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

[Dave Chinner <david@fromorbit.com>]

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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com