netdev - Re: [PATCH v2] 9p/trans_fd: perform read/write with TIF

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Message-ID: <38d892bd-8ace-c4e9-9d73-777d3828acbc@I-love.SAKURA.ne.jp>
Date:   Sun, 4 Sep 2022 09:27:22 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Christian Schoenebeck <linux_oss@...debyte.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        syzbot <syzbot+8b41a1365f1106fd0f33@...kaller.appspotmail.com>,
        v9fs-developer@...ts.sourceforge.net,
        syzkaller-bugs@...glegroups.com, netdev@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2] 9p/trans_fd: perform read/write with TIF_SIGPENDING
 set

On 2022/09/04 8:39, Dominique Martinet wrote:
> Is there any reason you spent time working on v2, or is that just
> theorical for not messing with userland fd ?

Just theoretical for not messing with userland fd, for programs generated
by fuzzers might use fds passed to the mount() syscall. I imagined that
syzbot again reports this problem when it started playing with fcntl().

For robustness, not messing with userland fd is the better. ;-)

> 
> unless there's any reason I'll try to find time to test v1 and queue it
> for 6.1

OK.

> We seem to check for EAGAIN where kernel_read/write end up being called
> and there's a poll for scheduling so it -should- work, but I assume this
> hasn't been tested much and might take a bit of time to test.

Right. Since the I/O in kernel side is poll based multiplexing, forcing
non-blocking I/O -should- work. (But I can't test e.g. changes in CPU time
usage because I don't have environment to test. I assume that poll based
multiplexing saves us from doing busy looping.)

We are currently checking for ERESTARTSYS and EAGAIN. The former is for
non-socket fds which do not have O_NONBLOCK flag, and the latter is for
socket fds which have O_NONBLOCK flag. If we enforce O_NONBLOCK flag,
the former will become redundant. I think we can remove the former check
after you tested that setting O_NONBLOCK flag on non-socket fds does not break.