backtrace support

Author: arihant2math

.cargo/config.toml

@@ -1,5 +1,6 @@
 [build]
 target = "aarch64-unknown-none-softfloat"
+rustflags = ["-C", "force-frame-pointers=yes"]
 
 [target.aarch64-unknown-none-softfloat]
 runner = "scripts/qemu_runner.py"

scripts/qemu_runner.py

@@ -1,7 +1,10 @@
 #!/usr/bin/env python3
 
 import argparse
+import re
+import shutil
 import subprocess
+import sys
 
 parser = argparse.ArgumentParser(description="QEMU runner")
 
@@ -14,6 +17,7 @@
 parser.add_argument("--memory", default="2G")
 parser.add_argument("--debug", action="store_true", help="Enable QEMU debugging")
 parser.add_argument("--display", action="store_true", help="Add a display device to the VM")
+parser.add_argument("--demangle", action="store_true", help="Add a demangled backtrace")
 
 
 
@@ -68,4 +72,59 @@
 
 qemu_command += extra_args
 
-subprocess.run(qemu_command, check=True)
+if args.demangle:
+    addr2line = (
+        shutil.which("aarch64-none-elf-addr2line")
+        or shutil.which("llvm-addr2line")
+        or shutil.which("addr2line")
+    )
+    pc_pattern = re.compile(r"\bPC ([0-9a-fA-F]+)\b")
+    decode_cache = {}
+
+
+    def decode_pc(pc: str) -> str | None:
+        if addr2line is None:
+            return None
+
+        pc = pc.lower()
+        if pc in decode_cache:
+            return decode_cache[pc]
+
+        result = subprocess.run(
+            [addr2line, "-e", elf_executable, "-f", "-C", "-p", f"0x{pc}"],
+            check=False,
+            capture_output=True,
+            text=True,
+        )
+
+        decoded = result.stdout.strip() or None
+        decode_cache[pc] = decoded
+        return decoded
+
+
+    with subprocess.Popen(
+        qemu_command,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        text=True,
+        bufsize=1,
+    ) as proc:
+        assert proc.stdout is not None
+
+        for line in proc.stdout:
+            sys.stdout.write(line)
+            sys.stdout.flush()
+
+            match = pc_pattern.search(line)
+            if match is None:
+                continue
+
+            decoded = decode_pc(match.group(1))
+            if decoded is not None:
+                print(f"\t=> {decoded}", flush=True)
+
+        ret = proc.wait()
+        if ret != 0:
+            raise subprocess.CalledProcessError(ret, qemu_command)
+else:
+    subprocess.run(qemu_command, check=True)

src/arch/arm64/mod.rs

@@ -45,6 +45,10 @@ mod proc;
 pub mod psci;
 pub mod ptrace;
 
+pub(crate) use self::boot::memory::{KERNEL_STACK_AREA, KERNEL_STACK_PG_ORDER};
+pub(crate) use self::exceptions::EMERG_STACK_END;
+pub(crate) use self::memory::IMAGE_BASE;
+
 pub struct Aarch64 {}
 
 impl CpuOps for Aarch64 {

src/arch/mod.rs

@@ -212,3 +212,7 @@ mod arm64;
 
 #[cfg(target_arch = "aarch64")]
 pub use self::arm64::Aarch64 as ArchImpl;
+#[cfg(target_arch = "aarch64")]
+pub(crate) use self::arm64::{
+    EMERG_STACK_END, IMAGE_BASE, KERNEL_STACK_AREA, KERNEL_STACK_PG_ORDER,
+};

src/backtrace/mod.rs

@@ -0,0 +1,133 @@
+use crate::arch::{EMERG_STACK_END, IMAGE_BASE, KERNEL_STACK_AREA, KERNEL_STACK_PG_ORDER};
+use core::arch::asm;
+use core::slice;
+use libkernel::memory::{PAGE_SIZE, address::VA, region::VirtMemoryRegion};
+use log::error;
+#[cfg(target_pointer_width = "32")]
+use object::elf::FileHeader32 as FileHeader;
+#[cfg(target_pointer_width = "64")]
+use object::elf::FileHeader64 as FileHeader;
+use object::{
+    NativeEndian, elf,
+    read::elf::{FileHeader as _, Sym as _},
+};
+
+const MAX_FRAMES: usize = 64;
+
+pub struct StackTrace {
+    pub fp: usize,
+    pub pc_ptr: *const usize,
+}
+
+impl StackTrace {
+    #[inline(always)]
+    pub unsafe fn start() -> Option<Self> {
+        unsafe {
+            let fp: usize;
+            asm!("mov {}, fp", out(reg) fp);
+            let pc_ptr = fp.checked_add(size_of::<usize>())?;
+            Some(StackTrace {
+                fp,
+                pc_ptr: pc_ptr as *const usize,
+            })
+        }
+    }
+
+    pub unsafe fn next(self) -> Option<Self> {
+        unsafe {
+            let fp = *(self.fp as *const usize);
+            let pc_ptr = fp.checked_add(size_of::<usize>())?;
+            Some(StackTrace {
+                fp,
+                pc_ptr: pc_ptr as *const usize,
+            })
+        }
+    }
+}
+
+fn emergency_stack_area() -> VirtMemoryRegion {
+    let stack_size = PAGE_SIZE << KERNEL_STACK_PG_ORDER;
+    VirtMemoryRegion::new(EMERG_STACK_END.sub_bytes(stack_size), stack_size)
+}
+
+fn is_valid_frame_pointer(frame: &StackTrace) -> bool {
+    let fp_virt = VA::from_value(frame.fp);
+    let pc_virt = VA::from_value(frame.pc_ptr.addr());
+    let emergency_stack = emergency_stack_area();
+
+    let in_kernel_stack =
+        |va| KERNEL_STACK_AREA.contains_address(va) || emergency_stack.contains_address(va);
+
+    in_kernel_stack(fp_virt)
+        && in_kernel_stack(pc_virt)
+        && (frame.fp as *const usize).is_aligned()
+        && frame.pc_ptr.is_aligned()
+}
+
+pub fn print_backtrace() {
+    unsafe {
+        let kernel_ptr = IMAGE_BASE.cast::<u8>().as_ptr();
+        let elf_header: &FileHeader<NativeEndian> = object::pod::from_bytes(slice::from_raw_parts(
+            kernel_ptr,
+            size_of::<FileHeader<NativeEndian>>(),
+        ))
+        .unwrap()
+        .0;
+
+        // This assumes that the linker places .shstrtab as last section. If it
+        // isn't, that just causes a recursive panic, not UB.
+        let kernel_size = elf_header.e_shoff(NativeEndian) as usize
+            + usize::from(elf_header.e_shnum(NativeEndian))
+                * usize::from(elf_header.e_shentsize(NativeEndian));
+        let kernel_slice = slice::from_raw_parts(kernel_ptr, kernel_size);
+
+        let symbols = elf_header
+            .sections(NativeEndian, kernel_slice)
+            .unwrap()
+            .symbols(NativeEndian, kernel_slice, elf::SHT_SYMTAB)
+            .unwrap();
+
+        let mut frame = StackTrace::start();
+
+        for _ in 0..MAX_FRAMES {
+            let Some(frame_) = frame else {
+                break;
+            };
+
+            if !is_valid_frame_pointer(&frame_) {
+                error!("  {:>016x}: INVALID FRAME", frame_.fp);
+                break;
+            }
+
+            let pc = *frame_.pc_ptr;
+            if pc == 0 {
+                error!(" {:>016x}: EMPTY RETURN", frame_.fp);
+                break;
+            }
+
+            error!("  FP {:>016x}: PC {:>016x}", frame_.fp, pc);
+
+            for sym in symbols.iter() {
+                if sym.st_type() != elf::STT_FUNC {
+                    continue;
+                }
+                let sym_addr = sym.st_value.get(NativeEndian) as usize;
+                if !(pc >= sym_addr && pc < sym_addr + sym.st_size.get(NativeEndian) as usize) {
+                    continue;
+                }
+
+                let sym_offset = pc - sym_addr;
+                if let Some(sym_name) = sym
+                    .name(NativeEndian, symbols.strings())
+                    .ok()
+                    .and_then(|name| core::str::from_utf8(name).ok())
+                {
+                    error!("    {sym_name} @ {sym_addr:>016X}+{sym_offset:>04X}");
+                } else {
+                    error!("    {sym_addr:>016X}+{sym_offset:>04X}");
+                }
+            }
+            frame = frame_.next();
+        }
+    }
+}

src/main.rs

@@ -42,6 +42,7 @@ extern crate alloc;
 extern crate moss_macros;
 
 mod arch;
+mod backtrace;
 mod clock;
 mod console;
 mod drivers;
@@ -74,6 +75,8 @@ fn on_panic(info: &PanicInfo) -> ! {
         error!("Kernel panicked at unknown location: {panic_msg}");
     }
 
+    backtrace::print_backtrace();
+
     ArchImpl::power_off();
 }