Fix example/subsystem_netconf.c

[Yuriy M. Kaminskiy]

On 11.09.2021 20:33, Christian Weisgerber via libssh2-devel wrote:
> In libssh2's example/subsystem_netconf.c, a pointer to a variable
> of the wrong size (on LP64) is passed.  Ouch.  The patch below fixes
> this and also eliminates the %n format specifier that is increasingly
> discouraged:
> 
> --- example/subsystem_netconf.c.orig
> +++ example/subsystem_netconf.c
> @@ -257,14 +257,14 @@ int main(int argc, char *argv[])
>      /* NETCONF: https://tools.ietf.org/html/draft-ietf-netconf-ssh-06 */
>  
>      fprintf(stderr, "Sending NETCONF client <hello>\n");
> -    snprintf(buf, sizeof(buf),
> +    len = snprintf(buf, sizeof(buf),
>        "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
>        "<hello>"
>        "<capabilities>"
>        "<capability>urn:ietf:params:xml:ns:netconf:base:1.0</capability>"
>        "</capabilities>"
>        "</hello>\n"
> -      "]]>]]>\n%n", (int *)&len);
> +      "]]>]]>\n");
>      if(-1 == netconf_write(channel, buf, len))
>          goto shutdown;

This is broken (with security implications).

snprintf can return value larger than sizeof(buf) or -1.

Same apply to %n (that is, original code was broken too).

> @@ -277,12 +277,12 @@ int main(int argc, char *argv[])
>              (int)len, buf);
>  
>      fprintf(stderr, "Sending NETCONF <rpc>\n");
> -    snprintf(buf, sizeof(buf),
> +    len = snprintf(buf, sizeof(buf),
>        "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
>        "<rpc xmlns=\"urn:ietf:params:xml:ns:netconf:base:1.0\">"
>        "<get-interface-information><terse/></get-interface-information>"
>        "</rpc>\n"
> -      "]]>]]>\n%n", (int *)&len);
> +      "]]>]]>\n");
>      if(-1 == netconf_write(channel, buf, len))
>          goto shutdown;
>  
>