CGSIZE inaccuracy?

[Nick Barnes]

I'm running 4.9-RELEASE, and writing some tools to navigate around my
UFS filesystems to figure out what I have lost when I get bad blocks.
There's not much detailed online documentation of UFS beyond fs(5) and
fs.h, so I'm feeling my way through the sources.

Looking at fs.h, I see this:

    /*
     * The size of a cylinder group is calculated by CGSIZE. The maximum size
     * is limited by the fact that cylinder groups are at most one block.
     * Its size is derived from the size of the maps maintained in the
     * cylinder group and the (struct cg) size.
     */
    #define CGSIZE(fs) \
        /* base cg */       (sizeof(struct cg) + sizeof(int32_t) + \
        /* blktot size */   (fs)->fs_cpg * sizeof(int32_t) + \
        /* blks size */     (fs)->fs_cpg * (fs)->fs_nrpos * sizeof(int16_t) + \
        /* inode map */     howmany((fs)->fs_ipg, NBBY) + \
        /* block map */     howmany((fs)->fs_cpg * (fs)->fs_spc / NSPF(fs), NBBY) +\
        /* if present */    ((fs)->fs_contigsumsize <= 0 ? 0 : \
        /* cluster sum */   (fs)->fs_contigsumsize * sizeof(int32_t) + \
        /* cluster map */   howmany((fs)->fs_cpg * (fs)->fs_spc / NSPB(fs), NBBY)))

In a typical filesystem (fs_fsize = 2048, fs_bsize = 16384, fs_ipg =
22528, fs_cpg = 89, fs_spc = 4096, fs_nrpos = 1, fs_contigsumsize =
7), the parts of this sum add up like this:

   172 struct cg;
     4 int32_t
   356 blktot: free blocks per cylinder;
   178 blks: free blocks per rpos per cylinder;
  2816 inode map, one bit per inode;
 11392 block map, one bit per fragment;
    28 cluster summary, one int32_t per contigsumsize+1;
  1424 cluster map, one bit per block;
------
 16370 CGSIZE

However, using the cg_* macros from fs.h (e.g. cg_clustersum), I get
offsets like this:

  base limit  size
     0-  168   168 struct cg less cg_space
   168-  524   356 cg_blktot (free blocks per cylinder)
   524-  702   178 cg_blks (free blocks per rpos per cylinder)
   702- 3518  2816 cg_inosused (inode bitmap)
  3518-14910 11392 cg_blksfree (fragment bitmap)
 14911-14912     2 padding
 14912-14940    28 cg_clustersum (block cluster summaries)
 14940-16364  1424 cg_clusteroff (block bitmap)
 16364             nextfreeoff

There are three discrepancies here:
+4: sizeof(struct cg) is used instead of offsetof(cg_space);
+4: sizeof(int32_t) is added, mysteriously;
-2: the padding for cg_clustersum is disregarded.

I don't *think* that this matters, because CGSIZE is only apparently
used (by newfs and fsck) as a conservative approximation of the size
of the cylinder group header.  But it seems odd, given that a correct
calculation is fairly easy.

Nick Barnes
Ravenbrook Limited