I have been on the 64 -current upgrades for quite a long time and have had rather few problems or issues and understand the procedures and how to fix minor problems.
I upgraded from the 20 Feb -current tree and the 4.19.24 kernel to the 4.19.25 kernel (Sat 23 Feb). I ran eliloconfig as I always need to do after kernel upgrade.

On reboot I got a login prompt with the keyboard disabled
and the message says that I am using 4.19.24 (not 4.19.25).
I tried the upgrade on a different partition on the same machine and had no problems at all upgrading to 4.19.25 .
I find nothing wrong with the disk and indeed the disk checks on boot are all ok but there are errors about missing kernel modules. The disk is not full.
It seems that the upgrade has failed. I have not encountered this problem before.

Can I fix the botched upgrade by chroot-ing into the root of the failed upgrade or do I need to reinstall slackware on that partition?

You shouldn't need to reinstall. My guess is that for some reason eliloconfig didn't work. You should be able to boot off your install media, mount the EFI partition, and manually copy over the kernel (and possibly initrd, if you need that) to the efi/Slackware/ folder on that partition.

1 members found this post helpful.

If you do the manual update as bassmadrigal suggested, make sure that the eliloconfig that sits next to kernel and image will use the newly copied kernel and image. In case you're using the generic names this won't be a problem, but in case your kernel and image carry the kernel-number -say when also using refind for booting- the config needs editing; this can be done very quickly as root in midnight commander (mc) via-F4/F2).

2 members found this post helpful.

BassMadrigal was right.
The problem was with eliloconfig even though I had carefully run it
after the upgrade.

I was able to boot the slackware on that disk with Grub
(no, I don't like Grub) so I am going again
and will get back to addressing efi issues after I do some other pressing work.

thanks much.
*
@brobr - I am not using refind as present but it
is an interesting tool and I have used it.
I learned a lot about bootloaders from
the refind website http://www.rodsbooks.com/refind/

thanks again.
I got the EFI fixed.
I was surprised by the failure of eliloconfig.
I had boot problems previously when I had forgotten to run lilo or eliloconfig but this was the first time that eliloconfig failed for me.

I'm glad you got it solved! EFI certainly does make things a bit easier in that you just need to have the file copied over to the EFI partition and referenced properly in the elilo.conf, but when the tools fail, it certainly does cause problems.

It would certainly be interesting to see why or how it failed. If you have any guesses as to why or if you remember any error messages that might've popped up, it might help get the program updated to catch the error and display it or to fix any broken code that might exist in the program.

I have continued to upgrade on the -current tree and no further issues. I am not sure why it eliloconfig failed that one time.