[PATCH] Fix buffer underflow in xdl_build_script

["Alex via GitGitGadget" <gitgitgadget@gmail.com>]

From: jinyaoguo <guo846@purdue.edu>

The loop in xdl_build_script used `i1 >= 0 || i2 >= 0`, causing
`i1` (or `i2`) to reach 0 and then access `rchg1[i1-1]` (or
`rchg2[i2-1]`), which underflows the buffer.
This commit adds explicit `i1 > 0` and `i2 > 0` checks around
those array accesses to prevent invalid negative indexing.

Signed-off-by: Alex Guo <alexguo1023@gmail.com>
---
    Fix buffer underflow in xdl_build_script

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1976%2Fmugitya03%2Fbuf-1-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1976/mugitya03/buf-1-v1
Pull-Request: https://github.com/git/git/pull/1976

 xdiff/xdiffi.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
index 5a96e36dfbe..2e983965328 100644
--- a/xdiff/xdiffi.c
+++ b/xdiff/xdiffi.c
@@ -951,9 +951,10 @@ int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr) {
 	 * Trivial. Collects "groups" of changes and creates an edit script.
 	 */
 	for (i1 = xe->xdf1.nrec, i2 = xe->xdf2.nrec; i1 >= 0 || i2 >= 0; i1--, i2--)
-		if (rchg1[i1 - 1] || rchg2[i2 - 1]) {
-			for (l1 = i1; rchg1[i1 - 1]; i1--);
-			for (l2 = i2; rchg2[i2 - 1]; i2--);
+		if ((i1 > 0 && rchg1[i1 - 1]) ||
+			(i2 > 0 && rchg2[i2 - 1])) {
+			for (l1 = i1; i1 > 0 && rchg1[i1 - 1]; i1--);
+            for (l2 = i2; i2 > 0 && rchg2[i2 - 1]; i2--);
 
 			if (!(xch = xdl_add_change(cscr, i1, i2, l1 - i1, l2 - i2))) {
 				xdl_free_script(cscr);

base-commit: 8613c2bb6cd16ef530dc5dd74d3b818a1ccbf1c0
-- 
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