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From: Michal Hocko <mhocko@kernel.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-api@vger.kernel.org, Khalid Aziz <khalid.aziz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org, Florian Weimer <fweimer@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH 1/2] mm: introduce MAP_FIXED_SAFE
Date: Wed, 6 Dec 2017 10:27:24 +0100
Message-ID: <20171206092724.GH16386@dhcp22.suse.cz> (raw)
In-Reply-To: <87tvx4e8sz.fsf@concordia.ellerman.id.au>

On Wed 06-12-17 16:15:24, Michael Ellerman wrote:
> Hi Michal,
> 
> Some comments below.
> 
> Michal Hocko <mhocko@kernel.org> writes:
> 
> > From: Michal Hocko <mhocko@suse.com>
> >
> > MAP_FIXED is used quite often to enforce mapping at the particular
> > range. The main problem of this flag is, however, that it is inherently
> > dangerous because it unmaps existing mappings covered by the requested
> > range. This can cause silent memory corruptions. Some of them even with
> > serious security implications. While the current semantic might be
> > really desiderable in many cases there are others which would want to
> > enforce the given range but rather see a failure than a silent memory
> > corruption on a clashing range. Please note that there is no guarantee
> > that a given range is obeyed by the mmap even when it is free - e.g.
> > arch specific code is allowed to apply an alignment.
> 
> I don't think this last sentence is correct. Or maybe I don't understand
> what you're referring to.
> 
> If you specifiy MAP_FIXED on a page boundary then the mapping must be
> made at that address, I don't think arch code is allowed to add any
> extra alignment.

The last sentence doesn't talk about MAP_FIXED. It talks about a hint
based mmap without MAP_FIXED ("there are others which would want to
enforce the given range but rather see a failure").
 
[...]
> > diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> > index 6bf730063e3f..ef3770262925 100644
> > --- a/arch/alpha/include/uapi/asm/mman.h
> > +++ b/arch/alpha/include/uapi/asm/mman.h
> > @@ -32,6 +32,8 @@
> >  #define MAP_STACK	0x80000		/* give out an address that is best suited for process/thread stacks */
> >  #define MAP_HUGETLB	0x100000	/* create a huge page mapping */
> >  
> > +#define MAP_FIXED_SAFE	0x200000	/* MAP_FIXED which doesn't unmap underlying mapping */
> > +
> 
> Why the new line before MAP_FIXED_SAFE? It should sit with the others.

will remove the empty line

> You're using a different value to other arches here, but that's OK, and
> alpha doesn't use asm-generic/mman.h or mman-common.h
> 
> > diff --git a/arch/powerpc/include/uapi/asm/mman.h b/arch/powerpc/include/uapi/asm/mman.h
> > index e63bc37e33af..3ffd284e7160 100644
> > --- a/arch/powerpc/include/uapi/asm/mman.h
> > +++ b/arch/powerpc/include/uapi/asm/mman.h
> > @@ -29,5 +29,6 @@
> >  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
> >  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
> >  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> > +#define MAP_FIXED_SAFE	0x800000	/* MAP_FIXED which doesn't unmap underlying mapping */
> 
> Why did you pick 0x800000?
> 
> I don't see any reason you can't use 0x8000 on powerpc.

Copy&paste I guess, will update it.

[...]

> >  #ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> >  # define MAP_UNINITIALIZED 0x4000000	/* For anonymous mmap, memory could be
> >  					 * uninitialized */
> > @@ -63,6 +64,7 @@
> >  # define MAP_UNINITIALIZED 0x0		/* Don't support this flag */
> >  #endif
> >  
> > +
> 
> Stray new line.

will remove

> >  /*
> >   * Flags for msync
> >   */
> > diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
> > index 2dffcbf705b3..56cde132a80a 100644
> > --- a/include/uapi/asm-generic/mman.h
> > +++ b/include/uapi/asm-generic/mman.h
> > @@ -13,6 +13,7 @@
> >  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
> >  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
> >  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> > +#define MAP_FIXED_SAFE	0x80000		/* MAP_FIXED which doesn't unmap underlying mapping */
> 
> So I think I proved above that all the arches that are using 0x80000 are
> also using mman-common.h, and vice-versa.
> 
> So you can put this in mman-common.h can't you?

Yes it seems I can. I would have to double check. It is true that
defining the new flag closer to MAP_FIXED makes some sense

[...]
> So it would be more accurate to say something like:
> 
> 	/*
> 	 * Internal to the kernel MAP_FIXED_SAFE is a superset of
> 	 * MAP_FIXED, so set MAP_FIXED in flags if MAP_FIXED_SAFE was
> 	 * set by the caller. This avoids all the arch code having to
> 	 * check for MAP_FIXED and MAP_FIXED_SAFE.
> 	 */
> 	if (flags & MAP_FIXED_SAFE)
> 		flags |= MAP_FIXED;

OK, I will use this wording.

Thanks for your review! Finally something that doesn't try to beat the
name to death ;)
-- 
Michal Hocko
SUSE Labs

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  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-29 14:42 [PATCH 0/2] " Michal Hocko
2017-11-29 14:42 ` [PATCH 1/2] " Michal Hocko
2017-12-06  5:15   ` Michael Ellerman
2017-12-06  9:27     ` Michal Hocko [this message]
2017-12-06 10:02       ` Michal Hocko
2017-12-07 12:07   ` Pavel Machek
2017-11-29 14:42 ` [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map Michal Hocko
2017-11-29 17:45   ` Khalid Aziz
2017-11-29 14:45 ` [PATCH] mmap.2: document new MAP_FIXED_SAFE flag Michal Hocko
2017-11-30  3:16   ` John Hubbard
2017-11-30  8:23     ` Michal Hocko
2017-11-30  8:24   ` [PATCH v2] " Michal Hocko
2017-11-30 18:31     ` John Hubbard
2017-11-30 18:39       ` Michal Hocko
2017-11-29 15:13 ` [PATCH 0/2] mm: introduce MAP_FIXED_SAFE Rasmus Villemoes
     [not found]   ` <b154b794-7a8b-995e-0954-9234b9446b31-rjjw5hvvQKZaa/9Udqfwiw@public.gmane.org>
2017-11-29 15:50     ` Michal Hocko
2017-11-29 22:15   ` Kees Cook
2017-11-29 22:12 ` Kees Cook
2017-11-29 22:25 ` Kees Cook
     [not found]   ` <CAGXu5jLa=b2HhjWXXTQunaZuz11qUhm5aNXHpS26jVqb=G-gfw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-30  6:58     ` Michal Hocko
2017-12-01 15:26       ` Cyril Hrubis
2017-12-06  4:51         ` Michael Ellerman
2017-12-06  4:54           ` Matthew Wilcox
2017-12-06  7:03             ` Matthew Wilcox
2017-12-06  7:33               ` John Hubbard
     [not found]                 ` <5f4fc834-274a-b8f1-bda0-5bcddc5902ed-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-06  7:35                   ` Florian Weimer
2017-12-06  8:06                     ` John Hubbard
     [not found]                       ` <27ee1755-76d8-f086-5760-9c973b31108a-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-06  8:54                         ` Florian Weimer
     [not found]               ` <20171206070355.GA32044-PfSpb0PWhxZc2C7mugBRk2EX/6BAtgUQ@public.gmane.org>
2017-12-07  5:46                 ` Michael Ellerman
2017-12-07 19:14                   ` Kees Cook
     [not found]                     ` <CAGXu5jLWRQn6EaXEEvdvXr+4gbiJawwp1EaLMfYisHVfMiqgSA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-12-07 19:57                       ` Matthew Wilcox
2017-12-08  8:33                         ` Michal Hocko
     [not found]                           ` <20171208083315.GR20234-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-12-08 20:13                             ` Kees Cook
     [not found]                               ` <CAGXu5j+VupGmKEEHx-uNXw27Xvndu=0ObsBqMwQiaYPyMGD+vw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-12-08 20:57                                 ` Matthew Wilcox
2017-12-08 11:08                         ` Michael Ellerman
2017-12-08 14:27                           ` Pavel Machek
2017-12-08 20:31                             ` Cyril Hrubis
2017-12-08 20:47                             ` Florian Weimer
2017-12-08 14:33                           ` David Laight
2017-12-06  4:50       ` Michael Ellerman
     [not found]         ` <87zi6we9z2.fsf-W0DJWXSxmBNbyGPkN3NxC2scP1bn1w/D@public.gmane.org>
2017-12-06  7:33           ` Rasmus Villemoes
     [not found]             ` <a3b3129a-2626-a65e-59b0-68aada523723-rjjw5hvvQKZaa/9Udqfwiw@public.gmane.org>
2017-12-06  9:08               ` Michal Hocko
2017-12-07  0:19                 ` Kees Cook
2017-12-07  1:08                   ` John Hubbard
2017-12-13  9:25 [PATCH v2 " Michal Hocko
2017-12-13  9:25 ` [PATCH 1/2] " Michal Hocko
     [not found]   ` <20171213092550.2774-2-mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2017-12-13 12:50     ` Matthew Wilcox
2017-12-13 13:01       ` Michal Hocko

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