Re: [PATCH 3/3] xfs: make struct xfs_buf_log_format have a consistent size

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jan 08, 2020 at 08:32:29AM -0800, Darrick J. Wong wrote:
> On Wed, Jan 08, 2020 at 12:54:02AM -0800, Christoph Hellwig wrote:
> > On Tue, Jan 07, 2020 at 08:18:25PM -0800, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > Increase XFS_BLF_DATAMAP_SIZE by 1 to fill in the implied padding at the
> > > end of struct xfs_buf_log_format.  This makes the size consistent so
> > > that we can check it in xfs_ondisk.h, and will be needed once we start
> > > logging attribute values.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > >  fs/xfs/libxfs/xfs_log_format.h |    9 +++++----
> > >  fs/xfs/xfs_ondisk.h            |    1 +
> > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > > 
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_log_format.h b/fs/xfs/libxfs/xfs_log_format.h
> > > index 8ef31d71a9c7..5d8eb8978c33 100644
> > > --- a/fs/xfs/libxfs/xfs_log_format.h
> > > +++ b/fs/xfs/libxfs/xfs_log_format.h
> > > @@ -462,11 +462,12 @@ static inline uint xfs_log_dinode_size(int version)
> > >  #define	XFS_BLF_GDQUOT_BUF	(1<<4)
> > >  
> > >  /*
> > > - * This is the structure used to lay out a buf log item in the
> > > - * log.  The data map describes which 128 byte chunks of the buffer
> > > - * have been logged.
> > > + * This is the structure used to lay out a buf log item in the log.  The data
> > > + * map describes which 128 byte chunks of the buffer have been logged.  Note
> > > + * that XFS_BLF_DATAMAP_SIZE is an odd number so that the structure size will
> > > + * be consistent between 32-bit and 64-bit platforms.
> > >   */
> > > -#define XFS_BLF_DATAMAP_SIZE	((XFS_MAX_BLOCKSIZE / XFS_BLF_CHUNK) / NBWORD)
> > > +#define XFS_BLF_DATAMAP_SIZE	(1 + ((XFS_MAX_BLOCKSIZE / XFS_BLF_CHUNK) / NBWORD))
> > 
> > I don't understand the explanation.  Why would the size differ for
> > 32-bit vs 64-bit architectures when it only uses fixed size types?
> 
> The structure is 84 bytes in length, which is not an even multiple of 8.
> The reason for this is that the end of the structure are 17 unsigned
> ints (blf_map_size + blf_map_data).
> 
> The blf_blkno field is int64_t, which on amd64 causes the compiler to
> round the the structure size up to the nearest 8-byte boundary, or 88
> bytes:
> 
> /* <1897d> /storage/home/djwong/cdev/work/linux-xfs/fs/xfs/libxfs/xfs_log_format.h:477 */
> struct xfs_buf_log_format {
>         short unsigned int         blf_type;                                             /*     0     2 */
>         short unsigned int         blf_size;                                             /*     2     2 */
>         short unsigned int         blf_flags;                                            /*     4     2 */
>         short unsigned int         blf_len;                                              /*     6     2 */
>         /* typedef int64_t -> s64 -> __s64 */ long long int              blf_blkno;      /*     8     8 */
>         unsigned int               blf_map_size;                                         /*    16     4 */
>         unsigned int               blf_data_map[17];                                     /*    20    68 */
>         /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> 
>         /* size: 88, cachelines: 2, members: 7 */
>         /* last cacheline: 24 bytes */
> };

And of course I forgot to pop the patch before building and pahole'ing,
so here's the correct version from x86_64:

/* <1897d> /storage/home/djwong/cdev/work/linux-xfs/fs/xfs/libxfs/xfs_log_format.h:476 */
struct xfs_buf_log_format {
        short unsigned int         blf_type;                                             /*     0     2 */
        short unsigned int         blf_size;                                             /*     2     2 */
        short unsigned int         blf_flags;                                            /*     4     2 */
        short unsigned int         blf_len;                                              /*     6     2 */
        /* typedef int64_t -> s64 -> __s64 */ long long int              blf_blkno;      /*     8     8 */
        unsigned int               blf_map_size;                                         /*    16     4 */
        unsigned int               blf_data_map[16];                                     /*    20    64 */
        /* --- cacheline 1 boundary (64 bytes) was 20 bytes ago --- */

        /* size: 88, cachelines: 2, members: 7 */
        /* padding: 4 */
        /* last cacheline: 24 bytes */
};

--D

> (Same thing with aarch64 and ppc64le gcc.)
> 
> i386 gcc doesn't do any of this rounding, so the size is 84 bytes:
> 
> /* <182ef> /storage/home/djwong/cdev/work/linux-xfs/fs/xfs/libxfs/xfs_log_format.h:476 */
> struct xfs_buf_log_format {
>         short unsigned int         blf_type;                                             /*     0     2 */
>         short unsigned int         blf_size;                                             /*     2     2 */
>         short unsigned int         blf_flags;                                            /*     4     2 */
>         short unsigned int         blf_len;                                              /*     6     2 */
>         /* typedef int64_t -> s64 -> __s64 */ long long int              blf_blkno;      /*     8     8 */
>         unsigned int               blf_map_size;                                         /*    16     4 */
>         unsigned int               blf_data_map[16];                                     /*    20    64 */
>         /* --- cacheline 1 boundary (64 bytes) was 20 bytes ago --- */
> 
>         /* size: 84, cachelines: 2, members: 7 */
>         /* last cacheline: 20 bytes */
> };
> 
> Since we accidentally write to blf_data_map[17] when invalidating a 68k
> buffer, that write will corrupt the slab's redzone, or worse, a live
> object packed in right after it.
> 
> --D