Fix gcc array subscript above bounds warning

In a debug build, certain GCC versions flag an array bounds warning in
the below code from dnode_sync.c

    } else {
            int i;
            ASSERT(dn->dn_next_nblkptr[txgoff] < dnp->dn_nblkptr);
            /* the blkptrs we are losing better be unallocated */
            for (i = dn->dn_next_nblkptr[txgoff];
                i < dnp->dn_nblkptr; i++)
                    ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[i]));

This usage is in fact safe, since the ASSERT ensures the index does
not exceed to maximum possible number of block pointers. However gcc
can't determine that the assignment 'i = dn->dn_next_nblkptr[txgoff];'
falls within the array bounds so it issues a warning.  To avoid this,
initialize i to zero to make gcc happy but skip the elements before
dn->dn_next_nblkptr[txgoff] in the loop body.  Since a dnode contains
at most 3 block pointers this overhead should be negligible.

Signed-off-by: Brian Behlendorf <[email protected]>
Closes #950

1 files changed, 4 insertions, 3 deletions

diff --git a/module/zfs/dnode_sync.c b/module/zfs/dnode_sync.c
index af636dcdb..f2dda867d 100644
--- a/module/zfs/dnode_sync.c
+++ b/module/zfs/dnode_sync.c
@@ -666,9 +666,10 @@ dnode_sync(dnode_t *dn, dmu_tx_t *tx)
 			int i;
 			ASSERT(dn->dn_next_nblkptr[txgoff] < dnp->dn_nblkptr);
 			/* the blkptrs we are losing better be unallocated */
-			for (i = dn->dn_next_nblkptr[txgoff];
-			    i < dnp->dn_nblkptr; i++)
-				ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[i]));
+			for (i = 0; i < dnp->dn_nblkptr; i++) {
+				if (i >= dn->dn_next_nblkptr[txgoff])
+					ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[i]));
+			}
 #endif
 		}
 		mutex_enter(&dn->dn_mtx);