Re: [PATCH 1/3] daemon: fix IPv6 address corruption in lookup_hostname()

[Junio C Hamano <gitster@pobox.com>]

"Sebastien Tardif via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Sebastien Tardif <sebtardif@ncf.ca>
>
> getaddrinfo() is called with AF_UNSPEC hints, so it may return IPv6
> results. However, the code unconditionally casts ai_addr to
> sockaddr_in and passes AF_INET to inet_ntop(). On IPv6-only hosts,
> this reads from the wrong struct offset, producing garbage IP
> addresses.
>
> Fix this by checking ai_family and extracting the address pointer
> into a local variable before calling inet_ntop() once with the
> correct family. Die on unexpected address families.
>
> Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>
> ---
>  daemon.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/daemon.c b/daemon.c
> index 0a7b1aae44..80fa0226d8 100644
> --- a/daemon.c
> +++ b/daemon.c
> @@ -674,9 +674,20 @@ static void lookup_hostname(struct hostinfo *hi)
>  
>  		gai = getaddrinfo(hi->hostname.buf, NULL, &hints, &ai);
>  		if (!gai) {
> -			struct sockaddr_in *sin_addr = (void *)ai->ai_addr;
> +			void *addr;
> +
> +			if (ai->ai_family == AF_INET) {
> +				struct sockaddr_in *sa = (void *)ai->ai_addr;
> +				addr = &sa->sin_addr;
> +			} else if (ai->ai_family == AF_INET6) {
> +				struct sockaddr_in6 *sa6 = (void *)ai->ai_addr;
> +				addr = &sa6->sin6_addr;
> +			} else {
> +				die("unexpected address family: %d",
> +				    ai->ai_family);
> +			}

The previous iteration used to more explicitly cast ai->ai_addr to
the target type, but the use of (void *) here is a cute way to make
the result shorter, which makes it a bit easier to read (it may take
readers a bit of practice to convince themselves that this type
conversion using (void *) as an intermediate type is perfectly fine,
though).

>  
> -			inet_ntop(AF_INET, &sin_addr->sin_addr,
> +			inet_ntop(ai->ai_family, addr,
>  				  addrbuf, sizeof(addrbuf));
>  			strbuf_addstr(&hi->ip_address, addrbuf);