linux-kernel - Re: [PATCH] mm/mincore: allow for making sys

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Message-Id: <9E337EA6-7CDA-457B-96C6-E91F83742587@amacapital.net>
Date:   Tue, 15 Jan 2019 21:25:38 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Snyder <joshs@...flix.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Dave Chinner <david@...morbit.com>,
        Jiri Kosina <jikos@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged



> On Jan 15, 2019, at 9:00 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Wed, Jan 16, 2019 at 12:42 PM Josh Snyder <joshs@...flix.com> wrote:
>> 
>> For Netflix, losing accurate information from the mincore syscall would
>> lengthen database cluster maintenance operations from days to months.  We
>> rely on cross-process mincore to migrate the contents of a page cache from
>> machine to machine, and across reboots.
> 
> Ok, this is the kind of feedback we need, and means I guess we can't
> just use the mapping existence for mincore.
> 
> The two other ways that we considered were:
> 
> (a) owner of the file gets to know cache information for that file.
> 
> (b) having the fd opened *writably* gets you cache residency information.
> 
> Sadly, taking a look at happycache, you open the file read-only, so
> (b) doesn't work.
> 
> Judging just from the source code, I can't tell how the user ownership
> works. Any input on that?
> 
> And if you're not the owner of the file, do you have another
> suggestion for that "Yes, I have the right to see what's in-core for
> this file". Because the problem is literally that if it's some random
> read-only system file, the kernel shouldn't leak access patterns to
> it..
> 
> 


Something like CAP_DAC_READ_SEARCH might not be crazy.