netdev - Re: [PATCH bpf v1] unix: fix an issue in unix_shutdown causing the other end read/write failures

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Date:   Thu, 11 Nov 2021 15:00:02 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Jiang Wang <jiang.wang@...edance.com>
CC:     <bpf@...r.kernel.org>, <cong.wang@...edance.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Dumazet <edumazet@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Rao Shoaib <Rao.Shoaib@...cle.com>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        Jakub Sitnicki <jakub@...udflare.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf v1] unix: fix an issue in unix_shutdown causing the
 other end read/write failures

On Mon, Oct 04, 2021 at 11:25:28PM +0000, Jiang Wang wrote:
> Commit 94531cfcbe79 ("af_unix: Add unix_stream_proto for sockmap")
> sets unix domain socket peer state to TCP_CLOSE
> in unix_shutdown. This could happen when the local end is shutdown
> but the other end is not. Then the other end will get read or write
> failures which is not expected.
> 
> Fix the issue by setting the local state to shutdown.
> 
> Fixes: 94531cfcbe79 (af_unix: Add unix_stream_proto for sockmap)
> Suggested-by: Cong Wang <cong.wang@...edance.com>
> Reported-by: Casey Schaufler <casey@...aufler-ca.com>
> Signed-off-by: Jiang Wang <jiang.wang@...edance.com>

This patch changed the behaviour of read(2) after a shutdown(2) on the
local end of a UDS.  Before this patch, reading from a UDS after a local
shutdown(SHUT_RDWR) would return the data written or EOF if there is no
data, but now it always returns -EINVAL.

For example, the following test program succeeds with "read 16 bytes" on
v5.14 but fails with "read: Invalid argument" on v5.15 and mainline:

#include <err.h>
#include <errno.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/unistd.h>

int main(int argc, char *argv[]) {
  int sock[2];
  int ret;

  ret = socketpair(AF_UNIX, SOCK_STREAM, 0, sock);
  if (ret < 0)
    err(1, "socketpair");

  char buf[16] = {};
  ret = write(sock[1], buf, sizeof(buf));
  if (ret < 0)
    err(1, "write");

  ret = shutdown(sock[0], SHUT_RDWR);
  if (ret < 0)
    err(1, "shutdown");

  ssize_t bytes = read(sock[0], buf, sizeof(buf));
  if (bytes < 0)
    err(1, "read");

  printf("read %zd bytes\n", bytes);

  return 0;
}