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From: Florian Weimer <fweimer@redhat.com>
To: Pavel Machek <pavel@ucw.cz>, Michael Ellerman <mpe@ellerman.id.au>
Cc: Matthew Wilcox <willy@infradead.org>,
	Kees Cook <keescook@chromium.org>, Cyril Hrubis <chrubis@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	Linux API <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 <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
	Joel Stanley <joel@jms.id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
Date: Fri, 8 Dec 2017 21:47:21 +0100	[thread overview]
Message-ID: <289f0cc8-9e24-fc38-cb83-4223f9923940@redhat.com> (raw)
In-Reply-To: <20171208142714.GB7793@amd>

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
>> If we had a time machine, the right set of flags would be:
>>
>>    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>>    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian

WARNING: multiple messages have this Message-ID (diff)
From: Florian Weimer <fweimer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
To: Pavel Machek <pavel-+ZI9xUNit7I@public.gmane.org>,
	Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>
Cc: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>,
	Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
	Cyril Hrubis <chrubis-AlSwsSmVLrQ@public.gmane.org>,
	Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Khalid Aziz <khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>,
	Andrew Morton
	<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
	Russell King - ARM Linux
	<linux-I+IVW8TIWO2tmTQ+vhA3Yw@public.gmane.org>,
	Andrea Arcangeli
	<aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Linux-MM <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>,
	LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	linux-arch <linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	John Hubbard <jhubbard-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>,
	Abdul Haleem
	<abdhalee-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
	Joel Stanley <joel-U3u1mxZcP9KHXe+LvDLADg@public.gmane.org>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
Date: Fri, 8 Dec 2017 21:47:21 +0100	[thread overview]
Message-ID: <289f0cc8-9e24-fc38-cb83-4223f9923940@redhat.com> (raw)
In-Reply-To: <20171208142714.GB7793@amd>

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
>> If we had a time machine, the right set of flags would be:
>>
>>    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>>    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian

WARNING: multiple messages have this Message-ID (diff)
From: Florian Weimer <fweimer@redhat.com>
To: Pavel Machek <pavel@ucw.cz>, Michael Ellerman <mpe@ellerman.id.au>
Cc: Matthew Wilcox <willy@infradead.org>,
	Kees Cook <keescook@chromium.org>, Cyril Hrubis <chrubis@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	Linux API <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 <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
	Joel Stanley <joel@jms.id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
Date: Fri, 8 Dec 2017 21:47:21 +0100	[thread overview]
Message-ID: <289f0cc8-9e24-fc38-cb83-4223f9923940@redhat.com> (raw)
In-Reply-To: <20171208142714.GB7793@amd>

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
>> If we had a time machine, the right set of flags would be:
>>
>>    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>>    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian

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  parent reply	other threads:[~2017-12-08 20:47 UTC|newest]

Thread overview: 117+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-29 14:42 [PATCH 0/2] mm: introduce MAP_FIXED_SAFE Michal Hocko
2017-11-29 14:42 ` Michal Hocko
2017-11-29 14:42 ` Michal Hocko
2017-11-29 14:42 ` [PATCH 1/2] " Michal Hocko
2017-11-29 14:42   ` Michal Hocko
2017-11-29 14:42   ` Michal Hocko
2017-12-06  5:15   ` Michael Ellerman
2017-12-06  5:15     ` Michael Ellerman
2017-12-06  9:27     ` Michal Hocko
2017-12-06  9:27       ` Michal Hocko
2017-12-06 10:02       ` Michal Hocko
2017-12-06 10:02         ` Michal Hocko
2017-12-07 12:07   ` Pavel Machek
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 14:42   ` Michal Hocko
2017-11-29 14:42   ` Michal Hocko
2017-11-29 17:45   ` Khalid Aziz
2017-11-29 17:45     ` Khalid Aziz
2018-05-29 22:21     ` Mike Kravetz
2018-05-30  8:02       ` Michal Hocko
2018-05-30 15:00         ` Mike Kravetz
2018-05-30 16:25           ` Michal Hocko
2018-05-31  0:51             ` Mike Kravetz
2018-05-31  9:24               ` Michal Hocko
2018-05-31 21:46                 ` Mike Kravetz
2017-11-29 14:45 ` [PATCH] mmap.2: document new MAP_FIXED_SAFE flag Michal Hocko
2017-11-29 14:45   ` Michal Hocko
2017-11-29 14:45   ` Michal Hocko
2017-11-30  3:16   ` John Hubbard
2017-11-30  3:16     ` John Hubbard
2017-11-30  3:16     ` John Hubbard
2017-11-30  8:23     ` Michal Hocko
2017-11-30  8:23       ` Michal Hocko
2017-11-30  8:24   ` [PATCH v2] " Michal Hocko
2017-11-30  8:24     ` Michal Hocko
2017-11-30  8:24     ` Michal Hocko
2017-11-30  8:24     ` Michal Hocko
2017-11-30 18:31     ` John Hubbard
2017-11-30 18:31       ` John Hubbard
2017-11-30 18:31       ` John Hubbard
2017-11-30 18:39       ` Michal Hocko
2017-11-30 18:39         ` Michal Hocko
2017-11-29 15:13 ` [PATCH 0/2] mm: introduce MAP_FIXED_SAFE Rasmus Villemoes
2017-11-29 15:13   ` Rasmus Villemoes
2017-11-29 15:13   ` Rasmus Villemoes
2017-11-29 15:50   ` Michal Hocko
2017-11-29 15:50     ` Michal Hocko
2017-11-29 15:50     ` Michal Hocko
2017-11-29 22:15   ` Kees Cook
2017-11-29 22:15     ` Kees Cook
2017-11-29 22:12 ` Kees Cook
2017-11-29 22:12   ` Kees Cook
2017-11-29 22:25 ` Kees Cook
2017-11-29 22:25   ` Kees Cook
2017-11-30  6:58   ` Michal Hocko
2017-11-30  6:58     ` Michal Hocko
2017-11-30  6:58     ` Michal Hocko
2017-12-01 15:26     ` Cyril Hrubis
2017-12-01 15:26       ` Cyril Hrubis
2017-12-06  4:51       ` Michael Ellerman
2017-12-06  4:51         ` Michael Ellerman
2017-12-06  4:54         ` Matthew Wilcox
2017-12-06  4:54           ` Matthew Wilcox
2017-12-06  7:03           ` Matthew Wilcox
2017-12-06  7:03             ` Matthew Wilcox
2017-12-06  7:33             ` John Hubbard
2017-12-06  7:33               ` John Hubbard
2017-12-06  7:35               ` Florian Weimer
2017-12-06  7:35                 ` Florian Weimer
2017-12-06  7:35                 ` Florian Weimer
2017-12-06  8:06                 ` John Hubbard
2017-12-06  8:06                   ` John Hubbard
2017-12-06  8:06                   ` John Hubbard
2017-12-06  8:06                   ` John Hubbard
2017-12-06  8:54                   ` Florian Weimer
2017-12-06  8:54                     ` Florian Weimer
2017-12-06  8:54                     ` Florian Weimer
2017-12-07  5:46             ` Michael Ellerman
2017-12-07  5:46               ` Michael Ellerman
2017-12-07  5:46               ` Michael Ellerman
2017-12-07 19:14               ` Kees Cook
2017-12-07 19:14                 ` Kees Cook
2017-12-07 19:57                 ` Matthew Wilcox
2017-12-07 19:57                   ` Matthew Wilcox
2017-12-07 19:57                   ` Matthew Wilcox
2017-12-08  8:33                   ` Michal Hocko
2017-12-08  8:33                     ` Michal Hocko
2017-12-08 20:13                     ` Kees Cook
2017-12-08 20:13                       ` Kees Cook
2017-12-08 20:13                       ` Kees Cook
2017-12-08 20:57                       ` Matthew Wilcox
2017-12-08 20:57                         ` Matthew Wilcox
2017-12-08 20:57                         ` Matthew Wilcox
2017-12-08 11:08                   ` Michael Ellerman
2017-12-08 11:08                     ` Michael Ellerman
2017-12-08 14:27                     ` Pavel Machek
2017-12-08 20:31                       ` Cyril Hrubis
2017-12-08 20:31                         ` Cyril Hrubis
2017-12-08 20:31                         ` Cyril Hrubis
2017-12-08 20:47                       ` Florian Weimer [this message]
2017-12-08 20:47                         ` Florian Weimer
2017-12-08 20:47                         ` Florian Weimer
2017-12-08 14:33                     ` David Laight
2017-12-08 14:33                       ` David Laight
2017-12-06  4:50     ` Michael Ellerman
2017-12-06  4:50       ` Michael Ellerman
2017-12-06  7:33       ` Rasmus Villemoes
2017-12-06  7:33         ` Rasmus Villemoes
2017-12-06  7:33         ` Rasmus Villemoes
2017-12-06  9:08         ` Michal Hocko
2017-12-06  9:08           ` Michal Hocko
2017-12-06  9:08           ` Michal Hocko
2017-12-07  0:19           ` Kees Cook
2017-12-07  0:19             ` Kees Cook
2017-12-07  1:08             ` John Hubbard
2017-12-07  1:08               ` John Hubbard

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