On Going 64-bit

Some History

My desktop system almost exclusively runs software that I compiled from source. It’s been that way for a long time, since I got frustrated with the lacklustre pace of Debian-stable software updates and with the package management system in general (this was 1998 or so). I began compiling packages from source rather than updating them via the Debian repository, and eventually decided to purge the remaining parts of Debian from my system, replacing Debian packages with compiled-from-source counterparts.

What can I say; I was young and optimistic. Surprisingly enough, I managed to get a working system, though compiling many packages was fraught with unwarranted difficulty and required various hacks to makefiles and buildscripts. Linux From Scratch didn’t even exist yet, so I was mostly on my own when I ran into problems, but I persevered and it payed off. I had a nice, fast, working system and I had a good understanding of how it all fit together. Later, when Fedora and Ubuntu appeared on the scene (and Debian had got its act together somewhat) I felt no desire to switch to these fancy new distributions because doing so would mean losing the strong connection with the system that I had pieced together by hand.

Sure, I faced some problems. Upgrading packages was a difficult, and often required upgrading several of the dependencies. Uninstalling packages was a nightmarish procedure of identifying which files belonged to the package and deleting them one by one. I learned that I could use “make DESTDIR=… install” to output many packages into their own root, and I eventually I wrote a shell script that would “install” packages by symbolically linking them into the root file system from a package specific root, and could uninstall them by removing those links – so I had a kind of rudimentary package management (I cursed those packages which didn’t support DESTDIR or an equivalent; I often ended up needing to edit the makefile by hand). I usually compiled without many of the optional dependencies and to a large extent I avoided the “dependency hell” that had plagued me while using Debian. I found Linux From Scratch at some point and used its guides as a starting point when I wanted to upgrade or install a new package. I kept notes of build options and any specific hacks or patches that I had used. Larger packages (the Mozilla suite and OpenOffice come to mind) were the most problematic, often incorporating non-standard build systems and having undocumented dependencies; to fix an obscure build problem I often had to tinker and then repeat the build process, which could run for hours, multiple times until I had managed to work around the issue (anyone who’s read this blog is now probably starting to understand why I have such a loathing for bad build documentation!).

Despite all the problems and the work involved, I maintained this system to the present day.

Modernising

When I started building my system, the processor was a 32-bit Pentium III. When an upgrade gave me a processor that was 64-bit capable, it didn’t seem like the effort of switching to the new architecture was justifiable, so I stuck with the 32-bit software system. Recently I decided that it’s time to change, so I begun the task of re-compiling the system for the 64-bit architecture. This required building GCC as a cross-compiler, that is, a compiler that runs on one architecture but that targets another.

Building a cross-compiler was not as easy as I had hope it would be. GCC requires a toolchain (linker, assembler etc) that supports the target architecture. GNU Binutils targeting on a 32-bit architecture cannot handle the production of 64-bit binaries, so the first step (and probably the easiest) was to build a cross-target Binutils. This was really about as simple as:

./configure --prefix=/opt/x86_64 --host=i686-pc-linux-gnu --target=x86_64-pc-linux-gnu
make
make install

However, building GCC as a cross compiler is nowhere near as trivial. The issue is that GCC includes both a compiler and a runtime-support library. Building the runtime-support library requires linking against an appropriate system C library, and of course I didn’t have one of those, and I couldn’t build one because I didn’t have a suitable cross-compiler. It turns out, however, that you can build just enough of GCC to be able to build just enough of Glibc that then you can then build a bit more of GCC and then the rest of Glibc and finally the rest of GCC. This process isn’t formally documented (which is a shame) but there’s a good rundown of it in a guide by Jeff Preshing, without which I’m not sure I would have succeeded. (Some additional notes on cross compiling GCC are included at the end of this post).

Now I had a working cross-compiler targeting the x86_64 platform. When built as a cross-compiler GCC and Binutils name their executables beginning with an architecture prefix, so for instance “gcc” becomes “x86_64-pc-linux-gnu-gcc” and “ld” becomes “x86_64-pc-linux-gnu-ld” (you actually get these names when building as a non-cross compiler, too, but in that case you also get the non-prefixed name installed). To build a 64-bit kernel, you simply supply some variables to the kernel make process:

make ARCH=x86_64 CROSS_COMPILE="/opt/x86_64/bin/x86_64-pc-linux-gnu-" menuconfig
make ARCH=x86_64 CROSS_COMPILE="/opt/x86_64/bin/x86_64-pc-linux-gnu-" bzImage
# and so on

The “CROSS_COMPILE” variable is the prefix used for any compiler/binutil utility when producing object for the target. So for example “gcc” is prefixed to become “/opt/x86_64/bin/x86_64-pc-linux-gnu-gcc”.

I built the kernel, and booted to it. It worked! … except that it refused to run “init”, because I hadn’t enabled “IA-32 emulation” (the ability to run 32-bit executables on a 64-bit kernel). That was easily resolved, however.

I then set about building some more 64-bit packages: build Binutils as a 64-bit native package, re-building Glibc without the special –prefix, building GCC as a 64-bit native compiler (which required first cross-compiling its prequisites, including MPFR, GMP and MPC). All seems to be going ok so far.

Multi-lib / Multi-arch

One issue with having 32-bit and 64-bit libraries in the same system is that you have name clashes. If I have a 32-bit /usr/lib/libgmp.so.10.1.3, where do I put the 64-bit version? It seems that the prevalent solution is to put 64-bit libraries in /usr/lib64 (or /lib64) and leave only 32-bit libraries in plain /usr/lib and /lib. The Glibc dynamic linker hard-codes these paths when compiled for x86_64, it seems. So, when I build 64-bit packages I’ll use –libdir=/usr/lib64, I guess, but I don’t like this much; the division seems pretty unnatural, mainly in that it favours the 32-bit architecture and is somewhat arbitrary. Debian and Ubuntu both appear to have similar reservations and are working on “Multiarch spec”, but for now I’ll go with the lib/lib64 division as it’s going to be less immediate hassle.

I also still have the issue that I can’t control which packages pkg-config will detect. I guess I can use the PKG_CONFIG_PATH environment variable to add the 64-bit paths in when doing a 64-bit build, but I can’t prevent it from picking up 32-bit packages in the case where the 64-bit package isn’t installed, which I imagine could lead to some pretty funky build errors.

That’s about where I’m at. It wasn’t particularly easy, but it is done and it seems to work.

Notes on building GCC as a Cross Compiler

First, read the guide by Jeff Preshing.

I have the following additional notes:

  • If it’s not clear from Jeff’s guide, –prefix for your GCC cross-compiler build should be /xyz (or whatever you like) and –prefix for Glibc should be /xyz/$TARGET, in my case I used /usr/x86-64 and /usr/x86-64/x86_64-pc-linux-gnu. The GCC cross-compiler will expect to find libraries and include files in the latter (under …/lib and …/include).
  • I wanted multilib support, whereas Jeff’s guide builds GCC without multilib. For this you need the 32-bit Glibc available to the cross-compiler, in $GLIBC_PREFIX/lib/32 (being /usr/x86-64/x86_64-pc-linux-gnu/lib/32 in my case). I symlinked various libraries, but you might get away with just symlinking your entire /usr/lib as $GLIBC_PREFIX/lib/32. You also need $GLIBC_PREFIX/include/gnu/stubs-32.h (which you can link from /usr/include/gnu/stubs-32.h).
  • During the Glibc build I got an error about an unresolved symbol, on something like __stack_chk_guard (foolishly I did not record the exact error). I got around this by configuring Glibc with ‘libc_cv_ssp=no’.
  • If you install 64-bit libraries into the standard /usr/lib and want to link against them when building with your cross-compiler, you’ll need to add -L/usr/lib to your linker flags (eg LDFLAGS=-L/usr/lib during configure).
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4 thoughts on “On Going 64-bit

  1. No mention of why you didn’t try Gentoo…? It gives you all the “do it myself” effect but saves you a ton of the time for tracking down dependencies and figuring out build options. Also keeps track of which file came from which package to make uninstalls possible, and revdep-rebuild to track down broken dependencies.

    Anyway, but I agree that a system where each package goes into its own folder and gets symlinked into the root FHS is a nice design. I’d love to see a distro that does that by default.

    1. I think I first started messing with Linux in ’98 or ’99, just before Gentoo appeared on the scene. By the time I was compiling all the packages on the system, it was probably 2001 or even a bit later; Gentoo existed then, but I’m not sure I’d heard of it (and probably would’ve been reluctant to abandon the system I had developed on my own). In hindsight, though, Gentoo would’ve probably been a very good fit.

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