Am 19.06.23 um 15:09 schrieb Stefan Metzmacher:
Am 19.06.23 um 15:05 schrieb Jens Axboe:
On 6/19/23 3:57?AM, Stefan Metzmacher wrote:
Hi Jens,
If the application sets ->msg_control and we have to later retry this
command, or if it got queued with IOSQE_ASYNC to begin with, then we
need to retain the original msg_control value. This is due to the net
stack overwriting this field with an in-kernel pointer, to copy it
in. Hitting that path for the second time will now fail the copy from
user, as it's attempting to copy from a non-user address.
I'm not 100% sure about the impact of this change.
But I think the logic we need is that only the
first __sys_sendmsg_sock() that returns > 0 should
see msg_control. A retry because of MSG_WAITALL should
clear msg_control[len] for a follow up __sys_sendmsg_sock().
And I fear the patch below would not clear it...
Otherwise the receiver/socket-layer will get the same msg_control twice,
which is unexpected.
Yes agree, if we do transfer some (but not all) data and WAITALL is set,
it should get cleared. I'll post a patch for that.
Thanks!
Note that it was also broken before, just differently broken. The most
likely outcome here was a full retry and now getting -EFAULT.
Yes, I can see that it was broken before...
I haven't checked myself, but I'm wondering about the recvmsg case,
I guess we would need to advance the msg_control buffer after each
iteration, in order to avoid overwritting the already received messages
on retry.
This all gets complicated with things like MSG_CTRUNC.
I guess it's too late to reject MSG_WAITALL together with msg_control
for io_recvmsg() because of compat reasons,
but as MSG_WAITALL is also processed in the socket layer, we could keep it
simple for now and skip the this retry logic:
if (flags & MSG_WAITALL)
min_ret = iov_iter_count(&kmsg->msg.msg_iter);
This might become something similar to this,
but likely more complex, as would need to record kmsg->controllen == 0
condition already in io_recvmsg_prep:
if (flags & MSG_WAITALL && kmsg->controllen == 0)
min_ret = iov_iter_count(&kmsg->msg.msg_iter);
metze
