Core-dump file format

I think you somehow don't write all the data to the core file.

Create a simple script, make it executable and set the core pattern to the script.

#!/bin/sh
cat > /tmp/core.$$

Now generate a core file (for example run sleep 1243 and press ctrl+\) and it should work.

I just tested it myself on my system and it works without a problem.

The first thing to check that comes to mind is the Elf header flag that indicates what kind of file it is. It has four values - shared object, unlinked object, executable and core dump. That's most likely what's causing gdb errors.

Also, try examining it with objdump - it can pull apart the entire ELF file for analysis what part of it is apparently not good.

You can find the ELF spec at https://refspecs.linuxbase.org/elf/elf.pdf