bug#46292: more info

[Lucas Nussbaum]

Hi,

This is not due to NFS, but due to the fact that the NFS mount is
mounted nosuid (and nodev, probably). I can reproduce it on a local
filesystem mounted nosuid.

It seems that, when remounting a bind mount which is originally nosuid
inside a mount ns, you need to specify explicitely the nosuid option, or
else can_change_locked_flags()[1] will return false.

[1] https://github.com/torvalds/linux/blame/master/fs/namespace.c#L2480

There's a concept of "locked mount flags" that cannot be cleared by a
less privileged user (see [2]). Our call to 'mount -o remount' ignores the
fact that the filesystem is mounted nosuid (and does not include this
flag), so the remount call tries to remove nosuid, and fails.

[2] 
https://github.com/torvalds/linux/commit/9566d6742852c527bf5af38af5cbb878dad75705

This probably needs to be fixed in Guix by fetching the current mount
flags and including them in the bind+remount+readonly call.
Unfortunately I did not find an easy way to convert mount flags in
/proc/$$/mountinfo to flags for the mount syscall...

Lucas