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Hi,
this has started as a follow up discussion [1][2] resulting in the
runtime failure caused by hardening patch [3] which removes MAP_FIXED
from the elf loader because MAP_FIXED is inherently dangerous as it
might silently clobber and 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 [4]. 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 ENOMEM if the given range
conflicts with an existing one. The flag is introduced as a completely
new flag 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
[5], though. Atomic address range probing in the multithreaded programs
sounds like an interesting thing to me as well, although I do not have
any specific usecase in mind.

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.

Does anybody see any fundamental reasons why this is a wrong approach?

Diffstat says
arch/alpha/include/uapi/asm/mman.h | 2 ++
arch/metag/kernel/process.c | 6 +++++-
arch/mips/include/uapi/asm/mman.h | 2 ++
arch/parisc/include/uapi/asm/mman.h | 2 ++
arch/powerpc/include/uapi/asm/mman.h | 1 +
arch/sparc/include/uapi/asm/mman.h | 1 +
arch/tile/include/uapi/asm/mman.h | 1 +
arch/xtensa/include/uapi/asm/mman.h | 2 ++
fs/binfmt_elf.c | 12 ++++++++----
include/uapi/asm-generic/mman.h | 1 +
mm/mmap.c | 11 +++++++++++
11 files changed, 36 insertions(+), 5 deletions(-)

[1] http://lkml.kernel.org/r/20171107162217.382cd754@canb.auug.org.au
[2] http://lkml.kernel.org/r/1510048229.12079.7.camel@abdul.in.ibm.com
[3] http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@kernel.org
[4] http://lkml.kernel.org/r/20171113094203.aofz2e7kueitk55y@dhcp22.suse.cz
[5] http://lkml.kernel.org/r/87efp1w7vy.fsf@concordia.ellerman.id.au


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