Re: ls input/output error ("NFS: readdir(/) returns -5") on krb5 NFSv4 client using SHA2

[Scott Mayhew]

On Mon, 17 Nov 2025, Tyler W. Ross wrote:

> Weird behavior I just discovered:
> 
> Explicitly setting allowed-enctypes in the gssd section of /etc/nfs.conf
> to exclude aes256-cts-hmac-sha1-96 makes both SHA2 ciphers work as
> expected (assuming each is allowed).
> 
> If allowed-enctypes is unset (letting gssd interrogate the kernel for
> supported enctypes) or includes aes256-cts-hmac-sha1-96, then the XDR
> overflow occurs.
> 
> Non-working configurations (first is the commented-out default in nfs.conf):
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes128-cts-hmac-sha256-128,camellia256-cts-cmac,camellia128-cts-cmac,aes256-cts-hmac-sha1-96,aes128-cts-hmac-sha1-96
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes256-cts-hmac-sha1-96
> allowed-enctypes=aes128-cts-hmac-sha256-128,aes256-cts-hmac-sha1-96
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes128-cts-hmac-sha256-128,aes256-cts-hmac-sha1-96
> 
> Working configurations (first is default sans aes256-cts-hmac-sha1-96):
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes128-cts-hmac-sha256-128,camellia256-cts-cmac,camellia128-cts-cmac,aes128-cts-hmac-sha1-96
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes128-cts-hmac-sha256-128
> allowed-enctypes=aes256-cts-hmac-sha384-192,aes128-cts-hmac-sha1-96
> allowed-enctypes=aes128-cts-hmac-sha256-128,aes128-cts-hmac-sha1-96
> 

That doesn't really make sense.  You should only need to use the
allowed-enctypes setting if you're talking to an NFS server that doesn't
have support for the new encryption types.

It basically works like the "permitted_enctypes" option in krb5.conf,
except it only affects NFS rather than affecting your krb5 configuration
as a whole.

Can you go back and re-do the tracepoint capture, except this time
umount your NFS filessytems before starting the capture (i.e. perform
the mount command while trace-cmd is running).  I'm curious what values
the rpcgss_update_slack tracepoint shows.

> 
> Is this gssd mishandling some setup/initialization?
> Or is there a miscalculation happening somewhere further up?
> 
> 
> TWR
>