In case you don't know what I mean by "pointer provenance", you can either read that previous blog post or the [Strict Provenance documentation](https://doc.rust-lang.org/nightly/core/ptr/index.html#provenance).
The gist of it is that a pointer consists not only of the address that it points to in memory, but also of its *provenance*: an extra piece of "shadow state" that is carried along with each pointer and that tracks which memory the pointer has permission to access and when.
-This is required to make sense of restrictions like "pointer arithmetic can never be used to construct a pointer that is valid for a different allocation than the one it started out in" (even with operations like Rust's [`wrapping_offset`](https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset) that *do* allow out-of-bounds pointer arithmetic), or "use-after-free is Undefined Behavior, even if you checked that there is a new allocation at the same address as the old one".
+This is required to make sense of restrictions like "use-after-free is Undefined Behavior, even if you checked that there is a new allocation at the same address as the old one".
Architectures like CHERI make this "shadow state" explicit (pointers are bigger than usual so that they can explicitly track which part of memory they are allowed to access),
but even when compiling for AMD64 CPUs, compilers act "as if" pointers had such extra state -- it is part of the specification, part of the Abstract Machine, even if it is not part of the target CPU.
int *y2 = y-1;
uintptr_t y2addr = (uintptr_t)y2;
- int *ptr = (int*)y2addr;
+ int *ptr = (int*)y2addr; // <-- using y2addr
*ptr = 1;
return *x;
int *ptr = (int*)y2addr;
*ptr = 1;
- return 0;
+ return 0; // <-- hard-coded return value
}
int main() {
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 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.
+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.[^with_addr]
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.
+[^with_addr]: `with_addr` has been unstably added to the Rust standard library very recently. Such an operation has been floating around in various discussions in the Rust community for quite a while, and it has even made it into [an academic paper](https://iris-project.org/pdfs/2022-popl-vip.pdf) under the name of `copy_alloc_id`. Who knows, maybe one day it will find its way into the C standard as well. :)
+
As part of Strict Provenance, Rust now has a second way of casting pointers to integers, `ptr.addr()`, which does *not* "expose" the permission of the underlying pointer, and hence can be treated like a pure operation![^experiment]
We can do shenanigans on the integer representation of a pointer *and* have all these juicy optimizations, as long as we don't expect bare integer-pointer casts to work.
As a bonus, this also makes Rust work nicely on CHERI *without* a 128bit wide `usize`, and it helps Miri, too.
Well, I would argue that the alternative is to treat the original program (after translation to Rust) as having Undefined Behavior.
There are, to my knowledge, generally two reasons why people might want to transmute a pointer to an integer:
- Chaining many `as` casts is annoying, so calling `mem::transmute` might be shorter.
-- The code doesn't actually care about the *integer* per se, it just needs *some way* to hold arbitrary data in a container of a given time.
+- The code doesn't actually care about the *integer* per se, it just needs *some way* to hold arbitrary data in a container of a given type.
The first kind of code should just use `as` casts, and we should do what we can (via lints, for example) to identify such code and get it to use casts instead.[^compat]
Maybe we can adjust the cast rules to remove the need for chaining, or add some [helper methods](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.expose_addr) that can be used instead.
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 now cautions against using this function to turn pointers into integers.
+**Update (2022-09-14):** After a lot more discussion, the current model pursued by the Unsafe Code Guidelines WG is to say that pointer-to-integer transmutation is permitted, but just strips provenance without exposing it.
+That means the program with the casts replaced by transmutation is UB, because the `ptr` it ends up dereferencing has invalid provenance.
+However, the transmutation itself is not UB.
+Basically, pointer-to-integer transmutation is equivalent to [the `addr` method](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.addr), with all its caveats -- in particular, transmuting a pointer to an integer and back is like calling `addr` and then calling [`ptr::invalid`](https://doc.rust-lang.org/nightly/std/ptr/fn.invalid.html).
+That is a *lossy* round-trip: it loses provenance information, making the resulting pointer invalid to dereference.
+It is lossy even if we use a regular integer-to-pointer cast (or `from_exposed_addr`) for the conversion back to a pointer, since the original provenance might never have been exposed.
+Compared to declaring the transmutation itself UB, this model has some nice properties that help compiler optimizations (such as removing unnecessary store-load round-trips). **/Update**
+
## A new hope for Rust
All in all, while the situation may be very complicated, I am actually more hopeful than ever that we can have both -- a precise memory model for Rust *and* all the optimizations we can hope for!
The three core pillars of this approach are:
- making pointer-integer casts "expose" the pointer's provenance,
- offering `ptr.addr()` to learn a pointer's address *without* exposing its provenance,
-- and disallowing pointer-integer transmutation.
+- and making pointer-integer transmutation round-trips lossy (such that the resulting pointer cannot be dereferenced).
Together, they imply that we can optimize "nice" code (that follows Strict Provenance, and does not "expose" or use integer-pointer casts) perfectly, without any risk of breaking code that does use pointer-integer round-trips.
In the easiest possible approach, the compiler can simply treat pointer-integer and integer-pointer casts as calls to some opaque external function.
Under this brave new world, I can entirely ignore pointer-integer round-trips when designing memory models for Rust.
Once that design is done, support for pointer-integer round-trips can be added as follows:
- When a pointer is cast to an integer, its provenance (whatever information it is that the model attaches to pointers -- in Stacked Borrows, this is called the pointer's *tag*) is marked as "exposed".
-- When an integer is cast to a pointer, we *guess* the provenance that the new pointer should have from among all the provenances that have been previously marked as "exposed". (And I mean *all* of them, not just the ones that have been exposed "at the same address" or anything like that. People will inevitably do imperfect round-trips where the integer is being offset before being cast back to a pointer, and we should support that. As far as I know, this doesn't really cost us anything in terms of optimizations.)
+- When an integer is cast to a pointer, we *guess* the provenance that the new pointer should have from among all the provenances that have been previously marked as "exposed".
+ (And I mean *all* of them, not just the ones that have been exposed "at the same address" or anything like that. People will inevitably do imperfect round-trips where the integer is being offset before being cast back to a pointer, and we should support that. As far as I know, this doesn't really cost us anything in terms of optimizations.)
This "guess" does not need to be described by an algorithm.
Through the magic that is formally known as [angelic non-determinism](https://en.wikipedia.org/wiki/Angelic_non-determinism), we can just wave our hands and say "the guess will be maximally in the programmer's favor": if *any* possible choice of (previously exposed) provenance makes the program work, then that is the provenance the new pointer will get.
It doesn't have to be perfect to be useful.
What I particularly like about this approach is that it makes pointer-integer round-trips a purely local concern.
-With an approach like Stacked Borrows "untagged pointers", *every* memory operation has to define how it handles such pointers -- complexity increases globally, and even when reasoning about Strict Provenance code we have to keep in mind that some pointers in other parts of the program might be "untagged".
+With an approach like Stacked Borrows "untagged pointers", *every* memory operation has to define how it handles such pointers.
+Complexity increases globally, and even when reasoning about Strict Provenance code we have to keep in mind that some pointers in other parts of the program might be "untagged".
In contrast, this "guessing maximally in your favor"-based approach is entirely local; code that does not syntactically contain exposing pointer-integer or integer-pointer casts can literally forget that such casts exist at all.
This is true both for programmers thinking about their `unsafe` code, and for compiler authors thinking about optimizations.
Compositionality at its finest!
I have talked a lot about my vision for "solving" pointer provenance in Rust.
What about other languages?
-As you might have heard, C is moving towards making [PNVI-ae-udi](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2577.pdf) an official recommendation for how to interpret the C memory model.
+As you might have heard, C is moving towards making [PNVI-ae-udi](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2676.pdf) an official recommendation for how to interpret the C memory model.
With C having so much more legacy code to care about and many more stakeholders than Rust does, this is an impressive achievement!
How does it compare to all I said above?
The "udi" part of the name means "user disambiguation", and is basically the mechanism by which an integer-pointer cast in C "guesses" the provenance it has to pick up.
The details of this are complicated, but the end-to-end effect is basically exactly the same as in the "best possible guess" model I have described above!
Here, too, my vision for Rust aligns very well with the direction C is taking.
-(The set of valid guesses in C is just a lot more restricted since they do not have `wrapping_offset`. That means they can actually feasibly give an algorithm for how to do the guessing.)
+(The set of valid guesses in C is just a lot more restricted since they do not have `wrapping_offset`, and the model does not cover `restrict`.
+That means they can actually feasibly give an algorithm for how to do the guessing.
+They don't have to invoke scary terms like "angelic non-determinism", but the end result is the same -- and to me, the fact that it is equivalent to angelic non-determinism is what justifies this as a reasonable semantics.
+Presenting this as a concrete algorithm to pick a suitable provenance is then just a stylistic choice.)
+Kudos go to Michael Sammler for opening my eyes to this interpretation of "user disambiguation", and arguing that angelic non-determinism might not be such a crazy idea after all.
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.
This was a long post, but I hope you found it worth reading. :)
To summarize, my concrete calls for action in Rust are:
-- Code that uses pointer-integer transmutation should migrate to regular casts or `MaybeUninit` transmutation ASAP.
- I think we should declare pointer-integer transmutation Undefined Behavior and not accept such code as well-defined.
+- Code that uses pointer-integer transmutation round-trips should migrate to regular casts or `MaybeUninit` transmutation ASAP.
+ I think we should declare pointer-integer transmutation as "losing" provenance, so code that assumes a lossless transmutation round-trip has Undefined Behavior.
- Code that uses pointer-integer or integer-pointer *casts* might consider migrating to the Strict Provenance APIs.
You can do this even on stable with [this polyfill crate](https://crates.io/crates/sptr).
However, such code *is and remains* well-defined. It just might not be optimized as well as one could hope, it might not compile on CHERI, and Miri will probably miss some bugs.
This would entirely lose the benefit I discussed above of making pointer-integer round-trips a *local* concern -- if these round-trips produce new, never-before-seen kinds of provenance, then the entire rest of the memory model has to define how it deals with those provenances.
We already have no choice but treat pointer-integer casts as an operation with side-effects; let's just do the same with integer-pointer casts and remain sure that no matter what the aliasing rules are, they will work fine even in the presence of pointer-integer round-trips.
+That said, under this model integer-pointer casts still have no side-effect, in the sense that just removing them (if their result is unused) is fine.
+Hence, it *could* make sense to implicitly perform integer-pointer casts in some situations, like when an integer value (without provenance) is used in a pointer operation (due to an integer-to-pointer transmutation).
+This breaks some optimizations like load fusion (turning two loads into one assumes the same provenance was picked both times), but most optimizations (in particular dead code elimination) are unaffected.
+
#### What about LLVM?
I discussed above how my vision for Rust relates to the direction C is moving towards.
What does that mean for the design space of LLVM?
-Which changes need to be made to fix (potential) miscompilations in LLVM and to make it compatible with these ideas for C and/or Rust?
+Which changes would have to be made to fix (potential) miscompilations in LLVM and to make it compatible with these ideas for C and/or Rust?
Here's the list of open problems I am aware of:
-- LLVM needs to stop [removing `inttoptr(ptrtoint(_))`](https://github.com/llvm/llvm-project/issues/33896) and stop doing [replacement of `==`-equal pointers](https://github.com/llvm/llvm-project/issues/34577).
+- LLVM would have to to stop [removing `inttoptr(ptrtoint(_))`](https://github.com/llvm/llvm-project/issues/33896) and stop doing [replacement of `==`-equal pointers](https://github.com/llvm/llvm-project/issues/34577).
- As the first example shows, LLVM also needs to treat `ptrtoint` as a side-effecting operation that has to be kept around even when its result is unused. (Of course, as with everything I say here, there can be special cases where the old optimizations are still correct, but they need extra justification.)
- I think LLVM should also treat `inttoptr` as a side-effecting (and, in particular, non-deterministic) operation, as per the last example. However, this could possibly be avoided with a `noalias` model that specifically accounts for new kinds of provenance being synthesized by casts. (I am being vague here since I don't know what that provenance needs to look like.)
So far, this all applies to LLVM as a Rust and C backend equally, so I don't think there are any good alternatives.
On the plus side, adapting this strategy for `inttoptr` and `ptrtoint` means that the recent LLVM ["Full Restrict Support"](https://lists.llvm.org/pipermail/llvm-dev/2019-March/131127.html) can also handle pointer-integer round-trips "for free"!
+Adding `with_addr`/`copy_alloc_id` to LLVM is not strictly necessary, since it can be implemented with `getelementptr` (without `inbounds`).
+However, optimizations don't seem to always deal well with that pattern, so it might still be a good idea to add this as a primitive operation to LLVM.
+
Where things become more subtle is around pointer-integer transmutation.
If LLVM wants to keep doing replacement of `==`-equal integers (which I strongly assume to be the case), *something* needs to give: my first example, with casts replaced by transmutation, shows a miscompilation.
If we focus on doing an `i64` load of a pointer value (e.g. as in the LLVM IR produced by `transmute_union`, or pointer-based transmutation in Rust), what are the options?
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