Re: [PATCH] statmount: add a new supported_mask field

[Jeff Layton]

On Wed, 2025-02-05 at 18:18 +0100, Jan Kara wrote:
> On Mon 03-02-25 12:09:48, Jeff Layton wrote:
> > Some of the fields in the statmount() reply can be optional. If the
> > kernel has nothing to emit in that field, then it doesn't set the flag
> > in the reply. This presents a problem: There is currently no way to
> > know what mask flags the kernel supports since you can't always count on
> > them being in the reply.
> > 
> > Add a new STATMOUNT_SUPPORTED_MASK flag and field that the kernel can
> > set in the reply. Userland can use this to determine if the fields it
> > requires from the kernel are supported. This also gives us a way to
> > deprecate fields in the future, if that should become necessary.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > I ran into this problem recently. We have a variety of kernels running
> > that have varying levels of support of statmount(), and I need to be
> > able to fall back to /proc scraping if support for everything isn't
> > present. This is difficult currently since statmount() doesn't set the
> > flag in the return mask if the field is empty.
> > ---
> >  fs/namespace.c             | 18 ++++++++++++++++++
> >  include/uapi/linux/mount.h |  4 +++-
> >  2 files changed, 21 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index a3ed3f2980cbae6238cda09874e2dac146080eb6..7ec5fc436c4ff300507c4ed71a757f5d75a4d520 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -5317,6 +5317,21 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
> >  	return 0;
> >  }
> >  
> > +/* This must be updated whenever a new flag is added */
> > +#define STATMOUNT_SUPPORTED (STATMOUNT_SB_BASIC | \
> > +			     STATMOUNT_MNT_BASIC | \
> > +			     STATMOUNT_PROPAGATE_FROM | \
> > +			     STATMOUNT_MNT_ROOT | \
> > +			     STATMOUNT_MNT_POINT | \
> > +			     STATMOUNT_FS_TYPE | \
> > +			     STATMOUNT_MNT_NS_ID | \
> > +			     STATMOUNT_MNT_OPTS | \
> > +			     STATMOUNT_FS_SUBTYPE | \
> > +			     STATMOUNT_SB_SOURCE | \
> > +			     STATMOUNT_OPT_ARRAY | \
> > +			     STATMOUNT_OPT_SEC_ARRAY | \
> > +			     STATMOUNT_SUPPORTED_MASK)
> > +
> >  static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
> >  			struct mnt_namespace *ns)
> >  {
> > @@ -5386,6 +5401,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
> >  	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
> >  		statmount_mnt_ns_id(s, ns);
> >  
> > +	if (!err && s->mask & STATMOUNT_SUPPORTED_MASK)
> > +		s->sm.supported_mask = STATMOUNT_SUPPORTED_MASK;
> 					^^^^ STATMOUNT_SUPPORTED here?

Ouch, yes. Good catch.

> 
> Otherwise the patch looks good to me so with this fixed feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> We could possibly also add:
> 	WARN_ON_ONCE(~s->sm.supported_mask & s->sm.mask);
> 
> to catch when we return feature that's not in supported mask. But maybe
> that's a paranoia :).
> 
> 

No, that's a good idea. I'll add that in and send a v2.

> 
> 
> > +
> >  	if (err)
> >  		return err;
> >  
> > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> > index c07008816acae89cbea3087caf50d537d4e78298..c553dc4ba68407ee38c27238e9bdec2ebf5e2457 100644
> > --- a/include/uapi/linux/mount.h
> > +++ b/include/uapi/linux/mount.h
> > @@ -179,7 +179,8 @@ struct statmount {
> >  	__u32 opt_array;	/* [str] Array of nul terminated fs options */
> >  	__u32 opt_sec_num;	/* Number of security options */
> >  	__u32 opt_sec_array;	/* [str] Array of nul terminated security options */
> > -	__u64 __spare2[46];
> > +	__u64 supported_mask;	/* Mask flags that this kernel supports */
> > +	__u64 __spare2[45];
> >  	char str[];		/* Variable size part containing strings */
> >  };
> >  
> > @@ -217,6 +218,7 @@ struct mnt_id_req {
> >  #define STATMOUNT_SB_SOURCE		0x00000200U	/* Want/got sb_source */
> >  #define STATMOUNT_OPT_ARRAY		0x00000400U	/* Want/got opt_... */
> >  #define STATMOUNT_OPT_SEC_ARRAY		0x00000800U	/* Want/got opt_sec... */
> > +#define STATMOUNT_SUPPORTED_MASK	0x00001000U	/* Want/got supported mask flags */
> >  
> >  /*
> >   * Special @mnt_id values that can be passed to listmount
> > 
> > ---
> > base-commit: 57c64cb6ddfb6c79a6c3fc2e434303c40f700964
> > change-id: 20250203-statmount-f7da9bec0f23
> > 
> > Best regards,
> > -- 
> > Jeff Layton <jlayton@xxxxxxxxxx>
> > 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>