[PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE

[Michal Hocko]

Hi,
I am resending with some minor updates based on Michael's review and
ask for inclusion. There haven't been any fundamental objections for
the RFC [1] nor the previous version [2].  The biggest discussion
revolved around the naming. There were many suggestions flowing
around MAP_REQUIRED, MAP_EXACT, MAP_FIXED_NOCLOBBER, MAP_AT_ADDR,
MAP_FIXED_NOREPLACE etc...

I am afraid we can bikeshed this to death and there will still be
somebody finding yet another better name. Therefore I've decided to
stick with my original MAP_FIXED_SAFE. Why? Well, because it keeps the
MAP_FIXED prefix which should be recognized by developers and _SAFE
suffix should also be clear that all dangerous side effects of the old
MAP_FIXED are gone.

If somebody _really_ hates this then feel free to nack and resubmit
with a different name you can find a consensus for. I am sorry to be
stubborn here but I would rather have this merged than go over few more
iterations changing the name just because it seems like a good idea
now. My experience tells me that chances are that the name will turn out
to be "suboptimal" anyway over time.

Some more background:
This has started as a follow up discussion [3][4] resulting in the
runtime failure caused by hardening patch [5] which removes MAP_FIXED
from the elf loader because MAP_FIXED is inherently dangerous as it
might silently clobber an existing underlying mapping (e.g. stack). The
reason for the failure is that some architectures enforce an alignment
for the given address hint without MAP_FIXED used (e.g. for shared or
file backed mappings).

One way around this would be excluding those archs which do alignment
tricks from the hardening [6]. The patch is really trivial but it has
been objected, rightfully so, that this screams for a more generic
solution. We basically want a non-destructive MAP_FIXED.

The first patch introduced MAP_FIXED_SAFE which enforces the given
address but unlike MAP_FIXED it fails with EEXIST if the given range
conflicts with an existing one. The flag is introduced as a completely
new one rather than a MAP_FIXED extension because of the backward
compatibility. We really want a never-clobber semantic even on older
kernels which do not recognize the flag. Unfortunately mmap sucks wrt.
flags evaluation because we do not EINVAL on unknown flags. On those
kernels we would simply use the traditional hint based semantic so the
caller can still get a different address (which sucks) but at least not
silently corrupt an existing mapping. I do not see a good way around
that. Except we won't export expose the new semantic to the userspace at
all. 

It seems there are users who would like to have something like that.
Jemalloc has been mentioned by Michael Ellerman [7]

Florian Weimer has mentioned the following:
: glibc ld.so currently maps DSOs without hints.  This means that the kernel
: will map right next to each other, and the offsets between them a completely
: predictable.  We would like to change that and supply a random address in a
: window of the address space.  If there is a conflict, we do not want the
: kernel to pick a non-random address. Instead, we would try again with a
: random address.

John Hubbard has mentioned CUDA example
: a) Searches /proc/<pid>/maps for a "suitable" region of available
: VA space.  "Suitable" generally means it has to have a base address
: within a certain limited range (a particular device model might
: have odd limitations, for example), it has to be large enough, and
: alignment has to be large enough (again, various devices may have
: constraints that lead us to do this).
: 
: This is of course subject to races with other threads in the process.
: 
: Let's say it finds a region starting at va.
: 
: b) Next it does: 
:     p = mmap(va, ...) 
: 
: *without* setting MAP_FIXED, of course (so va is just a hint), to
: attempt to safely reserve that region. If p != va, then in most cases,
: this is a failure (almost certainly due to another thread getting a
: mapping from that region before we did), and so this layer now has to
: call munmap(), before returning a "failure: retry" to upper layers.
: 
:     IMPROVEMENT: --> if instead, we could call this:
: 
:             p = mmap(va, ... MAP_FIXED_SAFE ...)
: 
:         , then we could skip the munmap() call upon failure. This
:         is a small thing, but it is useful here. (Thanks to Piotr
:         Jaroszynski and Mark Hairgrove for helping me get that detail
:         exactly right, btw.)
: 
: c) After that, CUDA suballocates from p, via: 
:  
:      q = mmap(sub_region_start, ... MAP_FIXED ...)
: 
: Interestingly enough, "freeing" is also done via MAP_FIXED, and
: setting PROT_NONE to the subregion. Anyway, I just included (c) for
: general interest.

Atomic address range probing in the multithreaded programs in general
sounds like an interesting thing to me.

The second patch simply replaces MAP_FIXED use in elf loader by
MAP_FIXED_SAFE. I believe other places which rely on MAP_FIXED should
follow. Actually real MAP_FIXED usages should be docummented properly
and they should be more of an exception.

Diffstat says
 arch/alpha/include/uapi/asm/mman.h     |  1 +
 arch/metag/kernel/process.c            |  6 +++++-
 arch/mips/include/uapi/asm/mman.h      |  2 ++
 arch/parisc/include/uapi/asm/mman.h    |  2 ++
 arch/sparc/include/uapi/asm/mman.h     |  1 -
 arch/xtensa/include/uapi/asm/mman.h    |  2 ++
 fs/binfmt_elf.c                        | 12 ++++++++----
 include/uapi/asm-generic/mman-common.h |  1 +
 mm/mmap.c                              | 11 +++++++++++
 9 files changed, 32 insertions(+), 6 deletions(-)

[1] http://lkml.kernel.org/r/20171116101900.13621-1-mhocko@xxxxxxxxxx
[2] http://lkml.kernel.org/r/20171129144219.22867-1-mhocko@xxxxxxxxxx
[3] http://lkml.kernel.org/r/20171107162217.382cd754@xxxxxxxxxxxxxxxx
[4] http://lkml.kernel.org/r/1510048229.12079.7.camel@xxxxxxxxxxxxxxxx
[5] http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@xxxxxxxxxx
[6] http://lkml.kernel.org/r/20171113094203.aofz2e7kueitk55y@xxxxxxxxxxxxxx
[7] http://lkml.kernel.org/r/87efp1w7vy.fsf@xxxxxxxxxxxxxxxxxxxxxxxx