[PATCH v2 2/3] xfs_repair: clear new memory after realloc

[Mike Grant]

longform_dir2_entry_check in phase6.c has a calloc'ed array of xfs_buf
pointers (bplist).  It reallocs this array if there turns out to be more
blocks than expected.  However, realloc doesn't zero the new memory like
calloc.  In unusual circumstances*, things may then blow up later due to
random data populating the new part of the array.

This patch zeros the new part of the array.

* (bit speculative) as dir_read_buf zeros the element it's looking at, I
think this can only happen if the realloc adds several members and one
of the first is corrupt.  In my case, the realloc went from 35 to 37
members, meaning db must have been 36 without being 35.  A read error
then caused it to goto out_fix. The crash then occurred in the
libxfs_putbuf when looping through the bplist structure, checking it for
NULL pointers (and presumably tripping over the non-zeroed data at
position 35?)

Signed-off-by: Mike Grant <mggr at pml.ac.uk>
---
 repair/phase6.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/repair/phase6.c b/repair/phase6.c
index 6520196..e24cf62 100644
--- a/repair/phase6.c
+++ b/repair/phase6.c
@@ -2327,6 +2327,7 @@ longform_dir2_entry_check(xfs_mount_t	*mp,
 		db = xfs_dir2_da_to_db(mp, da_bno);
 		if (db >= num_bps) {
 			/* more data blocks than expected */
+			int num_bps_prev = num_bps;
 			num_bps = db + 1;
 			bplist = realloc(bplist, num_bps * bpsz);
 			if (!bplist)
@@ -2334,6 +2335,9 @@ longform_dir2_entry_check(xfs_mount_t	*mp,
 					_("realloc failed in %s (%zu bytes)\n"),
 					__func__,
 					num_bps * bpsz);
+			/* clear new memory as previous bplist was calloc'ed */
+			memset((void *) bplist + num_bps_prev * bpsz, 0,
+				(num_bps - num_bps_prev) * bpsz);
 		}
 
 		if (isblock)
-- 
2.1.0

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