var/tmp is that the contents of /tmp should be deleted by the system init scripts on every reboot, but the contents of /var/tmp may be preserved across reboots. Except there's no guarantee that the contents of any temp directory won't be deleted. So any program that actually depends on the contents of /var/tmp being preserved across a reboot is obviously broken, and there's no reason not to just symlink them together.
(I case it hasn't become apparent yet, there's 30 years of accumulated cruft in the standards, convering a lot of cases that don't apply outside of supercomputing centers where 500 people share accounts on a mainframe that has a dedicated support staff. They serve no purpose on a laptop, let alone an embedded system.)
The corner case is /etc, which can be writeable (we symlink it to var/etc) or a read-only part of the / partition. It's really a question of whether you want to update configuration information and user accounts in a running system, or whether that stuff should be fixed before deploying. We're doing some cleanup, but leaving /etc writeable (as a symlink to /var/etc). Firmware Linux symlinks /etc/mtab->/proc/mounts, which is required by modern stuff like shared subtrees. If you want a read-only /etc, use "find /etc -type f | xargs ls -lt" to see what gets updated on the live system. Some specific cases are that /etc/adjtime was moved to /var by LSB and /etc/resolv.conf should be a symlink somewhere writeable.
The result of all this is that a running system can have / be mounted read only (with /usr living under that), /var can be ramfs or tmpfs with a tarball extracted to initialize it on boot, /dev can be ramfs/tmpfs managed by udev or mdev (with /dev/pts as devpts under that: note that /dev/shm naturally inherits /dev's tmpfs and some things like User Mode Linux get upset if /dev/shm is mounted noexec), /proc can be procfs, /sys can bs sysfs. Optionally, /home can be be an actual writeable filesystem on a hard drive or the network.
Remember to put root's home directory somewhere writeable (I.E. /root should move to either /var/root or /home/root, change the passwd entry to do this), and life is good.
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