From 43d11ec5f94d7e441182a3d6f484395dfb827ee4 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Mon, 11 Apr 2022 14:02:37 -0400 Subject: [PATCH] apply some feedback --- personal/_posts/2022-04-11-provenance-exposed.md | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/personal/_posts/2022-04-11-provenance-exposed.md b/personal/_posts/2022-04-11-provenance-exposed.md index 47160fa..ae03624 100644 --- a/personal/_posts/2022-04-11-provenance-exposed.md +++ b/personal/_posts/2022-04-11-provenance-exposed.md @@ -193,7 +193,7 @@ I moved the discussion of this point into the appendix below.) This may sound like bad news for low-level coding tricks like pointer tagging (storing a flag in the lowest bit of a pointer). Do we have to optimize this code less just because of corner cases like the above? As it turns out, no we don't -- there are some situations where it is perfectly fine to do a pointer-integer cast *without* having the "exposure" side-effect. -Specifically, this is the case if we never intent to cast the integer back to a pointer! +Specifically, this is the case if we never intend to cast the integer back to a pointer! That might seem like a niche case, but it turns out that most of the time, we can avoid 'bare' integer-pointer casts, and instead use an operation like [`with_addr`](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.with_addr) that explicitly specifies which provenance to use for the newly created pointer. This is more than enough for low-level pointer shenanigans like pointer tagging, as [Gankra demonstrated](https://gankra.github.io/blah/tower-of-weakenings/#strict-provenance-no-more-getting-lucky). Rust's [Strict Provenance experiment](https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance) aims to determine whether we can use operations like `with_addr` to replace basically all integer-pointer casts. @@ -242,12 +242,12 @@ But now we have the same contradiction as before! Either the original program already has Undefined Behavior, or one of the optimizations is incorrect. Previously, we resolved this conundrum by saying that removing the "dead cast" `(uintptr_t)x` whose result is unused was incorrect, because that cast had the side-effect of "exposing" the permission of `x` to be picked up by future integer-pointer casts. -We could apply the same solution again, but this time, we would have to say that a `union` access or a `memcpy` has an "expose" side-effect and hence cannot be entirely removed even if its result is unused. +We could apply the same solution again, but this time, we would have to say that a `union` access (at integer type) or a `memcpy` (to an integer) can have an "expose" side-effect and hence cannot be entirely removed even if its result is unused. And that sounds quite bad! `(uintptr_t)x` only happens in code that does tricky things with pointers, so urging the compiler to be careful and optimize a bit less seems like a good idea (and at least in Rust, `x.addr()` even provides a way to opt-out of this side-effect). However, `union` and `memcpy` are all over the place. Do we now have to treat *all* of them as having side-effects? -In Rust, due to the lack of a strict aliasing restriction, things get even worse, since literally *any* load of an integer from a raw pointer might be doing a pointer-integer transmutation and thus have the "expose" side-effect! +In Rust, due to the lack of a strict aliasing restriction (or in C with `-fno-strict-aliasing`), things get even worse, since literally *any* load of an integer from a raw pointer might be doing a pointer-integer transmutation and thus have the "expose" side-effect! To me, and speaking from a Rust perspective, that sounds like bad idea. Sure, we want to make it as easy as possible to write low-level code in Rust, and that code sometimes has to do unspeakable things with pointers. @@ -271,7 +271,7 @@ The right type to use for holding arbitrary data is `MaybeUninit`, so e.g. `[May Because of that, I think we should move towards discouraging, deprecating, or even entirely disallowing pointer-integer transmutation in Rust. That means a cast is the only legal way to turn a pointer into an integer, and after the discussion above we got our casts covered. -A [first careful step](https://github.com/rust-lang/rust/pull/95547) has recently been taken on this journey; the `mem::transmute` documentation no cautions against using this function to turn pointers into integers. +A [first careful step](https://github.com/rust-lang/rust/pull/95547) has recently been taken on this journey; the `mem::transmute` documentation now cautions against using this function to turn pointers into integers. ## A new hope for Rust @@ -340,16 +340,17 @@ Here, too, my vision for Rust aligns very well with the direction C is taking. What is left is the question of how to handle pointer-integer transmutation, and this is where the roads are forking. PNVI-ae-udi explicitly says loading from a union field at integer type exposes the provenance of the pointer being loaded, if any. -So, the example with `transmute_union` would be allowed, meaning the optimization of removing the "dead" load from the `union` would *not* be allowed. -Same for `transmute_memcpy`, where the proposal says that `memcpy` should *preserve* provenance, but later when we access the contents of `ret` at type `uintptr_t`, that will again implicitly expose the provenance of the pointer. +So, the example with `transmute_union` would be allowed, meaning the optimization of removing the "dead" load from the `union` would *not* (in general) be allowed. +Same for `transmute_memcpy`, where the proposal says that when we access the contents of `ret` at type `uintptr_t`, that will again implicitly expose the provenance of the pointer. I think there are several reasons why this choice makes sense for C, that do not apply to Rust: - There is a *lot* of legacy code. A *LOT*. - There is no alternative like `MaybeUninit` that could be used to hold data without losing provenance. - Strict aliasing means that not *all* loads at integer type have to worry about provenance; only loads at character type are affected. -On the other hand, I estimate the cost of this choice to be huge. +On the other hand, I am afraid that this choice might come with a significant cost in terms of lost optimizations. As the example above shows, the compiler has to be very careful when removing any operation that can expose a provenance, since there might be integer-pointer casts later that rely on this. +(Of course, until this is actually implemented in GCC or LLVM, it will be hard to know the actual cost.) Because of all that, I think it is reasonable for Rust to make a different choice here. ## Conclusion -- 2.30.2