Re: Possible NULL pointer dereference in lua_setlocal (Lua 5.4.6)

Hello,

The lua_getlocal and lua_setlocal functions seem to have three actual 
usages:

- Set a local variable for a given activation record.
- Get the value and name of a local variable for a given activation record.
- Get the name of a parameter of a function on the stack.

And so we have:

- lua_setlocal requires ar != NULL, and sets the value of a variable 
from an activation record.
- lua_getlocal with ar != NULL gets the name and value of a variable 
from an activation record.
- lua_getlocal with ar == NULL gets the name of the parameter of a 
function on the stack.

The only thing I would suggest is to make the documentation a little clearer
is to modify the section in the lua_setlocal documentation that 
currently says
'Parameters ar and n are as in the function lua_getlocal' to make it 
clear that
lua_setlocal does require 'ar' not NULL.

On 13/4/26 13:03, 武汉理工大学-周豪 wrote:
>
> Hi,
>
> I was going through the debug API in Lua 5.4.6 and noticed something 
> that might be a bug, or at least an inconsistency.
>
> In|ldebug.c|, the function|lua_getlocal|checks whether|ar|is NULL 
> before using it:
>
> c
> if (ar == NULL) {
> /* handle the NULL case */
> }
>
> But|lua_setlocal|right below it doesn't have that check. It just goes 
> ahead and accesses|ar->i_ci|directly:
>
> c
> name = luaG_findlocal(L, ar->i_ci, n, &pos);
>
> So if someone calls|lua_setlocal|with|ar = NULL|(which the API allows 
> as far as I can tell, since it's a public function 
> and|lua_getlocal|already handles it), it'll segfault.
>
> I haven't tested this in a real script yet, but it looks like an 
> oversight. Maybe|lua_setlocal|should have the same NULL guard 
> as|lua_getlocal|for consistency.
>
> Quick fix would be adding something like:
>
> c
> if (ar == NULL) {
> lua_unlock(L);
> return NULL;
> }
>
> at the beginning of the function.
>
> Not sure if this is intentional or just a missing check. Figured I'd 
> mention it.
>
> Thanks
>
>
>
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