Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

kernel/kernel.ld

@@ -0,0 +1,104 @@
+/* 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.*)
+  }
+
+  . = 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);