Re: NFS superblock sharing implies mount flags bug

[Aurelien Degremont]

Nobody is interested by this issue ?
It could be easily reproduced. All recent versions are concerned.

Aurelien


Aurelien Degremont wrote:
> 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
CEA/DAM - DIF/DSSI/SISR
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