UML on 2G/2G hosts
Most Linux machines are configured so that the kernel occupies the
upper 1G (0xc0000000 - 0xffffffff) of the 4G address space and
processes use the lower 3G (0x00000000 - 0xbfffffff). However, some
machine are configured with a 2G/2G split, with the kernel occupying
the upper 2G (0x80000000 - 0xffffffff) and processes using the lower
2G (0x00000000 - 0x7fffffff).
The prebuilt UML binaries on this site will not run on 2G/2G hosts
because UML occupies the upper .5G of the 3G process address space
(0xa0000000 - 0xbfffffff). Obviously, on 2G/2G hosts, this is right
in the middle of the kernel address space, so UML won't even load - it
will immediately segfault.
The fix for this is to rebuild UML from source after enabling
CONFIG_HOST_2G_2G (under 'General Setup'). This will cause UML to
load itself in the top .5G of that smaller process address space,
where it will run fine. See
this page if you need help building UML from source.