We have all used Box<T> before in our Rust code. It’s a glorious type, with great ergonomics and flexibitility. We can use it to put our values on the heap, but it can do even more than that!

struct Fields {
@@ -50,7 +50,7 @@ was affected by this (https://youtu.be/EY7
 for std::boxed. Well, the nightly or beta docs, because I only added this section very recently. For years, this behaviour was not really documented, and you had to
 belong to the arcane circles of the select few who were aware of it. So lots of code was written thinking that box was “just an
 RAII pointer” (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all
-pointers are concerned.
Stacked Borrows and Miri⌗
Miri is an interpreter for Rust code with the goal of finding undefined behaviour.
+pointers are concerned.
Stacked Borrows and Miri⌗
TODO: introduce UB by explaining how it allows optimizations like the one above, don’t talk in standardese
Miri is an interpreter for Rust code with the goal of finding undefined behaviour.
 Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed,
 anything can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly
 what you intended to happen. Examples of UB include use-after-free, out of bounds reads or data races.
I cannot recommend Miri highly enough for all unsafe code you’re writing (sadly support for some IO functions
@@ -84,10 +84,9 @@ note: inside `main` at src/main.rs:12:5
 12 |     takes_box_and_ptr_to_it(b, ptr);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This behaviour does indeed not look very defined at all. But what went wrong? There’s a lot of information here.

First of all, it says that we attempted a read access, and that this access failed because the tag does not exist in the -borrow stack. This is something about stacked borrows, the experimental memory model for Rust that is implemented -in Miri. For an excellent introduction, see this part of the great book Learning Rust With Entirely Too Many Linked Lists.

In short: each pointer has a unique tag attacked to it. Bytes in memory have a stack of such tags, and only the pointers -that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various -operations, for example borrowing.

In the code example above, we get a nice little hint where the tag was created. When we created a reference (that was then +borrow stack of the byte that was accessed. This is something about stacked borrows, the experimental memory model for Rust +that is implemented in Miri. For an excellent introduction, see this part of the great book Learning Rust With Entirely Too Many Linked Lists.

In short: each pointer has a unique tag attached to it. Each byte in memory has its own ‘borrow stack’ of these tags, +and only the pointers that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various operations, for example borrowing.

In the code example above, we get a nice little hint where the tag was created. When we created a reference (that was then coerced into a raw pointer) from our box, it got a new tag called <3314>. Then, when we moved the box into the function, something happened: The tag was invalidated and popped off the borrow stack. That’s because box invalidates all tags when it’s moved. The tag was popped off the borrow stack and we tried to read from it anyways - undefined behaviour happened!

And that’s how our code wasn’t a miscompilation, but undefined behaviour. Quite surprising, isn’t it?

noalias, nothanks⌗

Many people, myself included, don’t think that this is a good thing.

First of all, it introduces more UB that could have been defined behaviour instead. This is true for almost all UB, but usually, diff --git a/posts/index.xml b/posts/index.xml index 1b9c984..18ffc14 100644 --- a/posts/index.xml +++ b/posts/index.xml @@ -66,6 +66,7 @@ belong to the arcane circles of the select few who were aware of it. So lots of RAII pointer&rdquo; (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all pointers are concerned.</p> <h1 id="stacked-borrows-and-miri">Stacked Borrows and Miri</h1> +<p>TODO: introduce UB by explaining how it allows optimizations like the one above, don&rsquo;t talk in standardese</p> <p><a href="https://github.com/rust-lang/miri">Miri</a> is an interpreter for Rust code with the goal of finding undefined behaviour. Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed, <em>anything</em> can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly @@ -104,11 +105,10 @@ note: inside `main` at src/main.rs:12:5 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ </code></pre><p>This behaviour does indeed not look very defined at all. But what went wrong? There&rsquo;s a lot of information here.</p> <p>First of all, it says that we attempted a read access, and that this access failed because the tag does not exist in the -borrow stack. This is something about stacked borrows, the experimental memory model for Rust that is implemented -in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> -<p>In short: each pointer has a unique tag attacked to it. Bytes in memory have a stack of such tags, and only the pointers -that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various -operations, for example borrowing.</p> +borrow stack of the byte that was accessed. This is something about stacked borrows, the experimental memory model for Rust +that is implemented in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> +<p>In short: each pointer has a unique tag attached to it. Each byte in memory has its own &lsquo;borrow stack&rsquo; of these tags, +and only the pointers that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various operations, for example borrowing.</p> <p>In the code example above, we get a nice little hint where the tag was created. When we created a reference (that was then coerced into a raw pointer) from our box, it got a new tag called <code>&lt;3314&gt;</code>. Then, when we moved the box into the function, something happened: The tag was invalidated and popped off the borrow stack. That&rsquo;s because box invalidates all tags when it&rsquo;s diff --git a/tags/rust/index.xml b/tags/rust/index.xml index 316b3be..c73ee86 100644 --- a/tags/rust/index.xml +++ b/tags/rust/index.xml @@ -66,6 +66,7 @@ belong to the arcane circles of the select few who were aware of it. So lots of RAII pointer&rdquo; (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all pointers are concerned.</p> <h1 id="stacked-borrows-and-miri">Stacked Borrows and Miri</h1> +<p>TODO: introduce UB by explaining how it allows optimizations like the one above, don&rsquo;t talk in standardese</p> <p><a href="https://github.com/rust-lang/miri">Miri</a> is an interpreter for Rust code with the goal of finding undefined behaviour. Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed, <em>anything</em> can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly @@ -104,11 +105,10 @@ note: inside `main` at src/main.rs:12:5 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ </code></pre><p>This behaviour does indeed not look very defined at all. But what went wrong? There&rsquo;s a lot of information here.</p> <p>First of all, it says that we attempted a read access, and that this access failed because the tag does not exist in the -borrow stack. This is something about stacked borrows, the experimental memory model for Rust that is implemented -in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> -<p>In short: each pointer has a unique tag attacked to it. Bytes in memory have a stack of such tags, and only the pointers -that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various -operations, for example borrowing.</p> +borrow stack of the byte that was accessed. This is something about stacked borrows, the experimental memory model for Rust +that is implemented in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> +<p>In short: each pointer has a unique tag attached to it. Each byte in memory has its own &lsquo;borrow stack&rsquo; of these tags, +and only the pointers that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various operations, for example borrowing.</p> <p>In the code example above, we get a nice little hint where the tag was created. When we created a reference (that was then coerced into a raw pointer) from our box, it got a new tag called <code>&lt;3314&gt;</code>. Then, when we moved the box into the function, something happened: The tag was invalidated and popped off the borrow stack. That&rsquo;s because box invalidates all tags when it&rsquo;s diff --git a/tags/unsafe-code/index.xml b/tags/unsafe-code/index.xml index 10caa16..5a45a12 100644 --- a/tags/unsafe-code/index.xml +++ b/tags/unsafe-code/index.xml @@ -66,6 +66,7 @@ belong to the arcane circles of the select few who were aware of it. So lots of RAII pointer&rdquo; (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all pointers are concerned.</p> <h1 id="stacked-borrows-and-miri">Stacked Borrows and Miri</h1> +<p>TODO: introduce UB by explaining how it allows optimizations like the one above, don&rsquo;t talk in standardese</p> <p><a href="https://github.com/rust-lang/miri">Miri</a> is an interpreter for Rust code with the goal of finding undefined behaviour. Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed, <em>anything</em> can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly @@ -104,11 +105,10 @@ note: inside `main` at src/main.rs:12:5 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ </code></pre><p>This behaviour does indeed not look very defined at all. But what went wrong? There&rsquo;s a lot of information here.</p> <p>First of all, it says that we attempted a read access, and that this access failed because the tag does not exist in the -borrow stack. This is something about stacked borrows, the experimental memory model for Rust that is implemented -in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> -<p>In short: each pointer has a unique tag attacked to it. Bytes in memory have a stack of such tags, and only the pointers -that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various -operations, for example borrowing.</p> +borrow stack of the byte that was accessed. This is something about stacked borrows, the experimental memory model for Rust +that is implemented in Miri. For an excellent introduction, see this part of the great book <a href="https://rust-unofficial.github.io/too-many-lists/fifth-stacked-borrows.html">Learning Rust With Entirely Too Many Linked Lists</a>.</p> +<p>In short: each pointer has a unique tag attached to it. Each byte in memory has its own &lsquo;borrow stack&rsquo; of these tags, +and only the pointers that have their tag in the stack are allowed to access it. Tags can be pushed and popped from the stack through various operations, for example borrowing.</p> <p>In the code example above, we get a nice little hint where the tag was created. When we created a reference (that was then coerced into a raw pointer) from our box, it got a new tag called <code>&lt;3314&gt;</code>. Then, when we moved the box into the function, something happened: The tag was invalidated and popped off the borrow stack. That&rsquo;s because box invalidates all tags when it&rsquo;s