freebsd-update 6.2-R -> 6.3-B1 rollback failed

[Colin Percival]

Jan Henrik Sylvester wrote:
>> In short, as long as you don't build a custom kernel but call it
>> "GENERIC" or
>> "SMP", FreeBSD Update should automatically DTRT.
> 
> That is exactly my question. On 6.2-RELEASE, I sometimes used a modified
> ld-elf.so.1 or a single patched module without recompiling the kernel.
> What does using freebsd-update (accidentally or deliberately) do in that
> case?  By accident, I discovered that it does not always fail. Does it
> skip the modified files, overwrite them with new versions, or overwrite
> them with an unpredictable bdiff merge that is likely garbage?

Depending on the UpdateIfUnmodified option in freebsd-update.conf, it will
either update files to "clean" new versions or print a warning message and
not touch them.

There's also an IgnorePaths directive which you can use to tell FreeBSD
Update not to touch some files (even if they haven't been modified locally).

FreeBSD Update will never produce mangled files as a result of applying a
bsdiff patch to the wrong file -- it checks file hashes before and after
applying patches and gracefully falls back to downloading complete files
if it can't generate a file via patching.

Colin Percival