[PATCH] D35558: Use sys::Memory::AllocateRWX for JIT code

[Lang Hames via Phabricator via llvm-commits]

lhames added a comment.

> They are not allocated RWX, they are allocated RW with the option for later X. I.e. the kernel enforces W^X, but you can request additional protections for later use.

When you say "additional protections" do you mean you can add the 'X' permission later (to get RWX), or just that you can later toggle from RW- to R-X?

> That makes sense, but it seems like the right thing then is for LLVM to change its APIs to allocate RW pages everywhere and then flip them between RW and RX. We should remove this error-prone AllocateRWX API.

>From a quick grep it seems like AllocateRWX is only called from the llvm-rtdyld tool. I'll see how difficult it is to remove it. If it's easy enough we can kill it, then discuss more future-proof APIs.

> One important part LLVM needs to consider is whether it wants to enshrine the performance penalty of mprotect-after-commit in its APIs or not. The second part is whether platforms should aim to support hot-patchable JIT for multi-threaded environments or not.

For the JIT I'm hoping we can avoid having to choose by providing different implementations of a common higher-level API. Probably the StubsManager/CallbackManager level: If your platform supports hot-patching, use a HotPatchableStubsManager and you get the extra performance. If not, use a SlowButSafeStubsManager and everything still works. I hope to have time to prototype and sanity check that idea in a week or so.


Repository:
  rL LLVM

https://reviews.llvm.org/D35558