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//! Minimal startup / runtime for Cortex-M microcontrollers //! //! # Features //! //! This crate provides //! //! - Before main initialization of the `.bss` and `.data` sections. //! //! - Before main initialization of the FPU (for targets that have a FPU). //! //! - A `panic_fmt` implementation that just calls abort that you can opt into //! through the "abort-on-panic" Cargo feature. If you don't use this feature //! you'll have to provide the `panic_fmt` lang item yourself. Documentation //! [here](https://doc.rust-lang.org/unstable-book/language-features/lang-items.html) //! //! - A minimal `start` lang item to support the standard `fn main()` //! interface. (The processor goes to sleep (`loop { asm!("wfi") }`) after //! returning from `main`) //! //! - A linker script that encodes the memory layout of a generic Cortex-M //! microcontroller. This linker script is missing some information that must //! be supplied through a `memory.x` file (see example below). //! //! - A default exception handler tailored for debugging that lets you inspect //! what was the state of the processor at the time of the exception. By //! default, all exceptions are serviced by this handler but each exception //! can be individually overridden using the //! [`exception!`](macro.exception.html) macro. The default exception handler //! itself can also be overridden using the //! [`default_handler!`](macro.default_handler.html) macro. //! //! - A `_sheap` symbol at whose address you can locate a heap. //! //! - Zero cost stack overflow protection when using the `cortex-m-rt-ld` linker. //! //! # Example //! //! Creating a new bare metal project. (I recommend you use the //! [`cortex-m-quickstart`](https://docs.rs/cortex-m-quickstart/0.2.0/cortex_m_quickstart/) template //! as it takes of all the boilerplate shown here) //! //! ``` text //! $ cargo new --bin app && cd $_ //! //! $ # add this crate as a dependency //! $ $EDITOR Cargo.toml && tail $_ //! [dependencies.cortex-m-rt] //! features = ["abort-on-panic"] //! version = "0.3.0" //! //! $ # tell Xargo which standard crates to build //! $ $EDITOR Xargo.toml && cat $_ //! [dependencies.core] //! stage = 0 //! //! [dependencies.compiler_builtins] //! features = ["mem"] //! stage = 1 //! //! $ # memory layout of the device //! $ $EDITOR memory.x && cat $_ //! MEMORY //! { //! /* NOTE K = KiBi = 1024 bytes */ //! FLASH : ORIGIN = 0x08000000, LENGTH = 128K //! RAM : ORIGIN = 0x20000000, LENGTH = 8K //! } //! //! $ $EDITOR src/main.rs && cat $_ //! ``` //! //! ``` ignore,no_run //! #![feature(used)] //! #![no_std] //! //! extern crate cortex_m_rt; //! //! fn main() { //! // do something here //! } //! //! // As we are not using interrupts, we just register a dummy catch all //! // handler //! #[link_section = ".vector_table.interrupts"] //! #[used] //! static INTERRUPTS: [extern "C" fn(); 240] = [default_handler; 240]; //! //! extern "C" fn default_handler() { //! loop {} //! } //! ``` //! //! ``` text //! $ cargo install xargo //! //! $ xargo rustc --target thumbv7m-none-eabi -- \ //! -C link-arg=-Tlink.x -C linker=arm-none-eabi-ld -Z linker-flavor=ld //! //! $ arm-none-eabi-objdump -Cd $(find target -name app) | head //! //! Disassembly of section .text: //! //! 08000400 <cortex_m_rt::reset_handler>: //! 8000400: b580 push {r7, lr} //! 8000402: 466f mov r7, sp //! 8000404: b084 sub sp, #8 //! //! //! $ arm-none-eabi-size -Ax $(find target -name app) | head //! target/thumbv7m-none-eabi/debug/app : //! section size addr //! .vector_table 0x400 0x8000000 //! .text 0x24a 0x8000400 //! .rodata 0x0 0x800064c //! .stack 0x2000 0x20000000 //! .bss 0x0 0x20000000 //! .data 0x0 0x20000000 //! ``` //! //! ## Zero cost stack overflow protection //! //! Consider the following variation of the previous program: //! //! ``` ignore //! extern crate cortex_m_rt; //! //! const N: usize = 256; //! static mut XS: [u32; N] = [0; N]; //! //! fn main() { //! #[inline(never)] //! fn fib(n: u32) -> u32 { //! unsafe { assert!(XS.iter().all(|x| *x == 0)) } //! //! if n < 2 { //! 1 //! } else { //! fib(n - 1) + fib(n - 2) //! } //! } //! //! let x = fib(400); //! unsafe { *XS.iter_mut().first().unwrap() = x } //! } //! ``` //! //! This program allocates a 1KB array in `.bss`, recursively computes the 400th fibonacci number //! and stores the result in the head of the array. This program will hit a stack overflow at //! runtime because there's not enough memory to recursively call the `fib` function so many times. //! //! If you inspect the program using GDB you'll see that the assertion failed after `fib` was nested //! around 300 times. //! //! ``` console //! > continue //! Program received signal SIGTRAP, Trace/breakpoint trap. //! //! > backtrace //! #0 0x08000516 in cortex_m_rt::default_handler () //! #1 <signal handler called> //! #2 0x0800050a in rust_begin_unwind () //! #3 0x08000586 in core::panicking::panic_fmt () //! #4 0x0800055c in core::panicking::panic () //! #5 0x080004f6 in app::main::fib () //! #6 0x080004a0 in app::main::fib () //! (..) //! #301 0x080004a0 in app::main::fib () //! #302 0x080004a0 in app::main::fib () //! #303 0x08000472 in app::main () //! #304 0x08000512 in cortex_m_rt::lang_items::start () //! #305 0x08000460 in cortex_m_rt::reset_handler () //! ``` //! //! What this means is that the stack grew so much that it crashed into the `.bss` section and //! overwrote the memory in there. Continuing the GDB session you can confirm that the `XS` variable //! has been modified: //! //! ``` console //! > x/4 0x20000000 # start of .bss //! 0x20000000 <app::XS>: 0x00000000 0x00000000 0x00000000 0x00000000 //! //! > x/4 0x200003f0 # end of .bss //! 0x200003f0 <app::XS+1008>: 0x20000400 0x080004f5 0x00000000 0x00000001 //! ``` //! //! The problem is that the stack is growing towards the `.bss` section and both sections overlap as //! shown below: //! //! ``` console //! $ arm-none-eabi-size -Ax $(find target -name app) //! section size addr //! .vector_table 0x400 0x8000000 //! .text 0x186 0x8000400 //! .rodata 0x50 0x8000590 //! .stack 0x2000 0x20000000 //! .bss 0x400 0x20000000 //! .data 0x0 0x20000400 //! ``` //! //! Graphically the RAM sections look like this: //! //! <p align="center"> //! <img alt="Stack overflow" src="https://i.imgur.com/haJKXr4.png"> //! </p> //! //! To prevent memory corruption due to stack overflows in this scenario it suffices to switch the //! sections so that the `.bss` section is near the end of the RAM region and the `.stack` comes //! *before* `.bss`, at a lower address. //! //! To swap the sections you can use the [`cortex-m-rt-ld`] linker to link the program. //! //! ``` console //! $ cargo install cortex-m-rt-ld //! //! $ xargo rustc --target thumbv7m-none-eabi -- \ //! -C link-arg=-Tlink.x -C linker=cortex-m-rt-ld -Z linker-flavor=ld //! ``` //! //! Now you get non overlapping linker sections: //! //! ``` console //! section size addr //! .vector_table 0x400 0x8000000 //! .text 0x186 0x8000400 //! .rodata 0x50 0x8000590 //! .stack 0x1c00 0x20000000 //! .bss 0x400 0x20001c00 //! .data 0x0 0x20002000 //! ``` //! //! Note that the `.stack` section is smaller now. Graphically, the memory layout now looks like //! this: //! //! <p align="center"> //! <img alt="Swapped sections" src="https://i.imgur.com/waOKpHw.png"> //! </p> //! //! On stack overflows `.stack` will hit the lower boundary of the RAM region raising a hard fault //! exception, instead of silently corrupting the `.bss` section. //! //! You can confirm this by inspecting the program in GDB. //! //! ``` console //! > continue //! Program received signal SIGTRAP, Trace/breakpoint trap. //! //! > p $sp //! $1 = (void *) 0x1ffffff0 //! ``` //! //! The failure mode this time was the `.stack` crashing into the RAM boundary. The variable `XS` is //! unaffected this time: //! //! ``` console //! > x/4x app::XS //! 0x20001c00 <app::XS>: 0x00000000 0x00000000 0x00000000 0x00000000 //! //! > x/4x app::XS+252 //! 0x20001ff0 <app::XS+1008>: 0x00000000 0x00000000 0x00000000 0x00000000 //! ``` //! //! ## `.heap` //! //! If your program makes use of a `.heap` section a similar problem can occur: //! //! <p align="center"> //! <img alt="Memory layout when `.heap` exists" src="https://i.imgur.com/kFHRGiF.png"> //! </p> //! //! The `.stack` can crash into the `.heap`, or vice versa, and you'll also get memory corruption. //! //! `cortex-m-rt-ld` can also be used in this case but the size of the `.heap` section must be //! specified via the `_heap_size` symbol in `memory.x`, or in any other linker script. //! //! ``` console //! $ $EDITOR memory.x && tail -n1 $_ //! _heap_size = 0x400; //! ``` //! //! ``` console //! $ xargo rustc --target thumbv7m-none-eabi -- \ //! -C link-arg=-Tlink.x -C linker=cortex-m-rt-ld -Z linker-flavor=ld //! //! $ arm-none-eabi-size -Ax $(find target -name app) | head //! section size addr //! .vector_table 0x400 0x8000000 //! .text 0x1a8 0x8000400 //! .rodata 0x50 0x80005b0 //! .stack 0x1800 0x20000000 //! .bss 0x400 0x20001800 //! .data 0x0 0x20001c00 //! .heap 0x400 0x20001c00 //! ``` //! //! Graphically the memory layout looks like this: //! //! <p align="center"> //! <img alt="Swapped sections when `.heap` exists" src="https://i.imgur.com/6Y5DaBp.png"> //! </p> //! //! Now both stack overflows and dynamic memory over-allocations (OOM) will generate hard fault //! exceptions, instead of running into each other. //! //! # Symbol interfaces //! //! This crate makes heavy use of symbols, linker sections and linker scripts to //! provide most of its functionality. Below are described the main symbol //! interfaces. //! //! ## `DEFAULT_HANDLER` //! //! This weak symbol can be overridden to override the default exception handler //! that this crate provides. It's recommended that you use the //! `default_handler!` to do the override, but below is shown how to manually //! override the symbol: //! //! ``` ignore,no_run //! #[no_mangle] //! pub extern "C" fn DEFAULT_HANDLER() { //! // do something here //! } //! ``` //! //! ## `.vector_table.interrupts` //! //! This linker section is used to register interrupt handlers in the vector //! table. The recommended way to use this section is to populate it, once, with //! an array of *weak* functions that just call the `DEFAULT_HANDLER` symbol. //! Then the user can override them by name. //! //! ### Example //! //! Populating the vector table //! //! ``` ignore,no_run //! // Number of interrupts the device has //! const N: usize = 60; //! //! // Default interrupt handler that just calls the `DEFAULT_HANDLER` //! #[linkage = "weak"] //! #[naked] //! #[no_mangle] //! extern "C" fn WWDG() { //! unsafe { //! asm!("b DEFAULT_HANDLER" :::: "volatile"); //! core::intrinsics::unreachable(); //! } //! } //! //! // You need one function per interrupt handler //! #[linkage = "weak"] //! #[naked] //! #[no_mangle] //! extern "C" fn WWDG() { //! unsafe { //! asm!("b DEFAULT_HANDLER" :::: "volatile"); //! core::intrinsics::unreachable(); //! } //! } //! //! // .. //! //! // Use `None` for reserved spots in the vector table //! #[link_section = ".vector_table.interrupts"] //! #[no_mangle] //! #[used] //! static INTERRUPTS: [Option<extern "C" fn()>; N] = [ //! Some(WWDG), //! Some(PVD), //! // .. //! ]; //! ``` //! //! Overriding an interrupt (this can be in a different crate) //! //! ``` ignore,no_run //! // the name must match the name of one of the weak functions used to //! // populate the vector table. //! #[no_mangle] //! pub extern "C" fn WWDG() { //! // do something here //! } //! ``` //! //! ## `memory.x` //! //! This file supplies the information about the device to the linker. //! //! ### `MEMORY` //! //! The main information that this file must provide is the memory layout of //! the device in the form of the `MEMORY` command. The command is documented //! [here](https://sourceware.org/binutils/docs/ld/MEMORY.html), but at a minimum you'll want to //! create two memory regions: one for Flash memory and another for RAM. //! //! The program instructions (the `.text` section) will be stored in the memory //! region named FLASH, and the program `static` variables (the sections `.bss` //! and `.data`) will be allocated in the memory region named RAM. //! //! ### `_stack_start` //! //! This symbol provides the address at which the call stack will be allocated. //! The call stack grows downwards so this address is usually set to the highest //! valid RAM address plus one (this *is* an invalid address but the processor //! will decrement the stack pointer *before* using its value as an address). //! //! If omitted this symbol value will default to `ORIGIN(RAM) + LENGTH(RAM)`. //! //! #### Example //! //! Allocating the call stack on a different RAM region. //! //! ``` ignore //! MEMORY //! { //! /* call stack will go here */ //! CCRAM : ORIGIN = 0x10000000, LENGTH = 8K //! FLASH : ORIGIN = 0x08000000, LENGTH = 256K //! /* static variables will go here */ //! RAM : ORIGIN = 0x20000000, LENGTH = 40K //! } //! //! _stack_start = ORIGIN(CCRAM) + LENGTH(CCRAM); //! ``` //! //! ### `_heap_size` //! //! The size of the `.heap` section. Only meaningful when using `cortex-m-rt-ld`. //! //! ### `_stext` //! //! This symbol indicates where the `.text` section will be located. If not //! specified in the `memory.x` file it will default to right after the vector //! table -- the vector table is always located at the start of the FLASH //! region. //! //! The main use of this symbol is leaving some space between the vector table //! and the `.text` section unused. This is required on some microcontrollers //! that store some configuration information right after the vector table. //! //! #### Example //! //! Locate the `.text` section 1024 bytes after the start of the FLASH region. //! //! ``` ignore //! _stext = ORIGIN(FLASH) + 0x400; //! ``` //! //! ### `_sheap` //! //! This symbol is located in RAM right after the `.bss` and `.data` sections. //! You can use the address of this symbol as the start address of a heap //! region. This symbol is 4 byte aligned so that address will be a multiple of 4. //! //! #### Example //! //! ``` ignore //! extern crate some_allocator; //! //! // Size of the heap in bytes //! const SIZE: usize = 1024; //! //! extern "C" { //! static mut _sheap: u8; //! } //! //! fn main() { //! unsafe { //! let start_address = &mut _sheap as *mut u8; //! some_allocator::initialize(start_address, SIZE); //! } //! } //! ``` //! //! *NOTE* if you are using `cortex-m-rt-ld` and/or have defined the `_heap_size` symbol then you should //! use the address of the `_eheap` to compute the size of the `.heap` section, instead of //! duplicating the value that you wrote in `memory.x`. //! //! [1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html //! [qs]: https://docs.rs/cortex-m-quickstart/0.2.0/cortex_m_quickstart/ //! [`cortex-m-rt-ld`]: https://crates.io/crates/cortex-m-rt-ld //! [2]: https://sourceware.org/binutils/docs/ld/MEMORY.html #![cfg_attr(any(target_arch = "arm", feature = "abort-on-panic"), feature(core_intrinsics))] #![deny(missing_docs)] #![deny(warnings)] #![feature(asm)] #![feature(compiler_builtins_lib)] #![feature(global_asm)] #![feature(lang_items)] #![feature(linkage)] #![feature(naked_functions)] #![feature(used)] #![no_std] extern crate compiler_builtins; #[cfg(target_arch = "arm")] extern crate cortex_m; #[cfg(target_arch = "arm")] extern crate r0; #[cfg(not(test))] mod lang_items; #[cfg(target_arch = "arm")] use core::intrinsics; #[cfg(target_arch = "arm")] use cortex_m::asm; #[cfg(target_arch = "arm")] use cortex_m::exception::ExceptionFrame; extern "C" { // NOTE `rustc` forces this signature on us. See `src/lang_items.rs` #[cfg(target_arch = "arm")] fn main(argc: isize, argv: *const *const u8) -> isize; // Boundaries of the .bss section static mut _ebss: u32; static mut _sbss: u32; // Boundaries of the .data section static mut _edata: u32; static mut _sdata: u32; // Initial values of the .data section (stored in Flash) static _sidata: u32; } #[cfg(target_arch = "arm")] #[link_section = ".vector_table.reset_vector"] #[used] static RESET_VECTOR: unsafe extern "C" fn() -> ! = reset_handler; /// The reset handler /// /// This is the entry point of all programs #[cfg(target_arch = "arm")] #[link_section = ".reset_handler"] unsafe extern "C" fn reset_handler() -> ! { r0::zero_bss(&mut _sbss, &mut _ebss); r0::init_data(&mut _sdata, &mut _edata, &_sidata); match () { #[cfg(not(has_fpu))] () => { // Neither `argc` or `argv` make sense in bare metal context so we // just stub them main(0, ::core::ptr::null()); } #[cfg(has_fpu)] () => { // NOTE(safe) no exception / interrupt that also accesses the FPU // can occur here let scb = &*cortex_m::peripheral::SCB.get(); scb.enable_fpu(); // Make sure the user main function never gets inlined into this // function as that may cause FPU related instructions like vpush to // be executed *before* enabling the FPU and that would generate an // exception #[inline(never)] fn main() { unsafe { ::main(0, ::core::ptr::null()); } } main() } } // If `main` returns, then we go into "reactive" mode and simply attend // interrupts as they occur. loop { asm!("wfi" :::: "volatile"); } } #[cfg(target_arch = "arm")] global_asm!( r#" .weak NMI NMI = DEFAULT_HANDLER .weak HARD_FAULT HARD_FAULT = DEFAULT_HANDLER .weak MEM_MANAGE MEM_MANAGE = DEFAULT_HANDLER .weak BUS_FAULT BUS_FAULT = DEFAULT_HANDLER .weak USAGE_FAULT USAGE_FAULT = DEFAULT_HANDLER .weak SVCALL SVCALL = DEFAULT_HANDLER .weak PENDSV PENDSV = DEFAULT_HANDLER .weak SYS_TICK SYS_TICK = DEFAULT_HANDLER "# ); #[cfg(not(armv6m))] global_asm!( r#" .weak DEBUG_MONITOR DEBUG_MONITOR = DEFAULT_HANDLER "# ); #[cfg(target_arch = "arm")] extern "C" { fn NMI(); fn HARD_FAULT(); fn MEM_MANAGE(); fn BUS_FAULT(); fn USAGE_FAULT(); fn SVCALL(); #[cfg(not(armv6m))] fn DEBUG_MONITOR(); fn PENDSV(); fn SYS_TICK(); } #[allow(private_no_mangle_statics)] #[cfg(target_arch = "arm")] #[doc(hidden)] #[link_section = ".vector_table.exceptions"] #[no_mangle] #[used] pub static EXCEPTIONS: [Option<unsafe extern "C" fn()>; 14] = [ Some(NMI), Some(HARD_FAULT), Some(MEM_MANAGE), Some(BUS_FAULT), Some(USAGE_FAULT), None, None, None, None, Some(SVCALL), #[cfg(armv6m)] None, #[cfg(not(armv6m))] Some(DEBUG_MONITOR), None, Some(PENDSV), Some(SYS_TICK), ]; /// `ef` points to the exception frame /// /// That exception frame is a snapshot of the program state right before the /// exception occurred. #[allow(unused_variables)] #[cfg(target_arch = "arm")] extern "C" fn default_handler(ef: &ExceptionFrame) -> ! { asm::bkpt(); loop {} #[export_name = "DEFAULT_HANDLER"] #[linkage = "weak"] #[naked] extern "C" fn trampoline() -> ! { unsafe { asm!("mrs r0, MSP b $0" : : "i"(default_handler as extern "C" fn(&ExceptionFrame) -> !) : : "volatile"); intrinsics::unreachable() } } #[used] static KEEP: extern "C" fn() -> ! = trampoline; } // make sure the compiler emits the DEFAULT_HANDLER symbol so the linker can // find it! #[cfg(target_arch = "arm")] #[used] static KEEP: extern "C" fn(&ExceptionFrame) -> ! = default_handler; /// This macro lets you override the default exception handler /// /// The first and only argument to this macro is the path to the function that /// will be used as the default handler. That function must have signature /// `fn()` /// /// # Examples /// /// ``` ignore /// default_handler!(foo::bar); /// /// mod foo { /// pub fn bar() { /// ::cortex_m::asm::bkpt(); /// loop {} /// } /// } /// ``` #[macro_export] macro_rules! default_handler { ($path:path) => { #[allow(non_snake_case)] #[doc(hidden)] #[no_mangle] pub unsafe extern "C" fn DEFAULT_HANDLER() { // type checking let f: fn() = $path; f(); } } } /// Fault and system exceptions #[allow(non_camel_case_types)] #[doc(hidden)] pub enum Exception { /// Non-maskable interrupt NMI, /// All class of fault. HARD_FAULT, /// Memory management. MEN_MANAGE, /// Pre-fetch fault, memory access fault. BUS_FAULT, /// Undefined instruction or illegal state. USAGE_FAULT, /// System service call via SWI instruction SVCALL, /// Debug monitor #[cfg(not(armv6m))] DEBUG_MONITOR, /// Pendable request for system service PENDSV, /// System tick timer SYS_TICK, } /// Assigns a handler to an exception /// /// This macro takes two arguments: the name of an exception and the path to the /// function that will be used as the handler of that exception. That function /// must have signature `fn()`. /// /// Optionally, a third argument may be used to declare exception local data. /// The handler will have exclusive access to these *local* variables on each /// invocation. If the third argument is used then the signature of the handler /// function must be `fn(&mut $NAME::Locals)` where `$NAME` is the first /// argument passed to the macro. /// /// # Example /// /// ``` ignore /// exception!(MEM_MANAGE, mpu_fault); /// /// fn mpu_fault() { /// panic!("Oh no! Something went wrong"); /// } /// /// exception!(SYS_TICK, periodic, locals: { /// counter: u32 = 0; /// }); /// /// fn periodic(locals: &mut SYS_TICK::Locals) { /// locals.counter += 1; /// println!("This function has been called {} times", locals.counter); /// } /// ``` #[macro_export] macro_rules! exception { ($NAME:ident, $path:path, locals: { $($lvar:ident:$lty:ident = $lval:expr;)+ }) => { #[allow(non_snake_case)] mod $NAME { pub struct Locals { $( pub $lvar: $lty, )+ } } #[allow(non_snake_case)] #[doc(hidden)] #[no_mangle] pub unsafe extern "C" fn $NAME() { // check that the handler exists let _ = $crate::Exception::$NAME; static mut LOCALS: self::$NAME::Locals = self::$NAME::Locals { $( $lvar: $lval, )* }; // type checking let f: fn(&mut self::$NAME::Locals) = $path; f(&mut LOCALS); } }; ($NAME:ident, $path:path) => { #[allow(non_snake_case)] #[doc(hidden)] #[no_mangle] pub unsafe extern "C" fn $NAME() { // check that the handler exists let _ = $crate::Exception::$NAME; // type checking let f: fn() = $path; f(); } } }