linux-user: Allow sendmsg() without IOV

Applications do call sendmsg() without any IOV, e.g.:
 sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=NULL, msg_iovlen=0,
            msg_control=[{cmsg_len=36, cmsg_level=SOL_ALG, cmsg_type=0x2}],
            msg_controllen=40, msg_flags=0}, MSG_MORE) = 0
 sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="The quick brown fox jumps over t"..., iov_len=183}],
            msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_ALG, cmsg_type=0x3}],
            msg_controllen=24, msg_flags=0}, 0) = 183

The function do_sendrecvmsg_locked() is used for sndmsg() and recvmsg()
and calls lock_iovec() to lock the IOV into memory. For the first
sendmsg() above it returns NULL and thus wrongly skips the call the host
sendmsg() syscall, which will break the calling application.

Fix this issue by:
- allowing sendmsg() even with empty IOV
- skip recvmsg() if IOV is NULL
- skip both if the return code of do_sendrecvmsg_locked() != 0, which
  indicates some failure like EFAULT on the IOV

Tested with the debian "ell" package with hppa guest on x86_64 host.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20221212173416.90590-2-deller@gmx.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

1 files changed, 7 insertions, 2 deletions

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index a0d2beddaa..1e868e9b0e 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -3293,7 +3293,10 @@ static abi_long do_sendrecvmsg_locked(int fd, struct target_msghdr *msgp,
                      target_vec, count, send);
     if (vec == NULL) {
         ret = -host_to_target_errno(errno);
-        goto out2;
+        /* allow sending packet without any iov, e.g. with MSG_MORE flag */
+        if (!send || ret) {
+            goto out2;
+        }
     }
     msg.msg_iovlen = count;
     msg.msg_iov = vec;
@@ -3345,7 +3348,9 @@ static abi_long do_sendrecvmsg_locked(int fd, struct target_msghdr *msgp,
     }
 
 out:
-    unlock_iovec(vec, target_vec, count, !send);
+    if (vec) {
+        unlock_iovec(vec, target_vec, count, !send);
+    }
 out2:
     return ret;
 }