SchOS/kernel/kernel.ld

/* Simple linker script for the JOS kernel.
   See the GNU ld 'info' manual ("info ld") to learn the syntax. */

/* OUTPUT_FORMAT("elf32-i386", "elf32-i386", "elf32-i386") */
/* OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64", "elf64-x86-64") */
OUTPUT_FORMAT("elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)

mboot_load_addr = 0x00100000;

SECTIONS
{
  /* Link the kernel at this address: "." means the current address */
  /* Must be equal to KERNLINK */
  . = 0xFFFF800000100000;

  PROVIDE(begin = .);

  /* The .l* section names are the x86-64 psABI's "large" sections, and they
   * are not optional decoration: with -mcmodel=large both LLVM and GCC 13+
   * put nearly all code and data into .ltext/.lrodata/.ldata/.lbss rather
   * than the classic names.
   *
   * A script that does not name them still links. The large sections become
   * orphans placed after everything the script did name, which puts them past
   * PROVIDE(end = .) -- so kinit1(end, ...) hands the page allocator a megabyte
   * of live .lbss and the kernel destroys itself within a few instructions of
   * its first printf. That is exactly how this was found: the boot printed one
   * line and triple-faulted in a loop.
   *
   * Matched alongside their small counterparts so the layout is unchanged for
   * a toolchain that does not use them. */
  .text : AT(mboot_load_addr) {
    *(.text .rela.text .stub .text.* .gnu.linkonce.t.*)
    *(.ltext .ltext.*)
  }

  PROVIDE(etext = .);	/* Define the 'etext' symbol to this value */

  .rodata : {
    *(.rodata .rodata.* .gnu.linkonce.r.*)
    *(.lrodata .lrodata.*)
  }

  /* Thread-local storage.
   *
   * The kernel really does use TLS: `__thread struct cpu *cpu` and
   * `__thread struct proc *proc` (kernel/arch/x86_64/vm.c), reached through
   * FS_BASE, which seginit() points at kernel_fs_base.
   *
   * These output sections are what make the linker emit a PT_TLS segment, and
   * without a PT_TLS the local-exec relocations have nothing to resolve
   * against: lld left every access as a bare `movq %fs:0x0`, so `cpu` and
   * `proc` both aliased the thread pointer itself and the kernel died a few
   * instructions into seginit. GNU ld happened to place an orphan .tbss well
   * enough to work; that was luck, not correctness.
   *
   * Order matters and is checked by more than C: trapasm.S hardcodes `proc` at
   * %fs:(-8). With cpu at TLS offset 0 and proc at 8 in a 16-byte block, the
   * local-exec offsets come out as -16 and -8, which is the layout that file
   * assumes. */
  .tdata : { *(.tdata .tdata.* .gnu.linkonce.td.*) }
  .tbss  : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }

  /* Adjust the address for the data segment to the next page */
  . = ALIGN(0x1000);

  /* Conventionally, Unix linkers provide pseudo-symbols
   * etext, edata, and end, at the end of the text, data, and bss.
   * For the kernel mapping, we need the address at the beginning
   * of the data section, but that's not one of the conventional
   * symbols, because the convention started before there was a
   * read-only rodata section between text and data. */
  PROVIDE(data = .);

  /* The data segment */
  .data : {
    *(.data .data.*)
    *(.ldata .ldata.*)
  }

  /* The kernel symbol table, generated by tools/ksyms.pl between the two link
   * passes. See kernel/core/ksym.c.
   *
   * The position is the whole trick and it is not cosmetic: everything the
   * table records is a *text* address, and .ksym sits after .text and .rodata,
   * so adding several tens of kilobytes here cannot move a single entry in it.
   * That is what makes two passes enough -- pass one is read to build the
   * table, pass two carries it, and pass two's text is identical to pass one's.
   * Put this before .text and the table describes a kernel that no longer
   * exists, subtly, with every address off by the size of the table.
   *
   * It is inside the loaded image (before edata), not in .bss, because it has
   * contents. mboot_load_end is computed from edata and would otherwise not
   * cover it. */
  . = ALIGN(8);
  .ksym : {
    *(.ksym .ksym.*)
  }

  . = ALIGN(0x1000);

  PROVIDE(edata = .);

  .bss : {
    *(.bss .bss.*)
    *(.lbss .lbss.*)
      *(COMMON)
  }

  . = ALIGN(0x1000);

  PROVIDE(end = .);

  /DISCARD/ : {
    *(.eh_frame .rela.eh_frame .note.GNU-stack)
  }
}

mboot_load_end = mboot_load_addr + (edata - begin);
mboot_bss_end = mboot_load_addr + (end - begin);
mboot_entry_addr = mboot_load_addr + (mboot_entry - begin);