Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <20180730203633.GC12962@bombadil.infradead.org>
References: <20180730143710.14413-1-christian@brauner.io> <20180730143710.14413-2-christian@brauner.io>
 <20180730163155.GA27761@infradead.org> <20180730203633.GC12962@bombadil.infradead.org>
From:   Martijn Coenen <maco@android.com>
Date:   Tue, 31 Jul 2018 10:44:33 +0200
Message-ID: <CAB0TPYG5T_d32iwt8Lgq_nDO_COWfWBfuSzErraASSuTYGm3yQ@mail.gmail.com>
Subject: Re: [PATCH 1/4] file: export __alloc_fd()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Christian Brauner <christian@brauner.io>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>, rlove@google.com,
        ben@decadent.org.uk
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Jul 30, 2018 at 10:36 PM, Matthew Wilcox <willy@infradead.org> wrote:
> I'm not entirely sure I understand the binder code (... does anyone?)
> but from what I can see, it intends to open a file descriptor in the
> process which is the target of the message being sent.

You're right.

> That strikes
> me as wrong-headed; it should be allocating a struct file and passing
> that file to the other process.  When that process receives the message,
> *it* allocates a file descriptor for itself.ho

We're looking into cleaning this up (historically it was done this way
because VIVT caches made this not very efficient), and this is indeed
a very good candidate for fixing it.

>
> But I think the binder user-space API relies on this.  The userspace API
> seems to rely on passing fd numbers around ... but I'm having trouble
> figuring most of this user API out.  Perhaps Martijn can help here.

The UAPI does expect a file descriptor, but we may be able to do the
mapping from fd to struct file (and vice-versa) in the kernel driver,
so userspace wouldn't notice.

Thanks,
Martijn