Re: luaO_pushvfstring reads past end of format string on trailing %

On Wed, 12 Aug 2026 at 16:48, Roberto Ierusalimschy
<[email protected]> wrote:
> The manual says this:
>     Every occurrence of % in the string fmt must form a valid conversion
>     specifier.
> So, this 'just "keep unknown format in the result"' is undocumented
> behavior. As you said, in C there are endless ways to shoot your own
> foot, for instance by calling 'lua_pushfstring(L, "%s", 42l)'.
> A '%' in the end of the format string is just one more way.

I know, in fact I think I quoted that from the manual too.

I would like to wrap up a proposal ( for consideration in future versions).

- keep default in switch as is.
- Add "&& e[1]" just after strchr in while. If strchr succeeds e[1] is not UB.
- Let the tail after the while handle the dangling %.
- Change the manual to something like "Any occurrence of % in the
string fmt not starting a valid conversion specifier will be copied
to the output."

IMO this is a small change which, getting rid of the MUST, leads to
less UB in the manual.

Besides, and this is a personal taste, the e[1] check makes me feel
better as it insures e is a "valid C string with at least two chars (
i.e., strlen>=2 ) starting with '%' " before the switch.

Francisco Olarte.

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