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). 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. -- Catalin
