Is gfcombinefs - a filesystem for n-of-m Shamir secret sharing - fundamentally read-only?
I am the author of gfcombinefs.

Instead of sharing your actual secret, I would suggest secret-sharing an immutable, securely-random bytestring, and using that bytestring as a symmetric encryption key. That way, you can use any standard encryption mechanism to change and re-encrypt your actual secret. That's what I now do instead of using gfcombinefs - I use the gfcombine tool to reconstitute a symmetric encryption key on a ramfs, and use it to decrypt an encrypted volume. This means I have the freedom to change the secret keys that are on the encrypted volume, back up the encrypted volume, and so on without needing to re-share anything (which is very inconvenient if you have written some of the shares to external media stored in a separate location, or to paper).


  In these circumstances, I do not really understand why writing to a Shamir filesystem would fundamentally be unsupported?


Three reasons:


I didn't need it when I wrote gfcombinefs, and I didn't implement what I didn't need :-)
I haven't done the mathematics to prove whether it would be safe
I would guess (although I have no proof for this assertion) that making it safe would require having all the shares online and modifiable at the same time


Another commenter wrote:


  AFAICT, it should be safe against attackers who only get one look at the shares, but it might leak information if the attacker can observe the same n′&lt; n shares before and after the change


You are welcome to do whatever you want with gfcombinefs (bearing in mind that it has absolutely no warranty, and I never actually did a proper release of it). However, if it was my secret being shared, I would not be comfortable with doing this until I had a security model, and a mathematical proof that the proposed scheme met the security model.

I wrote a mathematical proof that libgfshare had the properties I believed it did (https://sources.debian.net/src/libgfshare/2.0.0-2/doc/theory.tex/) before starting to use libgfshare for anything I valued. I would suggest doing the same if you use it differently.