Re: [PATCH 003/119] xfs: check offsets of variable length structures

Brian Foster
2016-06-20T12:38:42Z

On Sat, Jun 18, 2016 at 11:01:33AM -0700, Darrick J. Wong wrote:
> On Fri, Jun 17, 2016 at 01:34:27PM -0400, Brian Foster wrote:
> > On Thu, Jun 16, 2016 at 06:18:12PM -0700, Darrick J. Wong wrote:
> > > Some of the directory/attr structures contain variable-length objects,
> > > so the enclosing structure doesn't have a meaningful fixed size at
> > > compile time. We can check the offsets of the members before the
> > > variable-length member, so do those.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> >
> > I'm missing why this is necessary. Is the intent still to catch
> > alignment and/or padding issues? If so, isn't the size check sufficient,
> > regardless of trailing variable size fields?
> >
> > Perhaps the goal here is to reduce the scope of checking from where it
> > isn't needed..? For example, xfs_dir2_data_unused_t looks like it has a
> > field where the offset in the structure is irrelevant, so that's a
> > possible false positive if that changes down the road. On the flip side,
> > that doesn't appear to be the case for other structures such as
> > xfs_attr_leaf_name_[local|remote]_t.
>
> ISTR making this change to work around behavioral variances in how
> much padding gcc adds to structures across its various targets. The
> macros that go along with the variable sized structures work fine,
> but testing the sizeof() doesn't work reliably.
>

Ok, I take that to mean that we may or may not have padding in some of
the variable structures depending on architecture (and we really only
care about certain fields in those structures). Fair enough, thanks!

Brian

> --D
>
> >
> > Brian
> >
> > > fs/xfs/xfs_ondisk.h | 25 +++++++++++++++++++++++--
> > > 1 file changed, 23 insertions(+), 2 deletions(-)
> > >
> > >
> > > diff --git a/fs/xfs/xfs_ondisk.h b/fs/xfs/xfs_ondisk.h
> > > index 184c44e..0272301 100644
> > > --- a/fs/xfs/xfs_ondisk.h
> > > +++ b/fs/xfs/xfs_ondisk.h
> > > @@ -22,6 +22,11 @@
> > > BUILD_BUG_ON_MSG(sizeof(structname) != (size), "XFS: sizeof(" \
> > > #structname ") is wrong, expected " #size)
> > >
> > > +#define XFS_CHECK_OFFSET(structname, member, off) \
> > > + BUILD_BUG_ON_MSG(offsetof(structname, member) != (off), \
> > > + "XFS: offsetof(" #structname ", " #member ") is wrong, " \
> > > + "expected " #off)
> > > +
> > > static inline void __init
> > > xfs_check_ondisk_structs(void)
> > > {
> > > @@ -75,15 +80,28 @@ xfs_check_ondisk_structs(void)
> > > XFS_CHECK_STRUCT_SIZE(xfs_attr_leaf_name_remote_t, 12);
> > > */
> > >
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, valuelen, 0);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, namelen, 2);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, nameval, 3);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_remote_t, valueblk, 0);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_remote_t, valuelen, 4);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_remote_t, namelen, 8);
> > > + XFS_CHECK_OFFSET(xfs_attr_leaf_name_remote_t, name, 9);
> > > XFS_CHECK_STRUCT_SIZE(xfs_attr_leafblock_t, 40);
> > > - XFS_CHECK_STRUCT_SIZE(xfs_attr_shortform_t, 8);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, hdr.totsize, 0);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, hdr.count, 2);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, list[0].namelen, 4);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, list[0].valuelen, 5);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, list[0].flags, 6);
> > > + XFS_CHECK_OFFSET(xfs_attr_shortform_t, list[0].nameval, 7);
> > > XFS_CHECK_STRUCT_SIZE(xfs_da_blkinfo_t, 12);
> > > XFS_CHECK_STRUCT_SIZE(xfs_da_intnode_t, 16);
> > > XFS_CHECK_STRUCT_SIZE(xfs_da_node_entry_t, 8);
> > > XFS_CHECK_STRUCT_SIZE(xfs_da_node_hdr_t, 16);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_data_free_t, 4);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_data_hdr_t, 16);
> > > - XFS_CHECK_STRUCT_SIZE(xfs_dir2_data_unused_t, 6);
> > > + XFS_CHECK_OFFSET(xfs_dir2_data_unused_t, freetag, 0);
> > > + XFS_CHECK_OFFSET(xfs_dir2_data_unused_t, length, 2);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_free_hdr_t, 16);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_free_t, 16);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_ino4_t, 4);
> > > @@ -94,6 +112,9 @@ xfs_check_ondisk_structs(void)
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_leaf_t, 16);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_leaf_tail_t, 4);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_sf_entry_t, 3);
> > > + XFS_CHECK_OFFSET(xfs_dir2_sf_entry_t, namelen, 0);
> > > + XFS_CHECK_OFFSET(xfs_dir2_sf_entry_t, offset, 1);
> > > + XFS_CHECK_OFFSET(xfs_dir2_sf_entry_t, name, 3);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_sf_hdr_t, 10);
> > > XFS_CHECK_STRUCT_SIZE(xfs_dir2_sf_off_t, 2);
> > >
> > >
> > > _______________________________________________
> > > xfs mailing list
> > > xfs@oss.sgi.com
> > > http://oss.sgi.com/mailman/listinfo/xfs
>
> _______________________________________________
> xfs mailing list
> xfs@oss.sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs