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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.
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