bzimage: Rewrite Linux Kernel Loading code

This allows Linux to be booted with any boot protocol.

The old code took in the Zeropage passed in via the Linux Kernel Boot
Protocol, modified it, and passed it into the Linux Kernel. This is not
the correct way to boot Linux per the documentation:
    https://www.kernel.org/doc/Documentation/x86/boot.txt

This code now correctly:
  - Uses a brand-new Zeropage inside the `Kernel` struct
  - Adds in the E820 map and RSDP pointer from the boot::Info
  - Reads the header from the file and copies it into the Zeropage
  - Loads the kernel and initrd into avalible memory
  - Properly manages the command-line at a fixed memory location
  - Jumps to the appropriate starting address

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/bzimage.rs

@@ -11,13 +11,18 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+use atomic_refcell::AtomicRefCell;
 
-use crate::fat::{self, Read};
+use crate::{
+    boot::{E820Entry, Header, Info, Params},
+    fat::{self, Read},
+    mem::MemoryRegion,
+};
 
 #[derive(Debug)]
 pub enum Error {
     FileError(fat::Error),
-    KernelOld,
+    NoInitrdMemory,
     MagicMissing,
     NotRelocatable,
 }
@@ -28,185 +33,145 @@ impl From<fat::Error> for Error {
     }
 }
 
-// From firecracker
-/// Kernel command line start address.
-const CMDLINE_START: usize = 0x4b000;
-/// Kernel command line start address maximum size.
-const CMDLINE_MAX_SIZE: usize = 0x10000;
-/// The 'zero page', a.k.a linux kernel bootparams.
-pub const ZERO_PAGE_START: usize = 0x7000;
+const KERNEL_LOCATION: u64 = 0x20_0000;
 
-const KERNEL_LOCATION: u32 = 0x20_0000;
+#[repr(transparent)]
+pub struct Kernel(Params);
 
-const E820_RAM: u32 = 1;
-
-#[repr(C, packed)]
-struct E820Entry {
-    addr: u64,
-    size: u64,
-    entry_type: u32,
-}
-
-pub fn load_initrd(f: &mut dyn Read) -> Result<(), Error> {
-    let mut zero_page = crate::mem::MemoryRegion::new(ZERO_PAGE_START as u64, 4096);
-
-    let mut max_load_address = u64::from(zero_page.read_u32(0x22c));
-    if max_load_address == 0 {
-        max_load_address = 0x37ff_ffff;
+impl Kernel {
+    pub fn new(info: &dyn Info) -> Self {
+        let mut kernel = Self(Params::default());
+        kernel.0.acpi_rsdp_addr = info.rsdp_addr();
+        kernel.0.set_entries(info);
+        kernel
     }
 
-    let e820_count = zero_page.read_u8(0x1e8);
-    let e820_table = zero_page.as_mut_slice::<E820Entry>(0x2d0, u64::from(e820_count));
+    pub fn load_kernel(&mut self, f: &mut dyn Read) -> Result<(), Error> {
+        self.0.hdr = Header::from_file(f)?;
 
-    // Search E820 table for highest usable ram location that is below the limit.
-    let mut top_of_usable_ram = 0;
-    for entry in e820_table {
-        if entry.entry_type == E820_RAM {
-            let m = entry.addr + entry.size - 1;
-            if m > top_of_usable_ram && m < max_load_address {
-                top_of_usable_ram = m;
-            }
+        if self.0.hdr.boot_flag != 0xAA55 || self.0.hdr.header != *b"HdrS" {
+            return Err(Error::MagicMissing);
+        }
+        // Check relocatable
+        if self.0.hdr.version < 0x205 || self.0.hdr.relocatable_kernel == 0 {
+            return Err(Error::NotRelocatable);
         }
-    }
 
-    if top_of_usable_ram > max_load_address {
-        top_of_usable_ram = max_load_address;
+        // Skip over the setup sectors
+        let setup_sects = match self.0.hdr.setup_sects {
+            0 => 4,
+            n => n as u32,
+        };
+        let setup_bytes = (setup_sects + 1) * 512;
+        let remaining_bytes = f.get_size() - setup_bytes;
+
+        let mut region = MemoryRegion::new(KERNEL_LOCATION, remaining_bytes as u64);
+        f.seek(setup_bytes)?;
+        f.load_file(&mut region)?;
+
+        // Fill out "write/modify" fields
+        self.0.hdr.type_of_loader = 0xff; // Unknown Loader
+        self.0.hdr.code32_start = KERNEL_LOCATION as u32; // Where we load the kernel
+        self.0.hdr.cmd_line_ptr = CMDLINE_START as u32; // Where we load the cmdline
+        Ok(())
     }
 
-    // Align address to 2MiB boundary as we use 2 MiB pages
-    let initrd_address = (top_of_usable_ram - u64::from(f.get_size())) & !((2 << 20) - 1);
-    let mut initrd_region = crate::mem::MemoryRegion::new(initrd_address, u64::from(f.get_size()));
-
-    let mut offset = 0;
-    while offset < f.get_size() {
-        let bytes_remaining = f.get_size() - offset;
-
-        // Use intermediata buffer for last, partial sector
-        if bytes_remaining < 512 {
-            let mut data: [u8; 512] = [0; 512];
-            match f.read(&mut data) {
-                Err(crate::fat::Error::EndOfFile) => break,
-                Err(e) => return Err(Error::FileError(e)),
-                Ok(_) => {}
-            }
-            let dst = initrd_region.as_mut_slice(u64::from(offset), u64::from(bytes_remaining));
-            dst.copy_from_slice(&data[0..bytes_remaining as usize]);
-            break;
-        }
+    // Compute the load address for the initial ramdisk
+    fn initrd_addr(&self, size: u64) -> Option<u64> {
+        let initrd_addr_max = match self.0.hdr.initrd_addr_max {
+            0 => 0x37FF_FFFF,
+            a => a as u64,
+        };
+        let max_start = (initrd_addr_max + 1) - size;
 
-        let dst = initrd_region.as_mut_slice(u64::from(offset), 512);
-        match f.read(dst) {
-            Err(crate::fat::Error::EndOfFile) => break,
-            Err(e) => return Err(Error::FileError(e)),
-            Ok(_) => {}
+        let mut option_addr = None;
+        for i in 0..self.0.num_entries() {
+            let entry = self.0.entry(i);
+            if entry.entry_type != E820Entry::RAM_TYPE {
+                continue;
+            }
+            let addr = entry.addr + entry.size - size;
+            // Align address to 2MiB boundary as we use 2 MiB pages
+            let addr = addr & !((2 << 20) - 1);
+            // The ramdisk must fit in the region completely
+            if addr > max_start || addr < entry.addr {
+                continue;
+            }
+            // Use the largest address we can find
+            if let Some(load_addr) = option_addr {
+                if load_addr >= addr {
+                    continue;
+                }
+            }
+            option_addr = Some(addr)
         }
-
-        offset += 512;
+        option_addr
     }
 
-    // initrd pointer/size
-    zero_page.write_u32(0x218, initrd_address as u32);
-    zero_page.write_u32(0x21c, f.get_size());
-    Ok(())
-}
-
-pub fn append_commandline(addition: &str) -> Result<(), Error> {
-    let mut cmdline_region =
-        crate::mem::MemoryRegion::new(CMDLINE_START as u64, CMDLINE_MAX_SIZE as u64);
-    let zero_page = crate::mem::MemoryRegion::new(ZERO_PAGE_START as u64, 4096);
-
-    let cmdline = cmdline_region.as_mut_slice::<u8>(0, CMDLINE_MAX_SIZE as u64);
-
-    // Use the actual string length but limit to the orgiginal incoming size
-    let orig_len = zero_page.read_u32(0x238) as usize;
-
-    let orig_cmdline = unsafe {
-        core::str::from_utf8_unchecked(&cmdline[0..orig_len]).trim_matches(char::from(0))
-    };
-    let orig_len = orig_cmdline.len();
-
-    cmdline[orig_len] = b' ';
-    cmdline[orig_len + 1..orig_len + 1 + addition.len()].copy_from_slice(addition.as_bytes());
-    cmdline[orig_len + 1 + addition.len()] = 0;
-
-    // Commandline pointer/size
-    zero_page.write_u32(0x228, CMDLINE_START as u32);
-    zero_page.write_u32(0x238, (orig_len + addition.len() + 1) as u32);
-
-    Ok(())
-}
-
-pub fn load_kernel(f: &mut dyn Read) -> Result<u64, Error> {
-    f.seek(0)?;
-
-    let mut buf: [u8; 1024] = [0; 1024];
-
-    f.read(&mut buf[0..512])?;
-    f.read(&mut buf[512..])?;
-
-    let setup = crate::mem::MemoryRegion::from_bytes(&mut buf[..]);
+    pub fn load_initrd(&mut self, f: &mut dyn Read) -> Result<(), Error> {
+        let size = f.get_size() as u64;
+        let addr = match self.initrd_addr(size) {
+            Some(addr) => addr,
+            None => return Err(Error::NoInitrdMemory),
+        };
 
-    if setup.read_u16(0x1fe) != 0xAA55 {
-        return Err(Error::MagicMissing);
-    }
+        let mut region = MemoryRegion::new(addr, size);
+        f.seek(0)?;
+        f.load_file(&mut region)?;
 
-    if setup.read_u32(0x202) != 0x5372_6448 {
-        return Err(Error::MagicMissing);
+        // initrd pointer/size
+        self.0.hdr.ramdisk_image = addr as u32;
+        self.0.hdr.ramdisk_size = size as u32;
+        Ok(())
     }
 
-    // Need for relocation
-    if setup.read_u16(0x206) < 0x205 {
-        return Err(Error::KernelOld);
+    pub fn append_cmdline(&mut self, addition: &[u8]) {
+        if !addition.is_empty() {
+            CMDLINE.borrow_mut().append(addition);
+            assert!(CMDLINE.borrow().len() < self.0.hdr.cmdline_size);
+        }
     }
 
-    // Check relocatable
-    if setup.read_u8(0x234) == 0 {
-        return Err(Error::NotRelocatable);
+    pub fn boot(&mut self) {
+        // 0x200 is the startup_64 offset
+        let jump_address = self.0.hdr.code32_start as u64 + 0x200;
+        // Rely on x86 C calling convention where second argument is put into %rsi register
+        let ptr = jump_address as *const ();
+        let code: extern "C" fn(usize, usize) = unsafe { core::mem::transmute(ptr) };
+        (code)(0 /* dummy value */, &mut self.0 as *mut _ as usize);
     }
+}
 
-    let header_start = 0x1f1 as usize;
-    let header_end = 0x202 + buf[0x0201] as usize;
-
-    // Reuse the zero page that we were originally given
-    // TODO: Zero and fill it ourself but will need to save E820 details
-    let mut zero_page = crate::mem::MemoryRegion::new(ZERO_PAGE_START as u64, 4096);
-
-    let dst = zero_page.as_mut_slice(header_start as u64, (header_end - header_start) as u64);
-    dst.copy_from_slice(&buf[header_start..header_end]);
-
-    // Unknown loader
-    zero_page.write_u8(0x210, 0xff);
+// This is the highest region at which we can load the kernel command line.
+const CMDLINE_START: u64 = 0x4b000;
+const CMDLINE_MAX_LEN: u64 = 0x10000;
 
-    // Where we will load the kernel into
-    zero_page.write_u32(0x214, KERNEL_LOCATION);
+static CMDLINE: AtomicRefCell<CmdLine> = AtomicRefCell::new(CmdLine::new());
 
-    let mut setup_sects = buf[header_start] as usize;
+struct CmdLine {
+    region: MemoryRegion,
+    length: usize, // Does not include null pointer
+}
 
-    if setup_sects == 0 {
-        setup_sects = 4;
+impl CmdLine {
+    const fn new() -> Self {
+        Self {
+            region: MemoryRegion::new(CMDLINE_START, CMDLINE_MAX_LEN),
+            length: 0,
+        }
     }
 
-    setup_sects += 1; // Include the boot sector
-
-    let setup_bytes = setup_sects * 512; // Use to start reading the main image
-
-    let mut load_offset = u64::from(KERNEL_LOCATION);
-
-    f.seek(setup_bytes as u32)?;
-
-    loop {
-        let mut dst = crate::mem::MemoryRegion::new(load_offset, 512);
-        let dst = dst.as_mut_slice(0, 512);
+    const fn len(&self) -> u32 {
+        self.length as u32
+    }
 
-        match f.read(dst) {
-            Err(crate::fat::Error::EndOfFile) => {
-                // 0x200 is the startup_64 offset
-                return Ok(u64::from(KERNEL_LOCATION) + 0x200);
-            }
-            Err(e) => return Err(Error::FileError(e)),
-            Ok(_) => {}
-        };
+    fn append(&mut self, args: &[u8]) {
+        let bytes = self.region.as_bytes();
+        bytes[self.length] = b' ';
+        self.length += 1;
 
-        load_offset += 512;
+        bytes[self.length..self.length + args.len()].copy_from_slice(args);
+        self.length += args.len();
+        bytes[self.length] = 0;
     }
 }

src/loader.rs

@@ -13,7 +13,8 @@
 // limitations under the License.
 
 use crate::{
-    bzimage,
+    boot,
+    bzimage::{self, Kernel},
     common::ascii_strip,
     fat::{self, Read},
 };
@@ -121,7 +122,7 @@ fn default_entry_path(fs: &fat::Filesystem) -> Result<[u8; 260], fat::Error> {
     Ok(entry_path)
 }
 
-pub fn load_default_entry(fs: &fat::Filesystem) -> Result<u64, Error> {
+pub fn load_default_entry(fs: &fat::Filesystem, info: &dyn boot::Info) -> Result<Kernel, Error> {
     let default_entry_path = default_entry_path(&fs)?;
     let default_entry_path = ascii_strip(&default_entry_path);
 
@@ -132,19 +133,20 @@ pub fn load_default_entry(fs: &fat::Filesystem) -> Result<u64, Error> {
     let initrd_path = ascii_strip(&entry.initrd_path);
     let cmdline = ascii_strip(&entry.cmdline);
 
+    let mut kernel = Kernel::new(info);
+
     let mut bzimage_file = fs.open(bzimage_path)?;
-    let jump_address = bzimage::load_kernel(&mut bzimage_file)?;
+    kernel.load_kernel(&mut bzimage_file)?;
 
     if !initrd_path.is_empty() {
         let mut initrd_file = fs.open(initrd_path)?;
-        bzimage::load_initrd(&mut initrd_file)?;
+        kernel.load_initrd(&mut initrd_file)?;
     }
 
-    if !cmdline.is_empty() {
-        bzimage::append_commandline(cmdline)?
-    }
+    kernel.append_cmdline(info.cmdline());
+    kernel.append_cmdline(cmdline.as_bytes());
 
-    Ok(jump_address)
+    Ok(kernel)
 }
 
 #[cfg(test)]