We always remember which allocation a pointer points to, so we can differentiate a pointer "one past the end" of one allocation from a pointer to the beginning of another allocation.
That's how miri can detect that our second example (with `&x[8]`) is UB.
+## The Model Falls Apart
+
In this model, pointers are not integers, but they are at least simple.
However, this simple model starts to fall apart once you consider pointer-integer casts.
-In miri, casting a pointer to an integer does not actually do anything, we now just have an integer variable whose value is a pointer (i.e., an allocation-offset pair).
-Multiplying that integer by 2 leads to an error, because it is entirely unclear what it means to multiply such a pair by 2.
+In miri, casting a pointer to an integer does not actually do anything, we now just have an integer variable (i.e., its *type* says it is an integer) whose *value* is a pointer (i.e., an allocation-offset pair).
+However, multiplying that "integer" by 2 leads to an error, because it is entirely unclear what it means to multiply such an abstract pointer by 2.
+
+This is the most lazy thing to do, and we do it because it is not clear what else to do -- in our abstract machine, there is no single coherent "address space" that all allocations live in, that we could use to map every pointer to a distinct integer.
+Every allocation is just identified by an (unobservable) ID.
+We could now start to enrich this model with extra data like a base address for each allocation, and somehow use that when casting an integer back to a pointer... but that's where it gets really complicated, and anyway discussing such a model is not the point of this post.
+The point it to discuss the *need* for such a model.
+If you are interested, I suggest you read [this paper](http://www.cis.upenn.edu/%7Estevez/papers/KHM+15.pdf) that explores the above idea of adding a base address.
+
+Long story short, pointer-integer casts are messy and hard to define formally when also considering optimizations like we discussed above.
+There is a conflict between the high-level view that is required to enable optimizations, and the low-level view that is required to explain casting a pointer to an integer and back.
+We mostly just ignore the problem in miri and opportunistically do as much as we can, given the simple model we are working with.
A full definition of a language like C++ or Rust of course cannot take this shortcut, it has to explain what really happens here.
-To my knowledge, no satisfying solution exists, but we are [getting](http://www.cis.upenn.edu/%7Estevez/papers/KHM+15.pdf) [closer](http://sf.snu.ac.kr/publications/llvmtwin.pdf).
+To my knowledge, no satisfying solution exists, but academic research is [getting closer](http://sf.snu.ac.kr/publications/llvmtwin.pdf).
+
This is why pointers are not simple, either.
## From Pointers to Bytes
I hope I made a convincing argument that integers are not the only data one has to consider when formally specifying low-level languages such as C++ or (the unsafe parts of) Rust.
However, this means that a simple operation like loading a byte from memory cannot just return a `u8`.
+Imagine we [implement `memcpy`](https://github.com/alexcrichton/rlibc/blob/defb486e765846417a8e73329e8c5196f1dca49a/src/lib.rs#L39) by loading (in turn) every byte of the source into some local variable `v`, and then storing it to the target.
What if that byte is part of a pointer? When a pointer is a pair of allocation and offset, what is its first byte?
-We cannot represent this as a `u8`.
+We have to say what the value of `v` is, so we have to find some way to answer this question.
+(And this is an entirely separate issue from the problem with multiplication that came up in the last section. We just assume some abstract type `Pointer`.)
+
+We cannot represent a byte of a pointer as an element of `0..256`.
Instead, we will remember both the pointer, and which byte of the pointer we got.
+So, a byte is now *either* an element of `0..256` ("raw bits"), *or* the n-th byte of some abstract pointer.
If we were to implement our memory model in Rust, this might look as follows:
{% highlight rust %}
enum ByteV1 {
}
{% endhighlight %}
For example, a `PtrFragment(ptr, 0)` represents the first byte of `ptr`.
-This way, we can "take apart" a pointer into the individual bytes that represent this pointer in memory, and assemble it back together.
+This way, `memcpy` can "take apart" a pointer into the individual bytes that represent this pointer in memory, and copy them separately.
On a 32bit architecture, the full value representing `ptr` consists of the following 4 bytes:
```
[PtrFragment(ptr, 0), PtrFragment(ptr, 1), PtrFragment(ptr, 2), PtrFragment(ptr, 3)]
```
-Such a representation supports performing all byte-level "data moving" operations on pointers, like implementing `memcpy` by copying one byte at a time.
+Such a representation supports performing all byte-level "data moving" operations on pointers, which is sufficient for `memcpy`.
Arithmetic or bit-level operations are not fully supported; as already mentioned above, that requires a more sophisticated pointer representation.
## Uninitialized Memory
projects / web.git / blobdiff