diff: fix out-of-bounds reads and NULL deref in diffstat UTF-8 truncation

f85b49f (diff: improve scaling of filenames in diffstat to handle
UTF-8 chars, 2026-01-16) introduced a loop in show_stats() that calls
utf8_width() repeatedly to skip leading characters until the displayed
width fits.  However, utf8_width() can return problematic values:

  - For invalid UTF-8 sequences, pick_one_utf8_char() sets the name
    pointer to NULL and utf8_width() returns 0.  Since name_len does
    not change, the loop iterates once more and pick_one_utf8_char()
    dereferences the NULL pointer, crashing.

  - For control characters, utf8_width() returns -1, so name_len
    grows when it is expected to shrink.  This can cause the loop to
    consume more characters than the string contains, reading past
    the trailing NUL.

By default, fill_print_name() will C-quotes filenames which escapes
control characters and invalid bytes to printable text.  That avoids
this bug from being triggered; however, with core.quotePath=false,
raw bytes can reach this code.

Add tests exercising both failure modes with core.quotePath=false and
a narrow --stat-name-width to force truncation: one with a bare 0xC0
byte (invalid UTF-8 lead byte, triggers NULL deref) and one with a
0x01 byte (control character, causes the loop to read past the end
of the string).

Fix both issues by introducing utf8_ish_width(), a thin wrapper
around utf8_width() that guarantees the pointer always advances and
the returned width is never negative:

  - On invalid UTF-8 it restores the pointer, advances by one byte,
    and returns width 1 (matching the strlen()-based fallback used
    by utf8_strwidth()).
  - On a control character it returns 0 (matching utf8_strnwidth()
    which skips them).

Also add a "&& *name" guard to the while-loop condition so it
terminates at end-of-string even when utf8_strwidth()'s strlen()
fallback causes name_len to exceed the sum of per-character widths.

Signed-off-by: Elijah Newren <newren@gmail.com>

Author: newren

diff.c

@@ -2927,6 +2927,28 @@ void print_stat_summary(FILE *fp, int files,
 	print_stat_summary_inserts_deletes(&o, files, insertions, deletions);
 }
 
+/*
+ * Like utf8_width(), but guaranteed safe for use in loops that subtract
+ * per-character widths:
+ *
+ *   - utf8_width() sets *start to NULL on invalid UTF-8 and returns 0;
+ *     we restore the pointer and advance by one byte, returning width 1
+ *     (matching the strlen()-based fallback in utf8_strwidth()).
+ *
+ *   - utf8_width() returns -1 for control characters; we return 0
+ *     (matching utf8_strnwidth() which skips them).
+ */
+static int utf8_ish_width(const char **start)
+{
+	const char *old = *start;
+	int w = utf8_width(start, NULL);
+	if (!*start) {
+		*start = old + 1;
+		return 1;
+	}
+	return (w < 0) ? 0 : w;
+}
+
 static void show_stats(struct diffstat_t *data, struct diff_options *options)
 {
 	int i, len, add, del, adds = 0, dels = 0;
@@ -3093,8 +3115,8 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
 			if (len < 0)
 				len = 0;
 
-			while (name_len > len)
-				name_len -= utf8_width((const char**)&name, NULL);
+			while (name_len > len && *name)
+				name_len -= utf8_ish_width((const char**)&name);
 
 			slash = strchr(name, '/');
 			if (slash)

t/t4052-stat-output.sh

@@ -445,4 +445,29 @@ test_expect_success 'diffstat where line_prefix contains ANSI escape codes is co
 	test_grep "<RED>|<RESET>  ${FILENAME_TRIMMED} | 0" out
 '
 
+test_expect_success 'diffstat truncation with invalid UTF-8 does not crash' '
+	empty_blob=$(git hash-object -w --stdin </dev/null) &&
+	printf "100644 blob $empty_blob\taaa-\300-aaa\n" |
+	git mktree >tree_file &&
+	tree=$(cat tree_file) &&
+	empty_tree=$(git mktree </dev/null) &&
+	c1=$(git commit-tree -m before $empty_tree) &&
+	c2=$(git commit-tree -m after -p $c1 $tree) &&
+	git -c core.quotepath=false diff --stat --stat-name-width=5 $c1..$c2 >output &&
+	test_grep "| 0" output
+'
+
+test_expect_success FUNNYNAMES 'diffstat truncation with control chars does not crash' '
+	FNAME=$(printf "aaa-\x01-aaa") &&
+	git commit --allow-empty -m setup &&
+	>$FNAME &&
+	git add -- $FNAME &&
+	git commit -m "add file with control char name" &&
+	git -c core.quotepath=false diff --stat --stat-name-width=5 HEAD~1..HEAD >output &&
+	test_grep "| 0" output &&
+	rm -- $FNAME &&
+	git rm -- $FNAME &&
+	git commit -m "remove test file"
+'
+
 test_done