Re: [PATCH 2/2] linux-user: Allow sendmsg() without IOV

[Laurent Vivier <laurent@vivier.eu>]

Le 12/12/2022 à 18:34, Helge Deller a écrit :
> 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>
> ---
>   linux-user/syscall.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index a365903a3a..9e2c0a18fc 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -3330,7 +3330,10 @@ 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 */

why don't you check only for count is 0?
Somehing like:

if (vec == NULL && (count || !send)) {
...

Thanks,
Laurent