[PATCH] xfs_copy: simplify first_agbno calculation

[Eric Sandeen <sandeen@redhat.com>]

After ffe9a9a xfsprogs: xfs_copy: fix data corruption of target,
xfs_copy started hitting an ASSERT for a 4k sector / 4k blocksize
filesystem:

# dd if=/dev/zero of=test.img bs=1M count=1024
# mkfs.xfs -s size=4096 test.img
# xfs_copy test.img xfs.img
xfs_copy: xfs_copy.c:720: main: Assertion `((((((xfs_daddr_t)(3 << (mp)->m_sectbb_log)) + 1) * (1<<9)) + first_residue) % source_blocksize) == 0' failed.
Aborted

I started digging through all the calculations below, and realized
that in the end, all it wants is the first filesystem block after
the AG header.  XFS_AGFL_BLOCK(mp) + 1 suffices for this purpose;
rip out the rest which seems overly complex and apparently bug-prone.

I tested this by creating a 4g filesystem with combinations of
sector & block size between 512 and 4k, copying in /lib/modules,
running an xfs_copy of that, and running repair against the copy;
it all looks good.  It took a long time, but I will create a
simpler/shorter xfstest based on this.

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

diff --git a/copy/xfs_copy.c b/copy/xfs_copy.c
index 7ce5ec9..279527c 100644
--- a/copy/xfs_copy.c
+++ b/copy/xfs_copy.c
@@ -475,7 +475,7 @@ main(int argc, char **argv)
 	int		open_flags;
 	xfs_off_t	pos, end_pos;
 	size_t		length;
-	int		c, first_residue, tmp_residue;
+	int		c;
 	__uint64_t	size, sizeb;
 	__uint64_t	numblocks = 0;
 	int		wblocks = 0;
@@ -697,27 +697,13 @@ main(int argc, char **argv)
 	ASSERT(source_blocksize % source_sectorsize == 0);
 	ASSERT(source_sectorsize % BBSIZE == 0);
 
-	if (source_blocksize > source_sectorsize)  {
-		/* get number of leftover sectors in last block of ag header */
-
-		tmp_residue = ((XFS_AGFL_DADDR(mp) + 1) * BBSIZE)
-					% source_blocksize;
-		first_residue = (tmp_residue == 0) ? 0 :
-			source_blocksize - tmp_residue;
-		ASSERT(first_residue % source_sectorsize == 0);
-	} else if (source_blocksize == source_sectorsize)  {
-		first_residue = 0;
-	} else  {
+	if (source_blocksize < source_sectorsize)  {
 		do_log(_("Error:  filesystem block size is smaller than the"
 			" disk sectorsize.\nAborting XFS copy now.\n"));
 		exit(1);
 	}
 
-	first_agbno = (((XFS_AGFL_DADDR(mp) + 1) * BBSIZE)
-				+ first_residue) / source_blocksize;
-	ASSERT(first_agbno != 0);
-	ASSERT(((((XFS_AGFL_DADDR(mp) + 1) * BBSIZE)
-				+ first_residue) % source_blocksize) == 0);
+	first_agbno = XFS_AGFL_BLOCK(mp) + 1;
 
 	/* now open targets */
 

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