Re: [PATCH 1/1] mount_util: fix mount_flags entries for MS_RDONLY

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, May 08, 2026 at 12:43:14PM +0200, Bernd Schubert wrote:
> 
> 
> On 5/8/26 00:13, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > MS_RDONLY maps to MOUNT_ATTR_RDONLY in the new fsmount API, but the
> > table omitted that.  Fix that.
> > 
> > Fixes: 0d7e72541564a5 ("Unify mount flag structures and remove redundant is_mount_attr field")
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> >  lib/mount_util.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/lib/mount_util.c b/lib/mount_util.c
> > index 1a0aec9bfe1a70..7ab1dba91e28e3 100644
> > --- a/lib/mount_util.c
> > +++ b/lib/mount_util.c
> > @@ -110,8 +110,8 @@
> >  
> >  const struct mount_flags mount_flags[] = {
> >  /* opt            flag             on  safe  mount_attr */
> > -{"rw",           MS_RDONLY,        0,  1,    0},                      /* fsconfig */
> > -{"ro",           MS_RDONLY,        1,  1,    0},                      /* fsconfig */
> > +{"rw",           MS_RDONLY,        0,  1,    MOUNT_ATTR_RDONLY},     /* fsconfig */
> > +{"ro",           MS_RDONLY,        1,  1,    MOUNT_ATTR_RDONLY},     /* fsconfig */
> >  {"suid",         MS_NOSUID,        0,  0,    MOUNT_ATTR_NOSUID},     /* fsmount  */
> >  {"nosuid",       MS_NOSUID,        1,  1,    MOUNT_ATTR_NOSUID},     /* fsmount  */
> >  {"dev",          MS_NODEV,         0,  1,    MOUNT_ATTR_NODEV},      /* fsmount  */
> > 
> 
> Hi Darrick,
> 
> I had already pulled this the night before and it fails the new tests as
> well. I think issue is that ro needs to be done via fsmount(...,
> MOUNT_ATTR_RDONLY) *and* fsconfig(SET_FLAG, "ro"). I'm going to add a
> new column into const struct mount_flags mount_flags[].

Ewww, you're right.  util-linux also special-cases this, and in a weird
way:

       Since version 2.41, libmount has the ability to use optional
       arguments vfs and fs (e.g. ro=fs) to specify where the read-only
       setting should be applied. For example, using the command:

          mount -o ro=vfs /dev/sdc1 /A

       will mount the filesystem as read-write on the superblock level,
       but the /A node will be set as read-only. In previous versions,
       this required an additional "-o bind,remount,ro" operation to
       achieve the same result.

"ro=vfs" means apply it to the mount but don't tell the filesystem;
"ro=fs" means apply tell it to the filesystem, but not the mount;
and plain "ro" means apply it to both?  Apparently??

--D