view design.html @ 2:be48c60f9edb

Update it a bit. Talk about environmental dependencies, etc.
author landley@driftwood
date Sun, 13 Aug 2006 22:30:17 -0400
parents 9b6afefcc082
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<title>Flimsy rationalizations for all of my design mistakes</title>

<h1>Build Process</h1>

<h2>Executive summary</h2>

<p>Cross-compile just enough to get a native compiler for the new environment,
and then emulate the new environment with QEMU to build the final system
natively.</p>

<p>The intermediate system is built and run using only the following eight
packages:</p>

<ul>
<li>linux-kernel</li>
<li>uclibc</li>
<li>busybox</li>
<li>binutils</li>
<li>gcc</li>
<li>make</li>
<li>bash<li>
<li>QEMU</li>
</ul>

<h2>The basic theory</h2>

<p>What we want to do is 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.</p>

<p>A minimal build environment consists of a C library, a compiler, and BusyBox.
So in theory you just need three packages:</p>

<ul>
  <li>A C library (uClibc)</li>
  <li>A toolchain (tcc)</li>
  <li>BusyBox</li>
</ul>

<p>Unfortunately, that doesn't work yet.</p>

<h2>Some differences between theory and reality.</h2>

<h3>Environmental dependencies.</h2>

<p>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.)</p>

<p>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.</p>

<p>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.)</p>

<p>The Firmware Linux build process is structured the way it is to eliminate
environmental dependencies.  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 chroot into that and work our way back up from there
by building more packages in the new environment.</p>

<h3>Resolving environmental dependencies.</h2>

<p><b>To build uClibc you need kernel headers</b> 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.</p>

<p>The 0.8x series of Firmware Linux used
<a href=http://ep09.pld-linux.org/~mmazur/linux-libc-headers/>kernel
headers manually cleaned up by Mariusz Mazur</a>, but after the 2.6.12 kernel
he had an attack of real life and fell too far behind to catch up again.</p>

<p>The current practice is to use the 2.6.18 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, so as of 2.6.18 we can use the Linux Kernel tarball as a source of
headers again.</p>

<p>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
<b>substitute bash</b>.</p>

<p>Finally, <b>most packages expect gcc</b>.  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
<a href=http://fabrice.bellard.free.fr/tcc/tccboot.html>built a modified Linux
kernel with tcc</a>, and
<a href=http://fabrice.bellard.free.fr/tcc/tccboot_readme.html>listed</a>
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 BusyBox 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.</p>

<p>The gnu compiler actually consists of three packages <b>(binutils, gcc, and
make)</b>, 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.)</p>

<p>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).</p>

<p>This can actually be made to work.  The next question is how?</p>

<h2>Additional complications</h2>

<h3>Cross-compiling and avoiding root access</h2>

<p>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.</p>

<p>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)...</p>

<p>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.</p>

<p>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.</p>

<p>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.)</p>

<h2>Ways to make it all work</h2>

<h3>Cross compiling vs native compiling under emulation</h3>

<p>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.</p>

<p>I wrote an
<a href=https://crossdev.timesys.com/documentation/introduction-to-cross-compiling-for-linux/>introduction
to cross-compiling</a> 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 compiler for the
new platform, which we then run under emulation.</p>

<h3>Which emulator?</h3>

<p>The emulator Firmware Linux 0.8x used was User Mode Linux (here's a
<a href=http://www.landley.net/code/UML.html>UML mini-howto</a> 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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>It's possible to use other emulators instead of QEMU, and I have a todo
item to look at armulator.  (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.)</p>

<h3>Alternatives to emulation</h3>

<p>The main downsides of emulation are that is it's slow, can use a lot of
memory, and can be tricky to debug if something goes wrong in the emulated
environment.  Cross compiling is sufficiently harder than native compiling that
I consider it a good trade-off, but there are alternatives.</p>

<p>Some other build systems (such as uClibc's Buildroot) use a package called
<a href=http://freshmeat.net/projects/fakeroot/>fakeroot</a>, which is sort
of a halfway emulator.  It creates an environment where binaries run as if
they had root access, but without being able to do anything that actually
requires root access.  This is nice if you want to create tarballs with
device nodes and different ownership in them, but not so useful if you want
to actually use one of those device nodes, or twiddle mount points.  Firmware
Linux doesn't use fakeroot (we use a real emulator instead), but it's
an option.</p> 

<p>In theory, we could work around the "host hasn't got uClibc" problem by
statically linking our apps for the intermediate system, and work around the
"host kernel older than the kernel headers we're using" problem by either
building the intermediate version of uClibc with the host's kernel headers
or just linking against glibc instead of uClibc.</p>

<p>This has a number of
downsides: harvesting the host's kernel headers is distribution-specific, and
could easily leak bits of the host into the final system.  Linking the host
tools against glibc (or a temporary version of uClibc built with different
kernel headers) doesn't give us as much evidence that the resulting system
will be able to rebuild itself under itself, and statically linking against
glibc wastes a regrettable amount of space.  None of this works with real
cross-compiling between different processors (such as building an ARM system
from x86).</p>

<p>We'd still have to solve the other problems (such as gcc wanting absolute
paths) anyway, there just wouldn't be a switchover point where we could
run the binaries we were building and start native compiling.  Instead we'd
have to keep cross-compiling all the way to the final system, and if anything's
wrong with it we wouldn't find out until we tried to run it.  With the native
build, we've given the tools a bit of a workout during the build, so if the
build completes then the finished system shouldn't have anything too
fundamentally wrong with it.</p>

<p>(Note: QEMU can export a host directory to the target through the emulated
network card as an smb filesystem, but you don't want to run your root
filesystem on smb.)</p>

<h2>Filesystem Layout</h2>

<p>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.</p>

<h3>Simplifying the $PATH.</h3>

<p>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.)</p>

<p>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.</p>

<p>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.</p>

<p>I.E. The seperation is just a historical relic, and I've consolidated it in
the name of simplicity.</p>

<p>The one bit where this can cause a problem is merging /lib with /usr/lib,
which means that the same library can show up in the search path twice, and
when that happens binutils gets confused and bloats the resulting executables.
(They become as big as statically linked, but still refuse to run without
opening the shared libraries.)  This is really a bug in either binutils or
collect2, and has probably been fixed since I first noticed it.  In any case,
the proper fix is to take /lib out of the binutils search path, which we do.
The symlink is left there in case somebody's using dlopen, and for "standards
compliance".</p>

<p>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.</p>

<p>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.</p>

<h3>Consolidating writeable directories.</h3>

<p>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".</p>

<p>The theoretical difference between /tmp and /var/tmp is that the contents
of /var/tmp should definitely be deleted by the system init scripts on every
reboot, but the contents of /tmp may be preserved across reboots.  Except
deleting everyting out of /tmp during a reboot is a good idea anyway, and any
program that actually depends on the contents of /tmp being preserved across
a reboot is obviously broken, so there's no reason not to symlink them
together.</p>

<p>(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.)</p>

<p>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.</p>

<h3>The resulting mount points</h3>

<p>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.</p>

<p>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.</p>

</p>