On Mon, Dec 07, 2015 at 02:31:36PM +0100, Marc-André Lureau wrote:
Hi
On Thu, Dec 3, 2015 at 10:53 AM, Didier Pallard
<address@hidden> wrote:
unix_send_msgfds is used by vhost-user control socket. qemu_chr_fe_write_all
is used to send a message and retries as long as EAGAIN errno is set,
but write_msgfds buffer is freed after first EAGAIN failure, causing
message to be sent without proper fds attachment.
In case unix_send_msgfds is called through qemu_chr_fe_write, it will be
user responsability to resend message as is or to free write_msgfds
using set_msgfds(0)
Signed-off-by: Didier Pallard <address@hidden>
Reviewed-by: Thibaut Collet <address@hidden>
---
qemu-char.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/qemu-char.c b/qemu-char.c
index 5448b0f..26d5f2e 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -2614,6 +2614,16 @@ static int unix_send_msgfds(CharDriverState *chr, const
uint8_t *buf, int len)
r = sendmsg(s->fd, &msgh, 0);
} while (r < 0 && errno == EINTR);
+ /* Ancillary data are not sent if no byte is written
+ * so don't free msgfds buffer if return value is EAGAIN
+ * If called from qemu_chr_fe_write_all retry will come soon
+ * If called from qemu_chr_fe_write, it is the user responsibility
+ * to resend message or free fds using set_msgfds(0)
+ */
+ if (r < 0 && errno == EAGAIN) {
+ return r;
+ }
+
This looks reasonable to me. However, I don't know what happens with
partial write of ancillary data. Hopefully it's all or nothing.
Apparently, reading unix_stream_sendmsg() in kernel shows that as long
as a few bytes have been sent, the ancillary data is sent. So it looks
like it still does the right thing in case of a partial write.
If I may put my two cents in, it looks to me very similar to an
fd leakage on back-end side. When a new set_call_fd request
arrives, it is very easy to forget closing the previous file
descriptor. As result, if interrupts are actively maksed/unmasked
by the guest, the back-end can easily reach maximum fds, which
will cause receiving side silently drop new fds in aux data.
--Victor