Re: [PATCH] xfs_copy: fix up initial sb buffer read on CRC fs

[Brian Foster <bfoster@redhat.com>]

On Thu, Jul 16, 2015 at 01:20:30PM -0500, Eric Sandeen wrote:
> My prior commit, aaf90a2 xfs_copy: fix copy of hard 4k devices
> causes xfs_copy to emit a CRC error warning when copying a
> CRC filesystem.
> 
> This is because we are now reading the maximum sector size,
> and attempting to verify the CRC based on that (likely incorrect)
> length.
> 
> In xfs_db, we currently just don't verify this read, so it's
> not a problem.  In xfs_copy, we almost certainly want to verify.
> 
> So, first do the maximal read with no verifier; once it's read,
> drop that buffer, and re-read with the proper sector size and
> verifier.
> 
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
> 
> diff --git a/copy/xfs_copy.c b/copy/xfs_copy.c
> index 44a32e8..fd96e15 100644
> --- a/copy/xfs_copy.c
> +++ b/copy/xfs_copy.c
> @@ -654,11 +654,17 @@ main(int argc, char **argv)
>  
>  	memset(&mbuf, 0, sizeof(xfs_mount_t));
>  	libxfs_buftarg_init(&mbuf, xargs.ddev, xargs.logdev, xargs.rtdev);
> +	/* We don't yet know the sector size, so read maximal size */
>  	sbp = libxfs_readbuf(mbuf.m_ddev_targp, XFS_SB_DADDR,
> -			     1 << (XFS_MAX_SECTORSIZE_LOG - BBSHIFT),
> -			     0, &xfs_sb_buf_ops);
> +			     1 << (XFS_MAX_SECTORSIZE_LOG - BBSHIFT), 0, NULL);
>  	sb = &mbuf.m_sb;
>  	libxfs_sb_from_disk(sb, XFS_BUF_TO_SBP(sbp));
> +	/* Do it again, now with proper length and verifier */
> +	libxfs_putbuf(sbp);
> +	libxfs_purgebuf(sbp);

Why the purge? On a quick look, it looks like the buffer cache code
would handle this if the buffer size changes.

Hmm, is it to ensure the verification occurs if the buffer size doesn't
actually change? If so, I'd suggest to enhance the comment. :)

Brian

> +	sbp = libxfs_readbuf(mbuf.m_ddev_targp, XFS_SB_DADDR,
> +			     1 << (sb->sb_sectlog - BBSHIFT),
> +			     0, &xfs_sb_buf_ops);
>  
>  	/*
>  	 * For now, V5 superblock filesystems are not supported without -d;
> 
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