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). > Behaviour shouldn't be variable across architectures either, I would > hope. The behaviour has already been variable for a long time. Even on x86, AFAICT x86_32 differs from x86_64 in this respect. Anyway, the patch should be posted to linux-arch for a cross-arch discussion. -- Catalin
