linux-kernel - Re: linux-next: Tree for Nov 7

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Message-ID: <87h8txw87w.fsf@concordia.ellerman.id.au>
Date:   Tue, 14 Nov 2017 19:54:59 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Joel Stanley <joel@....id.au>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mips@...ux-mips.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org
Subject: Re: linux-next: Tree for Nov 7

Michal Hocko <mhocko@...nel.org> writes:

> On Mon 13-11-17 22:34:50, Michael Ellerman wrote:
>> Hi Michal,
>> 
>> Michal Hocko <mhocko@...nel.org> writes:
>> > On Mon 13-11-17 10:20:06, Michal Hocko wrote:
>> >> [Cc arm and ppc maintainers]
>> >
>> > Hmm, it turned out to be a problem on other architectures as well.
>> > CCing more maintainers. For your reference, we are talking about
>> > http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@kernel.org
>> > which has broken architectures which do apply aligning on the mmap
>> > address hint without MAP_FIXED applied. See below my proposed way
>> > around this issue because I belive that the above patch is quite
>> > valuable on its own to be dropped for all archs.
>> 
>> I don't really like your solution sorry :)  The fact that you've had to
>> patch seven arches seems like a red flag.
>> 
>> I think this is a generic problem with MAP_FIXED, which I've heard
>> userspace folks complain about in the past.
>
> The thing is that we canno  change MAP_FIXED behavior as it is carved in
> stone

Yes obviously. I didn't mean to imply we would change MAP_FIXED, rather
we would add a new flag with the new semantics.

>> Currently MAP_FIXED does two things:
>>   1. makes addr not a hint but the required address
>>   2. blasts any existing mapping
>> 
>> You want 1) but not 2).
>
> + fail if there is a clashing range

Yep. I thought that was implied :)

>> So the right solution IMHO would be to add a new mmap flag to request
>> that behaviour, ie. a fixed address but iff there is nothing already
>> mapped there.
>> 
>> I don't know the mm code well enough to know if that's hard for some
>> reason, but it *seems* like it should be doable.
>
> Yes, I have mentioned that in the previous email but the amount of code
> would be even larger. Basically every arch which reimplements
> arch_get_unmapped_area would have to special case new MAP_FIXED flag to
> do vma lookup.

I'd have to look, but my memory of the arch code is that it doesn't deal
with the vma so it wouldn't need any change.

> So this was the most simple solution I could come up
> with. If there was a general interest for MAP_FIXED_SAFE then we can
> introduce it later of course. I would just like the hardening merged
> sooner rather than later.

Sure. But in the scheme of things one more kernel release is not that
big a deal to get it right. Given that the simple approach of dropping
MAP_FIXED turns out to not be simple at all.

cheers