linux-kernel - Re: [PATCH 1/4] file: export __alloc

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Date:   Tue, 31 Jul 2018 10:44:33 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>, rlove@...gle.com,
        ben@...adent.org.uk
Subject: Re: [PATCH 1/4] file: export __alloc_fd()

On Mon, Jul 30, 2018 at 10:36 PM, Matthew Wilcox <willy@...radead.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