Re: [PATCH v6 1/5] vfs: add flags parameter to ->mmap() in 'struct file_operations'

From: Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
To: Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Cc: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>,
	linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org,
	David Airlie <airlied-cv59FeDIM0c@public.gmane.org>,
	linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Takashi Iwai <tiwai-IBi9RG/b67k@public.gmane.org>,
	dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Julia Lawall <julia.lawall-L2FTfq7BK8M@public.gmane.org>,
	luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org,
	linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Daniel Vetter
	<daniel.vetter-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>,
	akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org,
	linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org
Subject: Re: [PATCH v6 1/5] vfs: add flags parameter to ->mmap() in 'struct file_operations'
Date: Thu, 24 Aug 2017 09:58:38 -0700	[thread overview]
Message-ID: <20170824165838.GB3121@infradead.org> (raw)
In-Reply-To: <150353211985.5039.4333061601382775843.stgit-p8uTFz9XbKj2zm6wflaqv1nYeNYlB/vhral2JQCrhuEAvxtiuMwx3w@public.gmane.org>

On Wed, Aug 23, 2017 at 04:48:40PM -0700, Dan Williams wrote:
> We are running running short of vma->vm_flags. We can avoid needing a
> new VM_* flag in some cases if the original @flags submitted to mmap(2)
> is made available to the ->mmap() 'struct file_operations'
> implementation. For example, the proposed addition of MAP_DIRECT can be
> implemented without taking up a new vm_flags bit. Another motivation to
> avoid vm_flags is that they appear in /proc/$pid/smaps, and we have seen
> software that tries to dangerously (TOCTOU) read smaps to infer the
> behavior of a virtual address range.
> 
> This conversion was performed by the following semantic patch. There
> were a few manual edits for oddities like proc_reg_mmap.
> 
> Thanks to Julia for helping me with coccinelle iteration to cover cases
> where the mmap routine is defined in a separate file from the 'struct
> file_operations' instance that consumes it.

How are we going to check that an instance actually supports any
of those flags?