update-index: refuse to add working tree items beyond symlinks

When "sym" is a symbolic link that is inside the working tree, and it
points at a directory "dir" that has "path" in it, "update-index --add
sym/path" used to mistakenly add "sym/path" as if "sym" were a normal
directory.

"git apply", "git diff" and "git merge" have been taught about this issue
some time ago, but "update-index" and "add" have been left ignorant for
too long.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-update-index.c

@@ -194,14 +194,17 @@ static int process_path(const char *path)
 	int len;
 	struct stat st;
 
+	len = strlen(path);
+	if (has_symlink_leading_path(len, path))
+		return error("'%s' is beyond a symbolic link", path);
+
 	/*
 	 * First things first: get the stat information, to decide
 	 * what to do about the pathname!
 	 */
 	if (lstat(path, &st) < 0)
 		return process_lstat_error(path, errno);
 
-	len = strlen(path);
 	if (S_ISDIR(st.st_mode))
 		return process_directory(path, len, &st);
 

t/t0055-beyond-symlinks.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+test_description='update-index refuses to add beyond symlinks'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	>a &&
+	mkdir b &&
+	ln -s b c &&
+	>c/d &&
+	git update-index --add a b/d
+'
+
+test_expect_success 'update-index --add beyond symlinks' '
+	test_must_fail git update-index --add c/d &&
+	! ( git ls-files | grep c/d )
+'
+
+test_done