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+
+linux-user: sendmsg fails to send messages without iov
+Description of problem:
+When run via qemu `sendmsg` fails to send messages which contain a zero length `iov` but _do_ contain ancillary data. This works fine on plain Linux.
+
+A practical example: the `ell` library relies on this for setting the IV on a kernel crypto (`AF_ALG`) socket: https://git.kernel.org/pub/scm/libs/ell/ell.git/tree/ell/cipher.c#n526
+
+A message without data but only ancillary data is used to set the IV.
+Steps to reproduce:
+See [qemu_ancillary.c](/uploads/84ee20aa3b9178022847d6cd7fcf0048/qemu_ancillary.c) for a self contained testcase which sends two mesages (one with `msg_iovlen=0`, one with `msg_iovlen=1`).
+
+(Test case is to be considered GPL, as I've copied bits from `ell`)
+
+Native:
+```
+$ strace -esendmsg ./a.out 
+sendmsg(6, {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}, 0) = 0
+sendmsg(6, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", iov_len=16}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_ALG, cmsg_type=0x2}], msg_controllen=40, msg_flags=0}, 0) = 16
++++ exited with 0 +++
+```
+
+
+Qemu (observe missing sendmsg call):
+```
+$ strace -esendmsg ~/debug/qemu/build/qemu-x86_64 ./a.out 
+sendmsg(6, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", iov_len=16}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_ALG, cmsg_type=0x2}], msg_controllen=40, msg_flags=0}, 0) = 16
++++ exited with 0 +++
+```
+
+For a practical reproducer:
+
+1. Compile and run `ell`'s `test-cipher` test case:
+
+```
+$ ~/debug/qemu/build/qemu-x86_64 ./unit/test-cipher 
+TEST: unsupported
+TEST: aes
+TEST: aes_ctr
+test-cipher: unit/test-cipher.c:102: test_aes_ctr: Assertion `!r' failed.
+Aborted (core dumped)
+```
+
+A strace will look similar.