Message-ID: <3b97233b-5d05-5efc-4173-e3a1ef177cbc@kernel.dk>
Date:   Wed, 23 Oct 2019 08:11:29 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Wolfgang Bumiller <w.bumiller@...xmox.com>
Cc:     linux-block@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH 1/3] io_uring: add support for async work inheriting files
 table

On 10/23/19 6:04 AM, Wolfgang Bumiller wrote:
> On Thu, Oct 17, 2019 at 03:28:56PM -0600, Jens Axboe wrote:
>> This is in preparation for adding opcodes that need to modify files
>> in a process file table, either adding new ones or closing old ones.
>>
>> If an opcode needs this, it must set REQ_F_NEED_FILES in the request
>> structure. If work that needs to get punted to async context have this
>> set, they will grab a reference to the process file table. When the
>> work is completed, the reference is dropped again.
> 
> I think IORING_OP_SENDMSG and _RECVMSG need to set this flag due to
> SCM_RIGHTS control messages.
> Thought I'd reply here since I just now ran into the issue that I was
> getting ever-increasing wrong file descriptor numbers on pretty much
> ever "other" async recvmsg() call I did via io-uring while receiving
> file descriptors from lxc for the seccomp-notify proxy. (I'm currently
> running an ubuntu based 5.3.1 kernel)
> I ended up finding them in /proc - they show up in all kernel threads,
> eg.:
> 
> root:/root # grep Name /proc/9/status
> Name:   mm_percpu_wq
> root:/root # ls -l /proc/9/fd
> total 0
> lr-x------ 1 root root 64 Oct 23 12:00 0 -> '/proc/512 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 1 -> /proc/512/mem
> lr-x------ 1 root root 64 Oct 23 12:00 10 -> '/proc/11782 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 11 -> /proc/11782/mem
> lr-x------ 1 root root 64 Oct 23 12:00 12 -> '/proc/12210 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 13 -> /proc/12210/mem
> lr-x------ 1 root root 64 Oct 23 12:00 14 -> '/proc/12298 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 15 -> /proc/12298/mem
> lr-x------ 1 root root 64 Oct 23 12:00 16 -> '/proc/13955 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 17 -> /proc/13955/mem
> lr-x------ 1 root root 64 Oct 23 12:00 18 -> '/proc/13989 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 19 -> /proc/13989/mem
> lr-x------ 1 root root 64 Oct 23 12:00 2 -> '/proc/584 (deleted)'
> lr-x------ 1 root root 64 Oct 23 12:00 20 -> '/proc/15502 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 21 -> /proc/15502/mem
> lr-x------ 1 root root 64 Oct 23 12:00 22 -> '/proc/15510 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 23 -> /proc/15510/mem
> lr-x------ 1 root root 64 Oct 23 12:00 24 -> '/proc/17833 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 25 -> /proc/17833/mem
> lr-x------ 1 root root 64 Oct 23 12:00 26 -> '/proc/17836 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 27 -> /proc/17836/mem
> lr-x------ 1 root root 64 Oct 23 12:00 28 -> '/proc/21929 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 29 -> /proc/21929/mem
> lrwx------ 1 root root 64 Oct 23 12:00 3 -> /proc/584/mem
> lr-x------ 1 root root 64 Oct 23 12:00 30 -> '/proc/22214 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 31 -> /proc/22214/mem
> lr-x------ 1 root root 64 Oct 23 12:00 32 -> '/proc/22283 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 33 -> /proc/22283/mem
> lr-x------ 1 root root 64 Oct 23 12:00 34 -> '/proc/29795 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 35 -> /proc/29795/mem
> lr-x------ 1 root root 64 Oct 23 12:00 36 -> '/proc/30124 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 37 -> /proc/30124/mem
> lr-x------ 1 root root 64 Oct 23 12:00 38 -> '/proc/31016 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 39 -> /proc/31016/mem
> lr-x------ 1 root root 64 Oct 23 12:00 4 -> '/proc/1632 (deleted)'
> lr-x------ 1 root root 64 Oct 23 12:00 40 -> '/proc/4137 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 41 -> /proc/4137/mem
> lrwx------ 1 root root 64 Oct 23 12:00 5 -> /proc/1632/mem
> lr-x------ 1 root root 64 Oct 23 12:00 6 -> '/proc/3655 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 7 -> /proc/3655/mem
> lr-x------ 1 root root 64 Oct 23 12:00 8 -> '/proc/7075 (deleted)'
> lrwx------ 1 root root 64 Oct 23 12:00 9 -> /proc/7075/mem
> root:/root #
> 
> Those are the fds I expected to receive, and I get fd numbers
> consistently increasing with them.
> lxc sends the syscall-executing process' pidfd and its 'mem' fd via a
> socket, but instead of making it to the receiver, they end up there...
> 
> I suspect that an async sendmsg() call could potentially end up
> accessing those instead of the ones from the sender process, but I
> haven't tested it...

Might "just" be a case of the sendmsg() being stuck, we can't currently
cancel work. So if they never complete, the ring won't go away.

Actually working on a small workqueue replacement for io_uring which
allow us to cancel things like that. It's a requirement for accept() as
well, but also for basic read/write send/recv on sockets. So used to
storage IO operations that complete in a finite amount of time...

But yes, I hope with that, and the flush trick that Jann suggested, that
we can make this 100% reliable for any type of operation.

-- 
Jens Axboe

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