97 lines
3.4 KiB
Rust
97 lines
3.4 KiB
Rust
//! Debugging a crash (exception)
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//!
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//! Most crash conditions trigger a hard fault exception, whose handler is defined via
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//! `exception!(HardFault, ..)`. The `HardFault` handler has access to the exception frame, a
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//! snapshot of the CPU registers at the moment of the exception.
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//!
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//! This program crashes and the `HardFault` handler prints to the console the contents of the
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//! `ExceptionFrame` and then triggers a breakpoint. From that breakpoint one can see the backtrace
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//! that led to the exception.
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//!
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//! ``` text
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//! (gdb) continue
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//! Program received signal SIGTRAP, Trace/breakpoint trap.
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//! __bkpt () at asm/bkpt.s:3
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//! 3 bkpt
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//!
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//! (gdb) backtrace
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//! #0 __bkpt () at asm/bkpt.s:3
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//! #1 0x080030b4 in cortex_m::asm::bkpt () at $$/cortex-m-0.5.0/src/asm.rs:19
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//! #2 rust_begin_unwind (args=..., file=..., line=99, col=5) at $$/panic-semihosting-0.2.0/src/lib.rs:87
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//! #3 0x08001d06 in core::panicking::panic_fmt () at libcore/panicking.rs:71
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//! #4 0x080004a6 in crash::hard_fault (ef=0x20004fa0) at examples/crash.rs:99
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//! #5 0x08000548 in UserHardFault (ef=0x20004fa0) at <exception macros>:10
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//! #6 0x0800093a in HardFault () at asm.s:5
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//! Backtrace stopped: previous frame identical to this frame (corrupt stack?)
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//! ```
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//!
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//! In the console output one will find the state of the Program Counter (PC) register at the time
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//! of the exception.
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//!
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//! ``` text
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//! panicked at 'HardFault at ExceptionFrame {
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//! r0: 0x2fffffff,
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//! r1: 0x2fffffff,
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//! r2: 0x080051d4,
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//! r3: 0x080051d4,
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//! r12: 0x20000000,
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//! lr: 0x08000435,
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//! pc: 0x08000ab6,
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//! xpsr: 0x61000000
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//! }', examples/crash.rs:106:5
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//! ```
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//!
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//! This register contains the address of the instruction that caused the exception. In GDB one can
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//! disassemble the program around this address to observe the instruction that caused the
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//! exception.
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//!
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//! ``` text
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//! (gdb) disassemble/m 0x08000ab6
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//! Dump of assembler code for function core::ptr::read_volatile:
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//! 451 pub unsafe fn read_volatile<T>(src: *const T) -> T {
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//! 0x08000aae <+0>: sub sp, #16
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//! 0x08000ab0 <+2>: mov r1, r0
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//! 0x08000ab2 <+4>: str r0, [sp, #8]
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//!
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//! 452 intrinsics::volatile_load(src)
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//! 0x08000ab4 <+6>: ldr r0, [sp, #8]
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//! -> 0x08000ab6 <+8>: ldr r0, [r0, #0]
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//! 0x08000ab8 <+10>: str r0, [sp, #12]
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//! 0x08000aba <+12>: ldr r0, [sp, #12]
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//! 0x08000abc <+14>: str r1, [sp, #4]
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//! 0x08000abe <+16>: str r0, [sp, #0]
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//! 0x08000ac0 <+18>: b.n 0x8000ac2 <core::ptr::read_volatile+20>
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//!
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//! 453 }
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//! 0x08000ac2 <+20>: ldr r0, [sp, #0]
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//! 0x08000ac4 <+22>: add sp, #16
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//! 0x08000ac6 <+24>: bx lr
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//!
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//! End of assembler dump.
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//! ```
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//!
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//! `ldr r0, [r0, #0]` caused the exception. This instruction tried to load (read) a 32-bit word
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//! from the address stored in the register `r0`. Looking again at the contents of `ExceptionFrame`
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//! we see that the `r0` contained the address `0x2FFF_FFFF` when this instruction was executed.
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//!
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//! ---
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#![no_main]
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#![no_std]
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extern crate panic_halt;
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use core::ptr;
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use cortex_m_rt::entry;
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#[entry]
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fn main() -> ! {
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unsafe {
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// read an address outside of the RAM region; this causes a HardFault exception
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ptr::read_volatile(0x2FFF_FFFF as *const u32);
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}
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loop {}
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}
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