Begin setting up stack unwinding/tracing.

This actually worked so I am chuffed, but it doesn't read the necessary
DWARF structures just yet. Still a good step forwards.

Cargo.lock

@@ -99,6 +99,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "elf"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4445909572dbd556c457c849c4ca58623d84b27c8fff1e74b0b4227d8b90d17b"
+
 [[package]]
 name = "fnv"
 version = "1.0.7"
@@ -111,7 +117,9 @@ version = "0.1.0"
 dependencies = [
  "cc",
  "crossbeam",
+ "elf",
  "futures-util",
+ "gimli",
  "libm",
  "limine",
  "linked_list_allocator",
@@ -145,6 +153,12 @@ dependencies = [
  "pin-utils",
 ]
 
+[[package]]
+name = "gimli"
+version = "0.31.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "07e28edb80900c19c28f1072f2e8aeca7fa06b23cd4169cefe1af5aa3260783f"
+
 [[package]]
 name = "ident_case"
 version = "1.0.1"

kernel/Cargo.toml

@@ -19,13 +19,8 @@ futures-util = { version = "0.3.31", default-features = false, features = [
     "alloc",
 ] }
 linked_list_allocator = { version = "0.10.5", features = ["use_spin"] }
-# unwinding = { version = "0.2.5", default-features = false, features = [
+gimli = { version = "0.31.1", default-features = false, features = ["read"] }
-#     "unwinder",
+elf = { version = "0.7.4", default-features = false, features = ["nightly"] }
-#     "fde-static",
-#     "personality",
-#     "panic",
-#     "hide-trace",
-# ] }
 
 [build-dependencies]
 cc = "1.2.14"

kernel/linker.ld

@@ -46,12 +46,6 @@ SECTIONS
         *(.rodata .rodata.*)
     } :rodata
 
-    /* Adds support for stack unwinding using the unwinding crate. */
-    . = ALIGN(8);
-    PROVIDE(__eh_frame = .);
-    .eh_frame : { KEEP (*(.eh_frame)) *(.eh_frame.*) }
-
-
     /* Move to the next memory page for .data */
     . = ALIGN(CONSTANT(MAXPAGESIZE));
 

kernel/src/arch/x86_64/memory/mapping.rs

@@ -16,7 +16,7 @@ static HIGHER_HALF_DIRECT_MAP_REQUEST: HhdmRequest = HhdmRequest::new();
 
 #[used]
 #[unsafe(link_section = ".requests")]
-static KERNEL_ADDRESS_REQUEST: KernelAddressRequest =
+pub static KERNEL_ADDRESS_REQUEST: KernelAddressRequest =
     KernelAddressRequest::new();
 
 /// ```rs

kernel/src/lib.rs

@@ -19,6 +19,8 @@ use arch::x86_64::memory::allocation::page_alloc::FoundryOSFrameAllocator;
 use arch::x86_64::memory::mapping;
 use core::arch::asm;
 use limine::BaseRevision;
+use std::unwind;
+use std::unwind::eh_info::ELF;
 use x86_64::VirtAddr;
 
 pub mod arch;
@@ -64,7 +66,7 @@ pub fn hcf() -> ! {
 
 pub fn boot() -> Result<(), &'static str> {
     if !BASE_REVISION.is_supported() {
-        return Err("base revision not supported");
+        return Err("Base revision not supported");
     }
 
     use arch::x86_64::{gdt, interrupts};
@@ -118,5 +120,16 @@ pub fn boot() -> Result<(), &'static str> {
     x86_64::instructions::interrupts::enable();
     println_log!("[Success]");
 
+    print_log!("    Setting up stack unwinder, panic handler... ");
+    // Setup stack traces and proper panic handler. TODO: Handle panics
+    // differently if not initialised.
+    let eh_frame_ptr = ELF
+        .get_section_addr(".eh_frame_hdr")
+        .expect("Could not get `.eh_frame_hdr` address.");
+
+    let _eh_info =
+        unsafe { unwind::eh_info::EhInfo::from_hdr_ptr(eh_frame_ptr) };
+    println_log!("[Success]");
+
     Ok(())
 }

kernel/src/std/elf/mod.rs

@@ -0,0 +1,114 @@
+//! Basic ELF parsing functionality using the `elf` crate.
+//!
+//! This may be extended in the future to support loading programs, however we
+//! currently use this for getting the sizes of sections in our kernel ELF at
+//! runtime.
+//!
+//! This is used for implementing stacktraces in std::unwind.
+//!
+//! # TODO
+//!
+//! * Add support for loading binary programs (this should probably be written
+//!   in a different module)
+
+use alloc::format;
+use elf::{
+    ElfBytes, ParseError,
+    endian::LittleEndian,
+    parse::{ParseAt, ParsingTable},
+    section::{SectionHeader, SectionHeaderTable},
+    string_table::StringTable,
+};
+use limine::request::KernelFileRequest;
+
+use crate::prelude::*;
+
+#[cfg(target_arch = "x86_64")]
+/// The length of the ELF header in bytes.
+pub const ELF_HEADER_LEN: usize = 64;
+
+/// Information about our own ELF file to make ELF parsing easier, such as the
+/// length of the file and a pointer to the contents.
+#[used]
+pub static KERNEL_FILE_REQUEST: KernelFileRequest = KernelFileRequest::new();
+
+/// A list of errors that may occur when parsing ELF files.
+#[derive(Debug)]
+pub enum ElfError {
+    /// Returned if a section did not exist in [ElfReader::get_section_size].
+    SectionNotExists,
+    /// Parse errors returned by the `elf` crate.
+    OtherParseError(elf::ParseError),
+}
+
+impl From<elf::ParseError> for ElfError {
+    fn from(err: elf::ParseError) -> Self {
+        Self::OtherParseError(err)
+    }
+}
+
+pub struct ElfReader {
+    /// Structure returned by the `elf` crate having parsed the ELF header.
+    elf: ElfBytes<'static, LittleEndian>,
+}
+
+impl ElfReader {
+    /// Parses the ELF file for the kernel, this uses data from Limine's Kernel
+    /// File Request to get a slice over the whole executable file.
+    ///
+    /// # Safety
+    ///
+    /// Assumes a properly formed ELF file, and that Limine returns a correct
+    /// pointer to the start of the file as well as a valid length in bytes.
+    ///
+    /// Both of these should be satisfied, but this function is marked unsafe
+    /// just in case, because we are derefererencing arbitrary pointers.
+    pub unsafe fn new() -> Result<Self, ElfError> {
+        let response = KERNEL_FILE_REQUEST
+            .get_response()
+            .expect("Didn't get the kernel file from Limine. That's odd.");
+
+        // We fetch these from Limine and use them to parse our own ELF file.
+        let file = response.file();
+        let file_start_ptr = file.addr();
+        let file_size = file.size() as usize;
+
+        // Safety: This slice should contain the whole bytes of the ELF file.
+        let elf_hdr_slice =
+            unsafe { core::slice::from_raw_parts(file_start_ptr, file_size) };
+
+        let elf: ElfBytes<'static, LittleEndian> =
+            elf::ElfBytes::minimal_parse(elf_hdr_slice)?;
+
+        Ok(Self { elf })
+    }
+
+    /// Gets the section size of `section_name` in bytes.
+    pub fn get_section_size(
+        &self,
+        section_name: &'static str,
+    ) -> Result<u64, ElfError> {
+        Ok(self.get_section_header(section_name)?.sh_size)
+    }
+
+    /// Gets the start address of the section `section_name` in memory.
+    pub fn get_section_addr(
+        &self,
+        section_name: &'static str,
+    ) -> Result<*const u8, ElfError> {
+        Ok(self.get_section_header(section_name)?.sh_addr as *const u8)
+    }
+
+    /// Gets the section header of `section_name`.
+    pub fn get_section_header(
+        &self,
+        section_name: &'static str,
+    ) -> Result<SectionHeader, ElfError> {
+        let section_hdr = self
+            .elf
+            .section_header_by_name(section_name)
+            .map_err(|_e| ElfError::SectionNotExists)?;
+
+        section_hdr.ok_or(ElfError::SectionNotExists)
+    }
+}

kernel/src/std/mod.rs

@@ -1,4 +1,6 @@
 pub mod application;
 pub mod ascii;
+pub mod elf;
 pub mod io;
 pub mod maths;
+pub mod unwind;

kernel/src/std/unwind/eh_info.rs

@@ -0,0 +1,135 @@
+//! Contains a [EhInfo] struct that contains the parsed DWARF exception header
+//! data from the ELF .eh_frame and .eh_frame_hdr sections.
+
+use alloc::{boxed::Box, slice};
+use gimli::{
+    BaseAddresses, EhFrame, EhFrameHdr, EhHdrTable, EndianSlice, LittleEndian,
+    ParsedEhFrameHdr,
+};
+use spin::Lazy;
+
+use crate::{println_log, std::elf::ElfReader};
+
+/// Contains useful data parsed from the ELF file in question. In the kernel
+/// this will be our own process.
+///
+/// We use this to implement stack traces and potential unwinding.
+///
+/// # Sources
+///
+/// This code is reproduced from [lesenchal.fr](https://lesenechal.fr/en/linux/unwinding-the-stack-the-hard-way#h5.1-parsing-eh_frame-and-eh_frame_hdr-with-gimli)
+/// and will later be extended as required.
+pub struct EhInfo {
+    /// A set of base addresses used for relative addressing.
+    base_addrs: BaseAddresses,
+    /// The parsed `.eh_frame_hdr` section.
+    hdr: &'static ParsedEhFrameHdr<EndianSlice<'static, LittleEndian>>,
+    /// The lookup table in the parsed `.eh_frame_hdr` section.
+    /// This is a binary search table, it is optional but should be present as
+    /// we are linking with LLD(?) \[needs citation].
+    hdr_table: EhHdrTable<'static, EndianSlice<'static, LittleEndian>>,
+    /// The parsed `.eh_frame` containing the CFIs (call frame information).
+    eh_frame: EhFrame<EndianSlice<'static, LittleEndian>>,
+}
+
+/// Stores the [ElfReader] struct for this ELF file.
+pub static ELF: Lazy<ElfReader> =
+    Lazy::new(|| unsafe { ElfReader::new().unwrap() });
+
+impl EhInfo {
+    /// Gets the `.eh_frame_hdr` size in bytes.
+    ///
+    /// # Panics
+    ///
+    /// If we can't get the size of `.eh_frame_hdr`.
+    pub fn eh_frame_hdr_size() -> usize {
+        ELF.get_section_size(".eh_frame_hdr")
+            .expect("Cannot get size of `.eh_frame_hdr`.") as usize
+    }
+
+    /// Gets the `.eh_frame` size in bytes.
+    ///
+    /// # Panics
+    ///
+    /// If we can't get the size of `.eh_frame`.
+    pub fn eh_frame_size() -> usize {
+        ELF.get_section_size(".eh_frame")
+            .expect("Cannot get size of `.eh_frame`.") as usize
+    }
+
+    /// Constructs a [EhInfo] from the base address. This is defined for a
+    /// symbol in the linker script so we can initialise stack traces and -- in
+    /// the future -- unwinding.
+    ///
+    /// # Safety
+    ///
+    /// Assumes the `.eh_frame_hdr` pointer to be valid, as well as the sizes of
+    /// the containing sections. These sizes are computed using [ElfReader].
+    ///
+    /// # Panics
+    ///
+    /// This function panics if Gimli throws parsing errors -- for example due
+    /// to a malformed or corrupted binary, or because this is called in
+    /// release-mode, or on a stripped binary.
+    ///
+    /// # TODOs
+    ///
+    /// * Support external System.map files which list symbols and contain
+    ///   debugging information.
+    pub unsafe fn from_hdr_ptr(eh_frame_hdr: *const u8) -> Self {
+        let mut base_addrs = BaseAddresses::default();
+        // We add the `.eh_frame_hdr` pointer to the set of base addresses which
+        // are used by Gimli for later parsing. This may be used to compute a
+        // pointer to `.eh_frame`.
+        base_addrs = base_addrs.set_eh_frame_hdr(eh_frame_hdr as u64);
+
+        // Leaking the Box gives us a reference with `'static` lifetime to use
+        // in Self.
+        let hdr = Box::leak(Box::new(
+            // We need to construct a slice as input for `EhFrameHdr::new`.
+            // This is sound if data pointer and length are known to be
+            // correct.
+            EhFrameHdr::new(
+                unsafe {
+                    core::slice::from_raw_parts(
+                        eh_frame_hdr,
+                        Self::eh_frame_hdr_size(),
+                    )
+                },
+                LittleEndian,
+            ) // Parse the header using the base address we provided (virtual
+            // memory). Address size is how many bytes make an
+            // address (64 bits).
+            .parse(&base_addrs, 8)
+            .expect(
+                "Could not parse `.eh_frame_hdr`. The ELF must be malformed.",
+            ),
+        ));
+
+        // Create a pointer to the `.eh_frame` ready to parse it.
+        let eh_frame = match hdr.eh_frame_ptr() {
+            gimli::Pointer::Direct(addr) => addr as *mut u8,
+            _ => unimplemented!(),
+        };
+
+        // Add the `.eh_frame` address for addresses relative to this section.
+        base_addrs = base_addrs.set_eh_frame(eh_frame as u64);
+
+        // Finally parse the `.eh_frame` section of our ELF.
+        let eh_frame = EhFrame::new(
+            unsafe {
+                core::slice::from_raw_parts(eh_frame, Self::eh_frame_size())
+            },
+            LittleEndian,
+        );
+
+        Self {
+            base_addrs,
+            hdr,
+            hdr_table: hdr.table().expect(
+                "The CFI binary search table was not present in this binary, oh dear.",
+            ),
+            eh_frame,
+        }
+    }
+}

kernel/src/std/unwind/mod.rs

@@ -0,0 +1 @@
+pub mod eh_info;