Last week, I found myself trying to update the software on a server that was still running Red Hat Enterprise Linux 5. My goal was to run an ASAN build of zephyrd, the Zephyr server to catch a heap corruption bug. ASAN is a feature of the clang compiler, which unfortunately wasn't available.

RHEL5, as well as the free derivative distributions that offer compatibility, was first released in 2007 with kernel 2.6.18; gcc 4.1.2; and glibc 2.5. Although RHEL5 is still "in support", receiving bug and security updates, it's not receiving new versions of the software it shipped it with.

clang provides binary downloads---getting a new version of clang onto this system should be easy! The first problem strikes: glibc is too old to support the binary clang, it depends on features from the newer version. Let's try compiling clang from source. Off goes ./configure...gcc 4.7 or newer required?! Oh no. Building clang would require going through building gcc just to get a new enough compiler. That's not worth the time.

At this point, I stepped back to re-evaluate my options. Although I had root access to the server, it wasn't a machine that I had full control over; the operations team owned it. On one hand, I could ask the team to reinstall it with something a bit more modern, but it would probably not happen until fleet-wide upgrades start. On the other, I could try to press onward with a larger hammer: install a chroot with a newer Enterprise Linux myself. Since the chroot was the more timely option, I decided to go try that first:

mkdir -p /srv/chroot/centos7-x86_64/
rpm --root=/srv/chroot/centos7-x86_64/ --rebuilddb
rpm --root=/srv/chroot/centos7-x86_64/ -ivh centos-release-7.x86_64.rpm

which promptly fails because the version of rpm in EL5 is too old. This can easily be fixed by using another host to build the chroot, as well as running yum install yum to get a minimal base image. Copying the base image over and running chroot . results in the dreaded FATAL: kernel too old message. No chroot tricks are going to work now; the version of glibc in EL7 assumes that the kernel is going to be much newer than the 2.6.18 that's installed.

There's just one last thing to do before giving up and reinstalling the whole server: recompiling glibc for the older kernel. The glibc spec file around line 400 sets the minimum kernel version it supports:

%define enablekernel 2.6.32
by changing that to 
%define enablekernel 2.6.18

we can build an EL7 glibc package that will make a base system suitable for building chroots that work on EL5 systems! The resulting base system with this glibc is the "Enterprise Linux Emergency Modernization Kit" because it makes it easy to get new software on ancient EL5 hosts. You can easily build your own by following these instructions, or you could download a CentOS7-based one now!