Unstable GCC on Debian Stable

When I was experimenting with Fedora earlier in the year I worked around a lack of cross-compilers for userspace components by using buildroot. However, this only gets you so far. Complex userspace components tend to require many dependencies. Their existence tends to be discovered via pkg-config, and for reasons I didn’t explore buildroot may deploy the pkg-config files in varying places in its host build tree. It’s possible to specify multiple paths via PKG_CONFIG_PATH, but really we’re in kinda sketchy territory as it is. Also, buildroot explicitly doesn’t support compilers in the target environment. If it did life may have been slightly easier.

Anyway, I’m now running Debian, so the cross-build problem is solved more easily: Install the cross-compilers and register the host architecture in question with apt.

While Debian generally makes cross-compilation life easier, I’m running stable (currently Bookworm) on the P14s, and that yields a different challenge. Bookworm ships with GCC-12, which is unfortunate, because OpenBMC userspace needs to be compiled with GCC-13. So we’re back in the familiar territory of needing a different compiler. The Debian testing (trixie) and unstable (sid) suites ship GCC-13, but using the usual method of package pinning results in apt wanting to upgrade most of my system. And so again, we need a different solution.

Despite that, life continues to be easier on Debian because we have debootstrap at our disposal. Coupled with schroot and Meson’s cross-files we can get ourselves a relatively easy-to-use setup that still allows us to dodge the docker container used for OpenBMC CI.

So let’s set up the debootstrap chroot:

$ sudo debootstrap testing /home/andrew/Environments/debian/trixie http://ftp.au.debian.org/debian/

With that done we can create a small schroot config to make it easily usable:

$ cat <<EOF | sudo tee /etc/schroot/chroot.d/trixie.conf
[trixie]
type=directory
description=Debian Trixie (testing)
directory=/home/andrew/Environments/debian/trixie
users=andrew
root-groups=root,andrew
EOF

From there we can install GCC:

$ schroot -c trixie -u root
# apt install gcc

Back outside the chroot we then create a meson cross file to hook the new GCC from the chroot into meson setup ...:

$ cat <<EOF > ~/.local/share/meson/cross/gcc-13
[host_machine]
system = 'linux'
cpu_family = 'x86_64'
cpu = 'x86_64'
endian = 'little'

[constants]
trixie = '/home/andrew/Environments/debian/trixie'

[built-in options]
c_args = [ '--sysroot=' + trixie ]
cpp_args = [ '--sysroot=' + trixie ]
pkg_config_path = trixie + '/usr/lib/x86_64-linux-gnu/pkgconfig'

[properties]
ld_args = [ '--sysroot=' + trixie ]
needs_exe_wrapper = true

[binaries]
c = trixie + '/usr/bin/x86_64-linux-gnu-gcc-13'
cpp = trixie + '/usr/bin/x86_64-linux-gnu-g++-13'
ld = 'ld'
strip = 'strip'
pkg-config = 'pkg-config'
exe_wrapper = 'trixie-meson-exe-wrapper'
EOF

You’ll notice we specify both exe_wrapper and needs_exe_wrapper. These are necessary as the test binaries will link against glibc from the Trixie sysroot. The trixie-meson-exe-wrapper script sets up LD_LIBRARY_PATH such that the Trixie sysroot is visible to the dynamic linker:

$ cat <<EOF > ~/.local/bin/trixie-meson-exe-wrapper
#!/bin/sh

SYSROOT="/home/andrew/Environments/debian/trixie/usr/lib/x86_64-linux-gnu"
LD_LIBRARY_PATH="${SYSROOT}:$LD_LIBRARY_PATH" "$@"
EOF
$ chmod +x ~/.local/bin/trixie-meson-exe-wrapper

We need to force the use of the wrapper with needs_exe_wrapper as the auto-detection is based on comparing the system and cpu_family properties of the build and host systems. That is broadly fine, except for the case of this little stunt we’re trying to pull off with our Trixie sysroot.

With those pieces in place, building a meson-based project with GCC-13 outside the chroot is a matter of:

$ meson setup --cross-file=gcc-13 ...