Understanding kernel abi bumps for Ubuntu

[Luis R. Rodriguez]

I'd like to further understand the reasoning for kernel ABI bumps for
Ubuntu. From what I gather from the documentation [1] this is to keep
binary compatibility, so that if a package is built on an older kernel
we know when a newer kernel introduced _something_ which would make
the old binary package not work on a kernel with a newer ABI. Does
that sum it up?

If so I'd like to know if someone can elaborate on what could be that
_something_.

I run old userspace on newer kernels all the time, and I don't see any
issues upgrading, nor do I see a reason for old userspace to not work
on newer kernels. In fact when newer kernels break old userspace that
is a bug. An exception that I can think of is when we document
something old for removal on
Documentation/feature-removal-schedule.txt and then remove it from a
new kernel. But that's an exception and these exceptions get wide
review prior to removal.

Another change which could affect userspace is a change in the kernel
configuration used. A clear example here is disabling
CONFIG_WIRELESS_EXT which you can now do. This would mean you *do*
need new userspace utilities to allow you to use nl80211. But
distributions won't be disabling this for a long time, so
wireless-extensions will always be available.

When someone tells me they cannot upgrade their kernel due to old
tools, I consider this an excuse, I'd like to know if there are real
examples you come up with which do not fit in the above mentioned
exceptions.

[1] https://help.ubuntu.com/community/Kernel/Compile

  Luis