On 2017/4/24 23:58, Catalin Marinas wrote: > On Mon, Apr 24, 2017 at 04:40:23PM +0100, Will Deacon wrote: >> On Wed, Apr 19, 2017 at 11:33:14AM +0100, Catalin Marinas wrote: >>> On Tue, Apr 18, 2017 at 09:01:52PM +0100, Peter Maydell wrote: >>>> On 18 April 2017 at 18:01, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: >>>>> On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote: >>>>>> From: Dong Bo <dongbo4@xxxxxxxxxx> >>>>>> >>>>>> In load_elf_binary(), once the READ_IMPLIES_EXEC flag is set, >>>>>> the flag is propagated to its child processes, even the elf >>>>>> files are marked as not requiring executable stack. It may >>>>>> cause superfluous operations on some arch, e.g. >>>>>> __sync_icache_dcache on aarch64 due to a PROT_READ mmap is >>>>>> also marked as PROT_EXEC. >>>> >>>>> That's affecting most architectures with a risk of ABI breakage. We >>>>> could do it on arm64 only, though I'm not yet clear on the ABI >>>>> implications (at a first look, there shouldn't be any). >>>> >>>> Is there a reason why it isn't just straightforwardly a bug >>>> (which we could fix) to make READ_IMPLIES_EXEC propagate to >>>> child processes? >>> >>> While I agree that it looks like a bug, if there are user programs >>> relying on such bug we call it "ABI". On arm64, I don't think there is >>> anything relying on inheriting READ_IMPLIES_EXEC but I wouldn't change >>> the compat task handling without the corresponding change in arch/arm. >>> >>>> AFAICT this should be per-process: just because >>>> init happens not to have been (re)compiled to permit non-executable >>>> stacks doesn't mean every process on the system needs to have >>>> an executable stack. >>> >>> I think this also affects the heap if brk(2) is used (via >>> VM_DATA_DEFAULT_FLAGS though I guess malloc mostly uses mmap these >>> days). >> >> I think it also affects mprotect, which is more worrying imo, particularly >> for things like JIT code that is ported from 32-bit (although a quick look >> at v8, ionmonkey and art suggests they all pass PROT_EXEC when needed). > > As Peter said, the default behaviour is READ_IMPLIES_EXEC off, so JIT > code must already pass PROT_EXEC if it wants executable permission. The > question is whether any user code relies on READ_IMPLIES_EXEC being > passed down to child processes. I don't think so but I would be > reluctant to make an such cross-arch change (happy to do it for arm64 > though). > OK, I have re-built a patch for arm64 as you suggested. Thanks. > Since linux-arch was cc'ed in the middle of this thread, I doubt people > would reply. I suggest that the original patch is re-posted to > linux-arch directly. > Re-posted. Bo Dong .
