Imported document about undefined behavior and safe api in LLD

subcommittee/coding-guidelines/initiatives/safe-use-of-unsafe-guidelines/rust-embedded-lld-safe-definition.md

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+# Rust Embedded Low Level Drivers (LLD) and Safe Definition
+
+- [Rust Embedded Low Level Drivers (LLD) and Safe Definition](#markdown-header-rust-embedded-low-level-drivers-lld-and-safe-definition)
+  - [Introduction](#markdown-header-introduction)
+  - [What does mean safe for Rust language.](#markdown-header-what-does-mean-safe-for-rust-language)
+    - [What is “undefined behavior” in Rust?](#markdown-header-what-is-undefined-behavior-in-rust)
+  - [Hardware undefined behavior](#markdown-header-hardware-undefined-behavior)
+  - [Rust safe/unsafe are not well defined when hardware is involved.](#markdown-header-rust-safeunsafe-are-not-well-defined-when-hardware-is-involved)
+    - [Svd2rust](#markdown-header-svd2rust)
+    - [Is safe and unsafe not applicable to external world?](#markdown-header-is-safe-and-unsafe-not-applicable-to-external-world)
+  - [Proposal](#markdown-header-proposal)
+    - [Example](#markdown-header-example)
+  - [Conclusions](#markdown-header-conclusions)
+
+## Introduction
+
+Any real system is not just composed by a CPUs, memories and software.
+In a real system there are other hardware components that shall interact
+with software. They are usually called peripherals.
+
+Peripheral examples:
+
+  - UART
+  - GPU
+  - DMA
+  - Ethernet
+  - Timers
+  - Watchdog
+
+Peripherals are typically configured and operated by reading memory
+mapped registers or in some cases using special CPU instructions.
+
+Peripheral configuration and operation could be complex and cumbersome
+and for this reason an embedded software developer doesn't interact
+directly with registers but through a low-level drivers. Low-level
+drivers typically don`t support portability across different type of
+microcontrollers but they implement a minimal logic to configure and
+operate properly the device accordingly with microcontroller or
+microprocessor specification.
+
+Rust developers expect to [sound](https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library) libraries.
+
+When I tried to understand what need to be done to offer safe low level
+drivers, I found that there is no clear definition of what does mean
+`safe` or `unsafe` when hardware is involved.
+
+In this document, I try to analyze the reason of this unclarity and
+propose a solution.
+
+Before to proceed to this analysis we need to understand what does mean
+`safe` for Rust language:
+
+## What does mean safe for Rust language.
+
+Rust is considered a safe language and the terms `safe` and `unsafe` are
+widely used in its documentation.
+
+What does mean `safe` in the context of Rust? I was not able to find a
+good definition of what is safe in Rust but there is quite clear
+definition of what shall be considered `unsafe` in Rust.
+
+In Rust Ferrocene specification is written the following [^1]:
+
+*Unsafety is the presence of unsafe operations in program text.*
+
+*An unsafe operation is an operation that may result in undefined
+behavior that is not diagnosed as a static error. Unsafe operations are
+referred to as unsafe Rust.*
+
+This definition is aligned with what is written in Rustnomicon[^2]:
+
+*No matter what, Safe Rust can't cause Undefined Behavior.*
+
+Moreover, the keyword `unsafe` is used in multiple context[^3]:
+
+  - *unsafe fn*: calling this function means abiding by a contract the
+    compiler cannot enforce.
+
+  - *unsafe trait*: implementing the trait means abiding by a contract
+    the compiler cannot enforce.
+
+  - *unsafe {}*: the contract necessary to call the operations inside
+    the block has been checked by the programmer and is guaranteed to be
+    respected.
+
+  - *unsafe impl*: the contract necessary to implement the trait has
+    been checked by the programmer and is guaranteed to be respected.
+
+This discussion only covers unsafe blocks and unsafe functions. Unsafe
+functions can be called only in unsafe blocks and the user shall ensure
+to use unsafe functions fulfilling all requirements described in the
+documentation to avoid undefined behavior.
+
+This simplifies code review and bug identification. The developers shall
+inspect first unsafe blocks in case the application shows an undefined
+behavior. However, this doesn`t exclude that a function developer forgot
+to mark a function as “unsafe”.
+
+### What is “undefined behavior” in Rust? 
+
+Ferrocene specification defines it as “unbounded error”[^4] or in other
+words: everything could happen.
+
+Rust Reference manual provide a non-exhaustive list of “undefined
+behaviors”[^5] The following behavior are worth mentioning because it
+involves hardware:
+
+  - *Executing code compiled with platform features that the current
+    platform does not support (see target\_feature), except if the
+    platform explicitly documents this to be safe.*
+
+Before discussing safe/unsafe in Low Level Drivers let's see the meaning
+of “undefined behavior” in the context of microcontroller/microprocessor
+hardware.
+
+## Hardware undefined behavior
+
+In the data sheets or user manuals of many micro-controllers and 
+microprocessors the term *undefined behavior* is used mostly to describe
+how to avoid the conditions that could generate an undefined behavior.
+
+What does mean *undefined behavior* in the context of microcontrollers
+or microprocessor?
+
+  - undefined behavior is not tested in simulation and in production.
+
+      - Any behavior could happen.
+
+  - undefined behavior can be different from device to device.
+
+  - undefined behavior can be different at different instance of time.
+
+The last two characteristic it is very true for mixed signal peripherals
+and for clock system of microcontroller due to sensitivity to physical
+conditions of device and surrounding environments (e.g., variance of
+process manufacturing, operating temperature…). Therefore, undefined
+behavior is the worst-case scenario and there is no software test that
+can detect it.
+
+It shall be highlighted that an undefined behavior could be triggered
+not only by wrong value of registers but also by wrong order of reading
+and writing of registers.
+
+## Rust safe/unsafe are not well defined when hardware is involved.
+
+From previous discussion it could appear obvious that safe low level
+drivers shall protect from any misuses that could trigger undefined
+behavior even from hardware.
+
+Unfortunately, this is not generally accepted and there are different
+understandings. Let`s list them:
+
+### Svd2rust
+
+Svd2Rust[^6] generated peripheral access crates (PAC) from xml register
+description. Peripheral access crates provides API to write and read
+peripheral registers.
+
+Safe functions provided by generated crates protect users from following
+issue:
+
+  - Writing out of range value for bitfields.
+
+  - Concurrent access to same register from different interrupt/thread
+
+  - Read or write registers that cannot be read or written.
+
+_These functions cannot protect from undefined
+behavior triggered by improper order of calling functions_.
+
+e.g. Clock initialization, peripheral initialization.
+
+This approach seems conflicting with other approach:
+
+  - In Embedonomicon is described how to create safe API for DMA low
+    level driver[^7]. Why shall I take all these measures if all my
+    register access API are safe?
+
+  - All C library all functions are by considered by default unsafe. Why
+    a very low level access to external HW function is considered safer
+    than accessing a C library.
+
+On svd2rust and safety there is an ongoing discussion in an issue of
+GitHub project[^8] related to this topic.
+
+### Is safe and unsafe not applicable to external world?
+
+I have talked with some developers, and they don`t consider relevant HW
+undefined behavior because are not part of Rust developed software. In
+other words, they assume that unsafety shall be restricted to only rust
+code memory issues, thread issue but not to HW component.
+
+This position is similar to the one regarding the safety of *fs::open*
+in Unix system[^9]. The *fs::open* is considered safe function even if
+it can open special file */proc/self/mem* that allows unchecked full
+access to process memory. Moreover, it is stated the following in the
+documentation:
+
+*Rust's safety guarantees only cover what the program itself can do, and
+not what entities outside the program can do to it.*
+
+Previous sentence is conflicting with the following:
+
+  - C API of external libraries are by default unsafe.
+
+  - HW undefined behavior is already included in Rust Reference:
+    “*Executing code compiled with platform features that the current
+    platform does not support*”.
+
+  - Measures required for safe DMA API in Embednomicon. DMA is external
+    to Rust program but measures are taken to have safe API.
+
+## Proposal
+
+I think it is quite clear that a clear unambiguous and widely accepted
+definition of undefined behavior in Rust language especially when HW is
+involved is missing.
+
+While waiting for a vastly better definition in an upcoming Rust
+specification, I would propose the following to solve this issue:
+
+  - HW undefined behavior shall be considered when developing low level
+    drivers.
+
+  - In the documentation of safe API shall be stated the scope of
+    safety: which part of the system is free of undefined behavior.
+
+### Example
+
+Let`s assume a system where a microcontroller is connected to a voltage
+regulator that is the power supply of microcontroller and a software
+running in microcontroller changes the voltage and clock based on
+required system mode.
+
+<p align="center">
+<img src="./rust-embedded-lld-safe-definitions-images/safe_lld_fig1.png"  alt="Figure 1 HW System" height="200">
+</p>
+
+Software is structured as in the below picture. All components provide
+safe API.
+
+<p align="center">
+<img src="./rust-embedded-lld-safe-definitions-images/safe_lld_fig2.png"  alt="Figure 2 SW architecture" height="200">
+</p>
+
+For each software component shall be documented which part of system is
+considered regarding undefined behavior.
+
+Microcontroller low level driver should provide safe API only
+considering just the microcontroller but excluding the voltage regulator
+because they should be generic and usable for any system.
+
+<p align="center">
+<img src="./rust-embedded-lld-safe-definitions-images/safe_lld_fig3.png"  alt="Figure 3 Scope of microcontroller LLD" height="200">
+</p>
+
+Voltage regulator driver requires a SPI low level driver in order and
+their combination shall protect from undefined behavior that could arise
+from microcontroller + voltage regulator. Voltage regulator driver is
+supposed to be generic and independent of the device connected to the
+output of voltage regulator.
+<p align="center">
+<img src="./rust-embedded-lld-safe-definitions-images/safe_lld_fig4.png"  alt="Figure 4 Scope of voltage regulator driver and microcontroller LLD" height="200">
+</p>
+Finally, system mode manager API shall guarantee the freedom from
+undefined behavior of full system as described in Figure 1 HW system.
+
+## Conclusions
+
+In this document I tried to show why the measures required to have safe
+LLD are not clear. The reason derives from the definition of “undefined
+behavior”. This is a call for Rust community and Rust specification
+writers to have an unambiguous and well-defined description of
+“undefined behavior” that includes HW drivers.
+
+I described a proposal inspired by ISO26262 out of context element that
+is applicable to LLD and still preserve one of the main advantages of
+Rust:
+
+*No matter what, Safe Rust can't cause Undefined Behavior.*
+
+[^1]: https://spec.ferrocene.dev/unsafety.html
+[^2]: https://doc.rust-lang.org/nomicon/safe-unsafe-meaning.html
+[^3]: https://doc.rust-lang.org/std/keyword.unsafe.html#the-different-meanings-of-unsafe
+[^4]: https://spec.ferrocene.dev/glossary.html#term_undefined_behavior
+[^5]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+[^6]: https://docs.rs/svd2rust/latest/svd2rust/
+[^7]: https://docs.rust-embedded.org/embedonomicon/dma.html
+[^8]: https://github.com/rust-embedded/svd2rust/issues/714
+[^9]: https://doc.rust-lang.org/std/os/unix/io/index.html#procselfmem-and-similar-os-features