Re: [bug report] xfs: pass the goal of the incore inode walk to xfs_inode_walk()

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

On Fri, Aug 13, 2021 at 07:40:48AM +1000, Dave Chinner wrote:
> On Thu, Aug 12, 2021 at 09:42:22AM +0300, Dan Carpenter wrote:
> > Hello Darrick J. Wong,
> > 
> > The patch c809d7e948a1: "xfs: pass the goal of the incore inode walk
> > to xfs_inode_walk()" from Jun 1, 2021, leads to the following
> > Smatch static checker warning:
> > 
> > 	fs/xfs/xfs_icache.c:52 xfs_icwalk_tag()
> > 	warn: unsigned 'goal' is never less than zero.
> > 
> > fs/xfs/xfs_icache.c
> >     49 static inline unsigned int
> >     50 xfs_icwalk_tag(enum xfs_icwalk_goal goal)
> >     51 {
> > --> 52 	return goal < 0 ? XFS_ICWALK_NULL_TAG : goal;
> > 
> > This enum will be unsigned in GCC, so "goal" can't be negative.
> 
> I think this is incorrect. The original C standard defines enums as
> signed integers, not unsigned. And according to the GCC manual
> (section 4.9 Structures, Unions, Enumerations, and Bit-Fields)
> indicates that C90 first defines the enum type to be compatible with
> the declared values. IOWs, for a build using C89 like the kernel
> does, enums should always be signed.
> 
> This enum is defined as:
> 
> enum xfs_icwalk_goal {
>         /* Goals that are not related to tags; these must be < 0. */
>         XFS_ICWALK_DQRELE       = -1,
> 
>         /* Goals directly associated with tagged inodes. */
>         XFS_ICWALK_BLOCKGC      = XFS_ICI_BLOCKGC_TAG,
>         XFS_ICWALK_RECLAIM      = XFS_ICI_RECLAIM_TAG,
> };
> 
> i.e. the enum is defined to clearly contain negative values and so
> GCC should be defining it as a signed integer regardless of the
> version of C being used...
> 
> > Plus
> > we only pass 0-1 for goal (as far as Smatch can tell).
> 
> Yup, smatch has definitely got that one wrong:
> 
> xfs_dqrele_all_inodes()
>   xfs_icwalk(mp, XFS_ICWALK_DQRELE, &icw);
>     xfs_icwalk_get_perag(.... XFS_ICWALK_DQRELE)
>       xfs_icwalk_tag(... XFS_ICWALK_DQRELE, ...)
> 
> So this warning looks like an issue with smatch, not a bug in the
> code...

...unless Dan is running smatch against for-next, which removes
XFS_ICWALK_DQRELE and thus allows for an unsigned type to back the enum?

--D

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com