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@xxxxxxxxxxxxxxxx [2] http://lkml.kernel.org/r/1510048229.12079.7.camel@xxxxxxxxxxxxxxxx [3] http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@xxxxxxxxxx [4] http://lkml.kernel.org/r/20171113094203.aofz2e7kueitk55y@xxxxxxxxxxxxxx [5] http://lkml.kernel.org/r/87efp1w7vy.fsf@xxxxxxxxxxxxxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
