It's definitely the case that some of the ELF header fields can be repurposed to store additional data, the following articles all address that optimization:
There are also a few instructions in the article that are size-suboptimal (e.g. "movl $4, %eax" is 5 bytes (b804000000), while the equivalent "xor %eax, %eax; movb $4, %al" is only 4 bytes (31c0b004)).
I tried to do it as optimal as possible, but I think there is indeed more to be saved with putting code in the elf header, i'm not very familiar with the elf format. This is as small as I could get it without doing that, just using yasm, ld and strip.
http://www.muppetlabs.com/~breadbox/software/tiny/teensy.htm... https://www.pimzero.com/2020/04/19/golf_so.html https://rpis.ec/blog/plaidctf-golfso/
There are also a few instructions in the article that are size-suboptimal (e.g. "movl $4, %eax" is 5 bytes (b804000000), while the equivalent "xor %eax, %eax; movb $4, %al" is only 4 bytes (31c0b004)).