libarchive: do not extract unsafe symlinks unless $EXTRACT_UNSAFE_SYMLINKS=1

function                                             old     new   delta
unsafe_symlink_target                                  -     147    +147
unzip_main                                          2711    2732     +21
copy_file                                           1657    1678     +21
tar_main                                             999     971     -28
data_extract_all                                    1038     984     -54
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/2 up/down: 189/-82)           Total: 107 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

archival/libarchive/unsafe_symlink_target.c

@@ -0,0 +1,48 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+#include "libbb.h"
+#include "bb_archive.h"
+
+int FAST_FUNC unsafe_symlink_target(const char *target)
+{
+	const char *dot;
+
+	if (target[0] == '/') {
+		const char *var;
+ unsafe:
+		var = getenv("EXTRACT_UNSAFE_SYMLINKS");
+		if (var) {
+			if (LONE_CHAR(var, '1'))
+				return 0; /* pretend it's safe */
+			return 1; /* "UNSAFE!" */
+		}
+		bb_error_msg("skipping unsafe symlink to '%s' in archive,"
+			" set %s=1 to extract",
+			target,
+			"EXTRACT_UNSAFE_SYMLINKS"
+		);
+		/* Prevent further messages */
+		setenv("EXTRACT_UNSAFE_SYMLINKS", "0", 0);
+		return 1; /* "UNSAFE!" */
+	}
+
+	dot = target;
+	for (;;) {
+		dot = strchr(dot, '.');
+		if (!dot)
+			return 0; /* safe target */
+
+		/* Is it a path component starting with ".."? */
+		if ((dot[1] == '.')
+		 && (dot == target || dot[-1] == '/')
+		    /* Is it exactly ".."? */
+		 && (dot[2] == '/' || dot[2] == '\0')
+		) {
+			goto unsafe;
+		}
+		/* NB: it can even be trailing ".", should only add 1 */
+		dot += 1;
+	}
+}