[PATCH 05/76] libxfs: pack the agfl header structure so XFS_AGFL_SIZE is correct

[Darrick J. Wong]

On Mon, Jan 04, 2016 at 02:12:18PM -0800, Darrick J. Wong wrote:
> On Sun, Jan 03, 2016 at 04:15:25AM -0800, Christoph Hellwig wrote:
> > On Sat, Dec 19, 2015 at 12:56:55AM -0800, Darrick J. Wong wrote:
> > > Because struct xfs_agfl is 36 bytes long and has a 64-bit integer
> > > inside it, gcc will quietly round the structure size up to the nearest
> > > 64 bits -- in this case, 40 bytes.  This results in the XFS_AGFL_SIZE
> > > macro returning incorrect results for v5 filesystems on 64-bit
> > > machines (118 items instead of 119).  As a result, a 32-bit xfs_repair
> > > will see garbage in AGFL item 119 and complain.
> > > 
> > > Therefore, tell gcc not to pad the structure so that the AGFL size
> > > calculation is correct.
> > 
> > Do you have a testcase for this?
> 
> Not much aside from:
> 
> 0. Build kernel/xfsprogs with RFCv4 patches on a 64bit machine.
> 1. Build kernel/xfsprogs with RFCv4 patches on a 32bit machine.
> 2. Format a XFS with reflink and rmap on a 64-bit machine, so that the AGFL
>    size is maximized.
> 3. Mount FS and create a reflinked file.
> 4. Unmount and xfs_repair with the 32-bit build.
> 
> I guess we could create a program that compares all the known sizeof(struct
> xfs_disk_object) values against known good values and stuff that into the
> xfsprogs build process.

I created a patch that uses BUILD_BUG_ON to stop kbuild if the sizes of the
on-disk data structures don't match known values.  I've also ported it to
xfsprogs, so we can check both.

--D

> 
> --D
> 
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