difftool: fix use-after-free

The left and right base directories were pointed to the buf field of
two strbufs, which were subject to change.

A contrived test case shows the problem where a file with a long enough
name to force the strbuf to grow is up-to-date (hence the code path is
used where the work tree's version of the file is reused), and then a
file that is not up-to-date needs to be written (hence the code path is
used where checkout_entry() uses the previously recorded base_dir that
is invalid by now).

Let's just copy the base_dir strings for use with checkout_entry(),
never touch them until the end, and release them then. This is an easily
verifiable fix (as opposed to the next-obvious alternative: to re-set
base_dir after every loop iteration).

This fixes https://github.com/git-for-windows/git/issues/1124

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/difftool.c

@@ -318,6 +318,7 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	struct strbuf rpath = STRBUF_INIT, buf = STRBUF_INIT;
 	struct strbuf ldir = STRBUF_INIT, rdir = STRBUF_INIT;
 	struct strbuf wtdir = STRBUF_INIT;
+	char *lbase_dir, *rbase_dir;
 	size_t ldir_len, rdir_len, wtdir_len;
 	const char *workdir, *tmp;
 	int ret = 0, i;
@@ -351,11 +352,11 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	memset(&wtindex, 0, sizeof(wtindex));
 
 	memset(&lstate, 0, sizeof(lstate));
-	lstate.base_dir = ldir.buf;
+	lstate.base_dir = lbase_dir = xstrdup(ldir.buf);
 	lstate.base_dir_len = ldir.len;
 	lstate.force = 1;
 	memset(&rstate, 0, sizeof(rstate));
-	rstate.base_dir = rdir.buf;
+	rstate.base_dir = rbase_dir = xstrdup(rdir.buf);
 	rstate.base_dir_len = rdir.len;
 	rstate.force = 1;
 
@@ -625,6 +626,8 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 		exit_cleanup(tmpdir, rc);
 
 finish:
+	free(lbase_dir);
+	free(rbase_dir);
 	strbuf_release(&ldir);
 	strbuf_release(&rdir);
 	strbuf_release(&wtdir);

t/t7800-difftool.sh

@@ -393,6 +393,25 @@ test_expect_success 'setup change in subdirectory' '
 	git commit -m "modified both"
 '
 
+test_expect_success 'difftool -d with growing paths' '
+	a=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &&
+	git init growing &&
+	(
+		cd growing &&
+		echo "test -f \"\$2/b\"" | write_script .git/test-for-b.sh &&
+		one=$(printf 1 | git hash-object -w --stdin) &&
+		two=$(printf 2 | git hash-object -w --stdin) &&
+		git update-index --add \
+			--cacheinfo 100644,$one,$a --cacheinfo 100644,$two,b &&
+		tree1=$(git write-tree) &&
+		git update-index --add \
+			--cacheinfo 100644,$two,$a --cacheinfo 100644,$one,b &&
+		tree2=$(git write-tree) &&
+		git checkout -- $a &&
+		git difftool -d --extcmd .git/test-for-b.sh $tree1 $tree2
+	)
+'
+
 run_dir_diff_test () {
 	test_expect_success "$1 --no-symlinks" "
 		symlinks=--no-symlinks &&