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:

quoted

On Tue, Apr 18, 2017 at 09:01:52PM +0100, Peter Maydell wrote:

quoted

On 18 April 2017 at 18:01, Catalin Marinas [off-list ref] wrote:

quoted

On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote:

quoted

From: Dong Bo <redacted>

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.

quoted

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.

quoted

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