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

[Darrick J. Wong]

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.

--D

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