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From: Rich Felker <dalias@libc.org>
To: Markus Wichmann <nullplan@gmx.net>
Cc: musl@lists.openwall.com
Subject: Re: [musl] IPv4 fallback in __res_msend_rc not functional
Date: Wed, 24 Aug 2022 19:32:28 -0400	[thread overview]
Message-ID: <20220824233228.GM7074@brightrain.aerifal.cx> (raw)
In-Reply-To: <20220824232657.GL7074@brightrain.aerifal.cx>

On Wed, Aug 24, 2022 at 07:26:58PM -0400, Rich Felker wrote:
> On Wed, Aug 24, 2022 at 09:03:49PM +0200, Markus Wichmann wrote:
> > Hi all,
> > 
> > I noticed something while reading some code: There is a fallback in
> > __res_msend_rc(), in case an IPv6 socket is requested but cannot be
> > allocated. In that case, the function tries to create an IPv4 socket
> > instead. However, I do not think this code can work that way. For
> > reference, this is the code:
> > 
> > 	/* Get local address and open/bind a socket */
> > 	sa.sin.sin_family = family;
> > 	fd = socket(family, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
> > 
> > 	/* Handle case where system lacks IPv6 support */
> > 	if (fd < 0 && family == AF_INET6 && errno == EAFNOSUPPORT) {
> > 		fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
> > 		family = AF_INET;
> > 	}
> > 	if (fd < 0 || bind(fd, (void *)&sa, sl) < 0) {
> > 		if (fd >= 0) close(fd);
> > 		pthread_setcancelstate(cs, 0);
> > 		return -1;
> > 	}
> > 
> > The problem is, if the fallback is triggered, the local address is still
> > set to be an IPv6 address, and so the bind() must necessarily fail with
> > EINVAL.
> > 
> > The fix depends on whether the fallback is still intended functionality
> > or not. If not, then the easiest would be to just get rid of the entire
> > fallback block. If the fallback is still intended to work, then the
> > fallback block must reset sl to the length of an IPv4 socket, and the
> > setting of sa.sin.sin_family must be delayed until after that block.
> > 
> > There is also the issue of the sendto() loop further down in the
> > function. If it is intended that the socket can be an IPv4 socket but
> > there can be IPv6 addresses in the list, then it might be prudent to
> > prevent sendto() from sending to the wrong address family. Or not, I
> > mean, you do not test for errors from sendto(), and the sends to the
> > wrong address family are just going to fail. So they would only waste
> > time and change errno, but not much of a visible side effect.
> 
> Thanks for reporting this! It's intended to be functional, but it's
> probably of little consequence whether it works since the issue seems
> to arise only when resolv.conf requested IPv6 nameservers but the
> system doesn't support IPv6. I'll look at what it'll take to fix it...
> hopefully it won't be too bad.

Does this work?

diff --git a/src/network/res_msend.c b/src/network/res_msend.c
index 3e018009..105bf598 100644
--- a/src/network/res_msend.c
+++ b/src/network/res_msend.c
@@ -68,14 +68,15 @@ int __res_msend_rc(int nqueries, const unsigned char *const *queries,
 	}
 
 	/* Get local address and open/bind a socket */
-	sa.sin.sin_family = family;
 	fd = socket(family, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 
 	/* Handle case where system lacks IPv6 support */
 	if (fd < 0 && family == AF_INET6 && errno == EAFNOSUPPORT) {
 		fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 		family = AF_INET;
+		sl = sizeof sa.sin;
 	}
+	sa.sin.sin_family = family;
 	if (fd < 0 || bind(fd, (void *)&sa, sl) < 0) {
 		if (fd >= 0) close(fd);
 		pthread_setcancelstate(cs, 0);


  reply	other threads:[~2022-08-24 23:32 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-24 19:03 Markus Wichmann
2022-08-24 23:26 ` Rich Felker
2022-08-24 23:32   ` Rich Felker [this message]
2022-08-25  2:58     ` Markus Wichmann
2022-08-25 13:26       ` Rich Felker
2022-08-26 16:16         ` Rich Felker
2022-08-26 16:28           ` Rich Felker
2022-08-26 17:48           ` Markus Wichmann

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