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

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Message-ID: <20170824165838.GB3121@infradead.org>
Date:   Thu, 24 Aug 2017 09:58:38 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Jan Kara <jack@...e.cz>,
        linux-nvdimm@...ts.01.org, David Airlie <airlied@...ux.ie>,
        linux-api@...r.kernel.org, Takashi Iwai <tiwai@...e.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, Julia Lawall <julia.lawall@...6.fr>,
        luto@...nel.org, linux-fsdevel@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...el.com>, linux-mm@...ck.org
Subject: Re: [PATCH v6 1/5] vfs: add flags parameter to ->mmap() in 'struct
 file_operations'

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?