clone: error specifically with --local and symlinked objects

6f054f9fb3 (builtin/clone.c: disallow --local clones with
symlinks, 2022-07-28) gives a good error message when "git clone
--local" fails when the repo to clone has symlinks in
"$GIT_DIR/objects". In bffc762f87 (dir-iterator: prevent top-level
symlinks without FOLLOW_SYMLINKS, 2023-01-24), we later extended this
restriction to the case where "$GIT_DIR/objects" is itself a symlink,
but we didn't update the error message then - bffc762f87's tests show
that we print a generic "failed to start iterator over" message.

This is exacerbated by the fact that Documentation/git-clone.txt
mentions neither restriction, so users are left wondering if this is
intentional behavior or not.

Fix this by adding a check to builtin/clone.c: when doing a local clone,
perform an extra check to see if "$GIT_DIR/objects" is a symlink, and if
so, assume that that was the reason for the failure and report the
relevant information. Ideally, dir_iterator_begin() would tell us that
the real failure reason is the presence of the symlink, but (as far as I
can tell) there isn't an appropriate errno value for that.

Also, update Documentation/git-clone.txt to reflect that this
restriction exists.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -58,6 +58,11 @@ never use the local optimizations).  Specifying `--no-local` will
 override the default when `/path/to/repo` is given, using the regular
 Git transport instead.
 +
+If the repository's `$GIT_DIR/objects` has symbolic links or is a
+symbolic link, the clone will fail. This is a security measure to
+prevent the unintentional copying of files by dereferencing the symbolic
+links.
++
 *NOTE*: this operation can race with concurrent modification to the
 source repository, similar to running `cp -r src dst` while modifying
 `src`.

builtin/clone.c

@@ -331,8 +331,18 @@ static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest,
 
 	iter = dir_iterator_begin(src->buf, DIR_ITERATOR_PEDANTIC);
 
-	if (!iter)
+	if (!iter) {
+		if (errno == ENOTDIR) {
+			int saved_errno = errno;
+			struct stat st;
+
+			if (!lstat(src->buf, &st) && S_ISLNK(st.st_mode))
+				die(_("'%s' is a symlink, refusing to clone with --local"),
+				    src->buf);
+			errno = saved_errno;
+		}
 		die_errno(_("failed to start iterator over '%s'"), src->buf);
+	}
 
 	strbuf_addch(src, '/');
 	src_len = src->len;

t/t5604-clone-reference.sh

@@ -358,7 +358,7 @@ test_expect_success SYMLINKS 'clone repo with symlinked objects directory' '
 	test_must_fail git clone --local malicious clone 2>err &&
 
 	test_path_is_missing clone &&
-	grep "failed to start iterator over" err
+	grep "is a symlink, refusing to clone with --local" err
 '
 
 test_done