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

From: Vincent Whitchurch <[email-masked]>
Date: Thu, 11 Nov 2021 15:00:02 +0100

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 <[email-masked]>
> Reported-by: Casey Schaufler <[email-masked]>
> Signed-off-by: Jiang Wang <[email-masked]>

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;
}