Re: [RFC PATCH 2/5] doc: rust: safety standard: add examples

[Daniel Almeida]

Sorry, ended up replying to this using my personal email.

Sending it again.

—————

Hi Benno,

> On 17 Jul 2024, at 19:12, Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> 
> Add examples of good and bad safety documentation.
> 
> There aren't many examples at the moment, as I hope to add more during
> discussions, since coming up with examples on my own is very difficult.
> 
> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> ---
> .../rust/safety-standard/examples.rst         | 70 +++++++++++++++++++
> Documentation/rust/safety-standard/index.rst  | 23 ++++--
> 2 files changed, 86 insertions(+), 7 deletions(-)
> create mode 100644 Documentation/rust/safety-standard/examples.rst
> 
> diff --git a/Documentation/rust/safety-standard/examples.rst b/Documentation/rust/safety-standard/examples.rst
> new file mode 100644
> index 000000000000..d66ef3f8954a
> --- /dev/null
> +++ b/Documentation/rust/safety-standard/examples.rst
> @@ -0,0 +1,70 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +.. highlight:: rust
> +
> +Examples
> +========
> +
> +Unsound APIs
> +------------
> +
> +Simple Unsound Function
> +***********************
> +::
> +
> +    struct Data {
> +        a: usize,
> +    }
> +
> +    fn access_a(data: *mut Data) -> usize {
> +        unsafe { (*data).a }
> +    }
> +
> +One would normally call this function as follows, which does not trigger UB::
> +
> +    fn main() {
> +        let mut d = Data { a: 42 };
> +        println!("{}", access_a(&mut d));
> +    }
> +
> +However, a caller could also call it like this, which triggers UB using only safe code::
> +
> +    fn main() {
> +        println!("{}", access_a(core::ptr::null_mut()));
> +    }
> +
> +And this would result in a dereference of a null pointer.
> +
> +
> +Sound ``unsafe`` Code
> +---------------------
> +
> +The Importance of the API Boundary
> +**********************************
> +
> +Is the following API sound?::
> +
> +    fn foo(r: &mut u32) {
> +        let ptr: *mut u32 = r;
> +        let val;
> +        unsafe {
> +            val = *ptr;
> +            *ptr = 0;
> +        }
> +    }
> +
> +It better be sound, but one could argue that it is unsound, since one could replace the ptr
> +initialization by ``ptr = core::ptr::null_mut()``::
> +
> +    fn foo(r: &mut u32) {
> +        let ptr: *mut u32 = core::ptr::null_mut();
> +        let val;
> +        unsafe {
> +            val = *ptr;
> +            *ptr = 0;
> +        }
> +    }
> +
> +But this modification is not allowed, since it goes beyond the API boundary of ``foo``. This way
> +any ``unsafe`` code that relies on surrounding safe code could be shown to be unsound. Instead one
> +should only consider safe code using the API, in this case, there is no way to make the code
> +incorrect, since a reference is always valid to dereference during its lifetime.

I find this paragraph a bit confusing. Maybe this can be clarified a bit further?


— Daniel