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.

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)
+    }
+}