Re: Problem with io_uring splice and KTLS

[Jens Axboe <axboe@kernel.dk>]

On 10/10/23 8:19 AM, Sascha Hauer wrote:
> Hi,
> 
> I am working with a webserver using io_uring in conjunction with KTLS. The
> webserver basically splices static file data from a pipe to a socket which uses
> KTLS for encryption. When splice is done the socket is closed. This works fine
> when using software encryption in KTLS. Things go awry though when the software
> encryption is replaced with the CAAM driver which replaces the synchronous
> encryption with a asynchronous queue/interrupt/completion flow.
> 
> So far I have traced it down to tls_push_sg() calling tcp_sendmsg_locked() to
> send the completed encrypted messages. tcp_sendmsg_locked() sometimes waits for
> more memory on the socket by calling sk_stream_wait_memory(). This in turn
> returns -ERESTARTSYS due to:
> 
>         if (signal_pending(current))
>                 goto do_interrupted;
> 
> The current task has the TIF_NOTIFY_SIGNAL set due to:
> 
> io_req_normal_work_add()
> {
>         ...
>         /* This interrupts sk_stream_wait_memory() (notify_method == TWA_SIGNAL) */
>         task_work_add(req->task, &tctx->task_work, ctx->notify_method)))
> }
> 
> The call stack when sk_stream_wait_memory() fails is as follows:
> 
> [ 1385.428816]  dump_backtrace+0xa0/0x128
> [ 1385.432568]  show_stack+0x20/0x38
> [ 1385.435878]  dump_stack_lvl+0x48/0x60
> [ 1385.439539]  dump_stack+0x18/0x28
> [ 1385.442850]  tls_push_sg+0x100/0x238
> [ 1385.446424]  tls_tx_records+0x118/0x1d8
> [ 1385.450257]  tls_sw_release_resources_tx+0x74/0x1a0
> [ 1385.455135]  tls_sk_proto_close+0x2f8/0x3f0
> [ 1385.459315]  inet_release+0x58/0xb8
> [ 1385.462802]  inet6_release+0x3c/0x60
> [ 1385.466374]  __sock_release+0x48/0xc8
> [ 1385.470035]  sock_close+0x20/0x38
> [ 1385.473347]  __fput+0xbc/0x280
> [ 1385.476399]  ____fput+0x18/0x30
> [ 1385.479537]  task_work_run+0x80/0xe0
> [ 1385.483108]  io_run_task_work+0x40/0x108
> [ 1385.487029]  __arm64_sys_io_uring_enter+0x164/0xad8
> [ 1385.491907]  invoke_syscall+0x50/0x128
> [ 1385.495655]  el0_svc_common.constprop.0+0x48/0xf0
> [ 1385.500359]  do_el0_svc_compat+0x24/0x40
> [ 1385.504279]  el0_svc_compat+0x38/0x108
> [ 1385.508026]  el0t_32_sync_handler+0x98/0x140
> [ 1385.512294]  el0t_32_sync+0x194/0x198
> 
> So the socket is being closed and KTLS tries to send out the remaining
> completed messages.  From a splice point of view everything has been sent
> successfully, but not everything made it through KTLS to the socket and the
> remaining data is sent while closing the socket.
> 
> I vaguely understand what's going on here, but I haven't got the
> slightest idea what to do about this. Any ideas?

Two things to try:

1) Depending on how you use the ring, set it up with
IORING_SETUP_SINGLE_ISSUER | IORING_SETUP_DEFER_TASKRUN. The latter will
avoid using signal based task_work notifications, which may be messing
you up here.

2) io_uring will hold a reference to the file/socket. I'm unsure if this
is a problem in the above case, but sometimes it'll prevent the final
flush.

Do you have a reproducer that could be run to test? Sometimes easier to
see what's going on when you can experiment, it'll save some time.

-- 
Jens Axboe