linux-kernel - Re: sg_io HARDENED_USERCOPY

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Date:   Fri, 30 Dec 2016 07:10:49 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Jones <davej@...emonkey.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: sg_io HARDENED_USERCOPY_PAGESPAN trace

On Fri, Dec 30, 2016 at 10:01:39AM -0500, Dave Jones wrote:
> I threw this debug printk into the pagespan code to see what exactly
> it was complaining about..
> 
> ptr:ffff88042614cff8 end:ffff88042614d003 n:c
> 
> so it was copying 12 bytes that spanned two pages.
> >From my reading of the config option help text, this thing is
> complaining that wasn't allocated with __GFP_COMP maybe ?

If this is on a devie using blk-mq the block core will use high
order allocations (as high as possible) to allocate the requests
for each queue, so struct request could very well span multiple
pages.  But I don't see what __GFP_COMP would have to do with
user copy annoations.  As all requests for a queue are freed
togeth again there is no point in setting __GFP_COMP for the
request allocations.