NFS superblock sharing implies mount flags bug

[Aurelien Degremont]

Hello

I'm facing incorrect using of mount flags when dealing with NFS mounts 
and I think it could be seen as a bug.

The error occurs when mounting the same NFS export many times, on the 
same machine but *with different mount flags*, particularly concerning 
RO/RW flags.

As the NFS client code re-uses superblocks when it detects that it is 
the same export (same server/same port/same exported directory) and that 
the read-only flag is managed as a per-superblock flag, if a NFS exports 
is mounted a second time, the superblock of the first mount is re-used 
and the specified mount flag is ignored.

# mount foo:/bar /bar_ro -o ro
# mount foo:/bar /bar_rw -o rw
$ touch /bar_rw/bar
touch: cannot touch `/bar_rw/bar': Read-only file system

Ideally, the best solution to fix this is to move the RDONLY flag from 
its per-superblock basis to a per-mountpoint (vfsmount) basis. I do not 
know is there is a something that prevent that except that this implies 
many changes as many codes do not use macros but access s_flags directly.

It seems quite clear that the superblock sharing couldn't be changed (to 
avoid incoherency, inode aliasing and so on...) ?

Do you have a (better) solution ?
I can help if needed.


Cordially

--
Aurelien Degremont
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