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Subject: Re: [PATCH v6] fuse: Add support for passthrough read/write
To:     Jann Horn <jannh@google.com>, Alessio Balsini <balsini@android.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Nikhilesh Reddy <reddyn@codeaurora.org>,
        Akilesh Kailash <akailash@google.com>,
        David Anderson <dvander@google.com>,
        Eric Yan <eric.yan@oneplus.com>,
        Martijn Coenen <maco@android.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Lawrence <paullawrence@google.com>,
        Stefano Duo <stefanoduo@google.com>,
        Zimuzo Ezeozue <zezeozue@google.com>,
        kernel-team <kernel-team@android.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>
References: <20200812161452.3086303-1-balsini@android.com>
 <CAG48ez17uXtjCTa7xpa=JWz3iBbNDQTKO2hvn6PAZtfW3kXgcA@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <c135d701-4455-95c7-e58a-82e8075d2ea7@kernel.dk>
Date:   Thu, 13 Aug 2020 12:41:55 -0600
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On 8/12/20 12:29 PM, Jann Horn wrote:
>> +       passthrough_inode = file_inode(passthrough_filp);
>> +
>> +       iocb->ki_filp = passthrough_filp;
> 
> Hmm... so we're temporarily switching out the iocb's ->ki_filp here? I
> wonder whether it is possible for some other code to look at ->ki_filp
> concurrently... maybe Jens Axboe knows whether that could plausibly
> happen?

I looked into the io_uring use case, and we're using req->file (which is
the same as kiocb->ki_filp) after submission for the polled-IO case.
That's IOCB_HIPRI, not poll(2) related. So it's not safe for that case,
but that probably isn't supported by fuse. But something to keep in
mind...

In general, kiocb->ki_filp is used for setup, and then at IO completion.
That use case appears safe, as long as the ki_filp is restored before
->ki_complete() is called.

Only other exception should be the poll handlers. They arm at setup
time, which is still fine, but re-arm if we get triggered and the file
is still not ready. I _think_ this case is still fine without having
seen all of the bits for this use case, as we haven't actually called
read/write_iter at that point on it.

But in general, I'd say it looks a bit iffy to be fiddling with ki_filp.
Maybe use a new kiocb and stack them like that instead?

-- 
Jens Axboe