Re: [musl] [PATCH] getopt: fix null pointer arithmetic ub

[Alexander Monakov <amonakov@ispras.ru>]

Hi,

On Fri, 10 Mar 2023, Alexey Izbyshev wrote:

> When an option that requires an argument is the last character of
> argv[argc-1], getopt computes argv[argc] + optpos. While optpos
> is always zero in this case, adding it to null pointer is still
> undefined.
> ---
>  src/misc/getopt.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/src/misc/getopt.c b/src/misc/getopt.c
> index c3f66995..af12973a 100644
> --- a/src/misc/getopt.c
> +++ b/src/misc/getopt.c
> @@ -87,7 +87,8 @@ int getopt(int argc, char * const argv[], const char *optstring)
>  	if (optstring[i] == ':') {
>  		optarg = 0;
>  		if (optstring[i+1] != ':' || optpos) {
> -			optarg = argv[optind++] + optpos;
> +			optarg = argv[optind++];
> +			if (optarg) optarg += optpos;

Can this be written as 'if (optpos) optarg += optpos;' instead? That will be
folded back into plain addition by the compiler.

(also (unlike the quoted variant) would allow undefined behavior
instrumentation to catch attempted NULL pointer arithmetic)

Alexander