Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Apr 24, 2008 at 04:36:38PM +0200, Miklos Szeredi wrote:
> > RTFS.  permission() doesn't do "is that vfsmount read-only" checks, exactly
> > because it's 100% bogus - either you cover it with entire area where we
> > are guaranteed to stay r/w, or it's by definition racy.
> 
> I know that.
> 
> That does not mean, that fh_verify() needs to do vfsmount r/o checks.
> AFAICS it's perfectly OK to do that later, around the vfs_ call.

... and around everything else that happens to be done after fh_verify
for write access, surely?  Note that e.g. nfsd_setattr() does _not_ call
vfs_<anything>()...

BTW, that assumes we want to do these checks for nfsd at access time.
And not go for _exports_ being r/w or r/o, with protection applying to
the entire lifetime of export.