linux-user: implement TARGET_SO_PEERSEC

"The purpose of this option is to allow an application to obtain the
security credentials of a Unix stream socket peer.  It is analogous to
SO_PEERCRED (which provides authentication using standard Unix credentials
of pid, uid and gid), and extends this concept to other security
models." -- https://lwn.net/Articles/62370/

Until now it was passed to the kernel with an "int" argument and
fails when it was supported by the host because the parameter is
like a filename: it is always a \0-terminated string with no embedded
\0 characters, but is not guaranteed to be ASCII or UTF-8.

I've tested the option with the following program:

    /*
     * cc -o getpeercon getpeercon.c
     */

    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>

    int main(void)
    {
        int fd;
        struct sockaddr_in server, addr;
        int ret;
        socklen_t len;
        char buf[256];

        fd = socket(PF_INET, SOCK_STREAM, 0);
        if (fd == -1) {
            perror("socket");
            return 1;
        }

        server.sin_family = AF_INET;
        inet_aton("127.0.0.1", &server.sin_addr);
        server.sin_port = htons(40390);

        connect(fd, (struct sockaddr*)&server, sizeof(server));

        len = sizeof(buf);
        ret = getsockopt(fd, SOL_SOCKET, SO_PEERSEC, buf, &len);
        if (ret == -1) {
            perror("getsockopt");
            return 1;
        }
        printf("%d %s\n", len, buf);
        return 0;
    }

On host:

  $ ./getpeercon
  33 system_u:object_r:unlabeled_t:s0

With qemu-aarch64/bionic without the patch:

  $ ./getpeercon
  getsockopt: Numerical result out of range

With the patch:

  $ ./getpeercon
  33 system_u:object_r:unlabeled_t:s0

Bug: https://bugs.launchpad.net/qemu/+bug/1823790
Reported-by: Matthias Lüscher <lueschem@gmail.com>
Tested-by: Matthias Lüscher <lueschem@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200204211901.1731821-1-laurent@vivier.eu>

linux-user/syscall.c

@@ -2344,6 +2344,28 @@ static abi_long do_getsockopt(int sockfd, int level, int optname,
             }
             break;
         }
+        case TARGET_SO_PEERSEC: {
+            char *name;
+
+            if (get_user_u32(len, optlen)) {
+                return -TARGET_EFAULT;
+            }
+            if (len < 0) {
+                return -TARGET_EINVAL;
+            }
+            name = lock_user(VERIFY_WRITE, optval_addr, len, 0);
+            if (!name) {
+                return -TARGET_EFAULT;
+            }
+            lv = len;
+            ret = get_errno(getsockopt(sockfd, level, SO_PEERSEC,
+                                       name, &lv));
+            if (put_user_u32(lv, optlen)) {
+                ret = -TARGET_EFAULT;
+            }
+            unlock_user(name, optval_addr, lv);
+            break;
+        }
         case TARGET_SO_LINGER:
         {
             struct linger lg;