[tarantool-patches] Re: [PATCH v2 1/3] lua-yaml: verify arguments count

[Vladislav Shpilevoy]

Thanks for the patch!

On 22/01/2019 05:12, Alexander Turenko wrote:
> From: AKhatskevich <avkhatskevich at tarantool.org>
> 
> Added arguments count check for yaml.encode() and decode.decode()

Typo: 'decode.decode' -> 'yaml.decode'.

> functions.
> 
> Without these checks the functions could read garbage outside of a Lua
> stack when called w/o arguments.

Honestly, I do not understand how is it possible. Please,
provide a test for both functions. See my 3 doubts below.

> ---
>   third_party/lua-yaml/lyaml.cc | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/third_party/lua-yaml/lyaml.cc b/third_party/lua-yaml/lyaml.cc
> index c6d118a79..9b07992d8 100644
> --- a/third_party/lua-yaml/lyaml.cc
> +++ b/third_party/lua-yaml/lyaml.cc
> @@ -400,7 +400,8 @@ static void load(struct lua_yaml_loader *loader) {
>    */
>   static int l_load(lua_State *L) {
>      struct lua_yaml_loader loader;
> -   if (! lua_isstring(L, 1)) {
> +   int top = lua_gettop(L);
> +   if (!(top == 1 || top == 2) || !lua_isstring(L, 1)) {

1. How could the old code lead to a bug, if there was a
check if the first argument is a string? The second argument
is not used until the next hunk, about which see my next
comment

>   usage_error:
>         return luaL_error(L, "Usage: yaml.decode(document, "\
>                           "[{tag_only = boolean}])");
> @@ -416,7 +417,7 @@ usage_error:
>         return luaL_error(L, OOM_ERRMSG);
>      yaml_parser_set_input_string(&loader.parser, (yaml_char_t *) document, len);
>      bool tag_only;
> -   if (lua_gettop(L) > 1) {
> +   if (lua_gettop(L) == 2) {

2. This function never touches anything beyond second value on
the stack, so here lua_gettop(L) > 1 means the same as
lua_gettop(L) == 2 - the second argument exist. Third and next
values do not matter.

>         if (! lua_istable(L, 2))
>            goto usage_error;
>         lua_getfield(L, 2, "tag_only");
> @@ -794,7 +795,7 @@ error:
>   static int l_dump(lua_State *L) {
>      struct luaL_serializer *serializer = luaL_checkserializer(L);
>      int top = lua_gettop(L);
> -   if (top > 2) {
> +   if (!(top == 1 || top == 2)) {

3. Here my reasoning is the same - the previous checking works
as well.

>   usage_error:
>         return luaL_error(L, "Usage: encode(object, {tag_prefix = <string>, "\
>                           "tag_handle = <string>})");
>