On Thu, 2007-10-11 at 20:31 +0200, Miklos Szeredi wrote:
> > On Thu, 2007-10-11 at 17:08 +0200, Miklos Szeredi wrote:
> > > > diff -puN fs/namei.c~get-write-in-__dentry_open fs/namei.c
> > > > --- lxc/fs/namei.c~get-write-in-__dentry_open 2007-10-03
> > > > 14:44:52.000000000 -0700
> > > > +++ lxc-dave/fs/namei.c 2007-10-04 18:02:48.000000000 -0700
> > > > @@ -1621,14 +1621,6 @@ int may_open(struct nameidata *nd, int a
> > > > return -EACCES;
> > > >
> > > > flag &= ~O_TRUNC;
> > > > - } else if (flag & FMODE_WRITE) {
> > > > - /*
> > > > - * effectively: !special_file()
> > > > - * balanced by __fput()
> > > > - */
> > > > - error = mnt_want_write(nd->mnt);
> > > > - if (error)
> > > > - return error;
> > > > }
> > >
> > > Maybe readonly should still be checked here, so that the order of
> > > error checking doesn't change. If racing with a read-only remount the
> > > order is irrelevant anyway. Something like this?
> > >
> > > } else if (flag & FMODE_WRITE && __mnt_is_readonly(nd->mnt)) {
> > > return -EROFS
> > > }
> >
> > I think that would be a bug if anything actually managed to trip that
> > code. all of the may_open() calls should have been covered by the
> > __dentry_open() mnt writer.
>
> AFACIS, __dentry_open() will normally be called later than may_open().
> And we don't want it earlier, because ->open() may have side affects,
> that could be unsafe if done before permission checking.
I actually check the mount write count before the ->open() in
__dentry_open(). The truncates are also definitely wrapped in their own
mnt_want_write() calls now.
> > > And they should be added around do_truncate() as well, since you
> > > remove the protection from may_open().
> > >
> > > This one introduces an interesting race between ro-remount and
> > > open(O_TRUNC), where the truncate can succeed but the open fail with
> > > EROFS. Is that a problem?
> >
> > You're right, this does introduce that race, and it is relatively hard
> > to fix properly. But, the 'return a filp' patch makes it easy to fix.
> > I've put a temporary kludge in the updated version of this patch, and
> > fixed it properly in that later patch.
>
> If you fix this properly, that should take care of the first problem
> as well.
Yup. New series coming up.
-- Dave
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