Merge pull request #34 from achilleasa/remove-hardcoded-g-offsets-from-rt0-code

Remove hardcoded g offsets from rt0 code and use runtime.g0/m0

Makefile

@@ -22,7 +22,7 @@ GOROOT := $(shell $(GO) env GOROOT)
 
 GC_FLAGS ?=
 LD_FLAGS := -n -T $(BUILD_DIR)/linker.ld -static --no-ld-generated-unwind-info
-AS_FLAGS := -g -f elf64 -F dwarf -I arch/$(ARCH)/asm/ -dNUM_REDIRECTS=$(shell $(GO) run tools/redirects/redirects.go count)
+AS_FLAGS := -g -f elf64 -F dwarf -I $(BUILD_DIR)/ -I arch/$(ARCH)/asm/ -dNUM_REDIRECTS=$(shell $(GO) run tools/redirects/redirects.go count)
 
 MIN_OBJCOPY_VERSION := 2.26.0
 HAVE_VALID_OBJCOPY := $(shell objcopy -V | head -1 | awk -F ' ' '{print "$(MIN_OBJCOPY_VERSION)\n" $$NF}' | sort -ct. -k1,1n -k2,2n && echo "y")
@@ -38,7 +38,7 @@ kernel_image: $(kernel_target)
 	@echo "[tools:redirects] populating kernel image redirect table"
 	@$(GO) run tools/redirects/redirects.go populate-table $(kernel_target)
 	
-$(kernel_target): $(asm_obj_files) linker_script go.o
+$(kernel_target): asm_files linker_script go.o
 	@echo "[$(LD)] linking kernel-$(ARCH).bin"
 	@$(LD) $(LD_FLAGS) -o $(kernel_target) $(asm_obj_files) $(BUILD_DIR)/go.o
 
@@ -65,6 +65,9 @@ go.o:
 	@objcopy \
 		--add-symbol kernel.Kmain=.text:0x`nm $(BUILD_DIR)/go.o | grep "kmain.Kmain$$" | cut -d' ' -f1` \
 		--globalize-symbol _rt0_interrupt_handlers \
+		--globalize-symbol runtime.g0 \
+		--globalize-symbol runtime.m0 \
+		--globalize-symbol runtime.physPageSize \
 		 $(BUILD_DIR)/go.o $(BUILD_DIR)/go.o
 
 binutils_version_check:
@@ -86,11 +89,19 @@ linker_script:
 		-E -x \
 		c arch/$(ARCH)/script/linker.ld.in | grep -v "^#" > $(BUILD_DIR)/linker.ld
 
+$(BUILD_DIR)/go_asm_offsets.inc:
+	@mkdir -p $(BUILD_DIR)
+	
+	@echo "[tools:offsets] calculating OS/arch-specific offsets for g, m and stack structs"
+	@$(GO) run tools/offsets/offsets.go -target-os $(GOOS) -target-arch $(GOARCH) -go-binary $(GO) -out $@
+
 $(BUILD_DIR)/arch/$(ARCH)/asm/%.o: arch/$(ARCH)/asm/%.s
 	@mkdir -p $(shell dirname $@)
 	@echo "[$(AS)] $<"
 	@$(AS) $(AS_FLAGS) $< -o $@
 
+asm_files: $(BUILD_DIR)/go_asm_offsets.inc $(asm_obj_files)
+
 iso: $(iso_target)
 
 $(iso_target): iso_prereq kernel_image

arch/x86_64/asm/rt0_64.s

@@ -20,12 +20,9 @@ _rt0_idt_desc:
 ; Allocates space for the IRQ handlers pointers registered by the IRQ package 
 _rt0_irq_handlers resq IDT_ENTRIES
 
-r0_g_ptr:	  resq 1 ; fs:0x00 is a pointer to the current g struct
+; The FS register is loaded with the address of r0_g_ptr. fs:0x00 should contain 
-r0_g:
+; a pointer to the currently active g struct (in this case runtime.g0)
-r0_g_stack_lo:    resq 1
+r0_g_ptr:	  resq 1 
-r0_g_stack_hi:    resq 1
-r0_g_stackguard0: resq 1  ; rsp compared to this value in go stack growth prologue
-r0_g_stackguard1: resq 1  ; rsp compared to this value in C stack growth prologue
 
 section .text 
 
@@ -42,34 +39,7 @@ global _rt0_64_entry
 _rt0_64_entry:
 	call _rt0_install_redirect_trampolines
 	call _rt0_64_load_idt
-
+	call _rt0_64_setup_go_runtime_structs
-	; According to the x86_64 ABI, the fs:0 should point to the address of 
-	; the user-space thread structure. The actual TLS structure is located 
-	; just before that (aligned). Go code tries to fetch the address to the 
-	; active go-routine's g struct by accessing fs:-8. What we need to do 
-	; is to setup a mock g0 struct, populate its stack_lo/hi/guard fields 
-	; and then use wrmsr to update the FS register
-	extern stack_top 
-	extern stack_bottom
-	
-	; Setup r0_g
-	mov rax, stack_bottom
-	mov rbx, stack_top
-	mov rsi, r0_g
-	mov qword [rsi+0], rax   ; stack_lo
-	mov qword  [rsi+8], rbx  ; stack_hi
-	mov qword [rsi+16], rax  ; stackguard0
-	mov rax, r0_g_ptr
-	mov qword [rax], rsi
-	
-	; Load 64-bit FS register address 
-	; rax -> lower 32 bits 
-	; rdx -> upper 32 bits
-	mov ecx, 0xc0000100  ; fs_base
-	mov rax, rsi         ; lower 32 bits 
-	shr rsi, 32
-	mov rdx, rsi         ; high 32 bits
-	wrmsr
 
 	; Call the kernel entry point passing a pointer to the multiboot data
 	; copied by the 32-bit entry code
@@ -93,6 +63,58 @@ _rt0_64_entry:
 	cli
 	hlt
 
+;------------------------------------------------------------------------------
+; Setup m0, g0 and other symbols required for bootstrapping the Go runtime.
+; For the definitions of g and m see the Go runtime src: src/runtime/runtime2.go
+;------------------------------------------------------------------------------
+_rt0_64_setup_go_runtime_structs:
+	%include "go_asm_offsets.inc" ; generated by tools/offsets
+
+	; The Go allocator expects this symbol to be set to the system page size
+	; As the kernel bypass osinit() this needs to be set here.
+	extern runtime.physPageSize
+	mov rax, runtime.physPageSize
+	mov qword [rax], 0x1000 ; 4096
+	
+	; Setup r0_g stack limits using the reserved stack 
+	extern stack_top
+	extern stack_bottom
+	extern runtime.g0
+	mov rax, stack_bottom
+	mov rbx, stack_top
+	mov rsi, runtime.g0
+	mov qword [rsi+GO_G_STACK+GO_STACK_LO], rax   ; g.stack.lo
+	mov qword [rsi+GO_G_STACK+GO_STACK_HI], rbx   ; g.stack.hi
+	mov qword [rsi+GO_G_STACKGUARD0], rax         ; g.stackguard0
+
+	; Link m0 to the g0
+	extern runtime.m0
+	mov rbx, runtime.m0
+	mov qword [rbx+GO_M_G0], rsi       ; m.g0 = g0
+	mov qword [rsi+GO_G_M], rbx        ; g.m = m
+
+	; Store the address of g0 in r0_g_ptr 
+	mov rax, r0_g_ptr
+	mov qword [rax], rsi
+	
+	; According to the x86_64 ABI, the fs register should contain the
+	; address after the pointer to the pointer to the user-space thread
+	; structure. This allows the Go runtime to retrieve the address of 
+	; the currently active g structure by accessing fs:-0x8.
+	;
+	; Load 64-bit FS register address 
+	; eax -> lower 32 bits 
+	; edx -> upper 32 bits
+	mov ecx, 0xc0000100  ; fs_base
+	mov rsi, r0_g_ptr
+	add rsi, 8	     ; fs -> r0_g_ptr + 0x8
+	mov rax, rsi         ; lower 32 bits 
+	shr rsi, 32
+	mov rdx, rsi         ; high 32 bits
+	wrmsr
+
+	ret
+
 
 ;------------------------------------------------------------------------------
 ; Setup and load IDT. We preload each IDT entry with a pointer to a gate handler 

tools/offsets/offsets.go

@@ -0,0 +1,192 @@
+package main
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func exit(err error) {
+	fmt.Fprintf(os.Stderr, "[offsets] error: %s\n", err.Error())
+	os.Exit(1)
+}
+
+func genBuildScript(targetOS, targetArch, goBinary, workDir string) ([]byte, error) {
+	// Write a dummy program in workDir so "go build" does not complain
+	dummyGoProgram := []byte("package main\n func main(){}")
+	err := ioutil.WriteFile(fmt.Sprintf("%s/main.go", workDir), dummyGoProgram, os.ModePerm)
+	if err != nil {
+		return nil, err
+	}
+
+	// Run "go build -a -n" in workDir and capture the output. The -a flag
+	// ensures that the generated build script includes steps to always
+	// rebuild the runtime packages.
+	cmd := exec.Command(goBinary, "build", "-a", "-n")
+	cmd.Dir = workDir
+	cmd.Env = append(cmd.Env, fmt.Sprintf("GOOS=%s", targetOS))
+	cmd.Env = append(cmd.Env, fmt.Sprintf("GOARCH=%s", targetArch))
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate build script\nMore info:\n%s", out)
+	}
+
+	return out, nil
+}
+
+func patchBuildScript(script []byte, workDir, targetOS, targetArch, goBinary string) ([]byte, error) {
+	lines := strings.Split(string(script), "\n")
+
+	// Inject os/arch and workdir to the top of the build file
+	header := []string{
+		fmt.Sprintf("export GOOS=%s", targetOS),
+		fmt.Sprintf("export GOARCH=%s", targetArch),
+		fmt.Sprintf("WORK=%q", workDir),
+		fmt.Sprintf("alias pack='%s tool pack'", goBinary),
+	}
+	lines = append(header, lines...)
+
+	// We are only interested in building the runtime as this block generates
+	// the asm headers. Scan the lines till we find "# runtime" comment and
+	// stop at next comment
+	var stopOnNextComment bool
+	for lineIndex := 0; lineIndex < len(lines); lineIndex++ {
+		// Ignore empty comments
+		if lines[lineIndex] == "#" {
+			continue
+		}
+
+		if stopOnNextComment && strings.HasPrefix(lines[lineIndex], "#") {
+			return []byte(strings.Join(lines[:lineIndex], "\n")), nil
+		}
+
+		if lines[lineIndex] == "# runtime" {
+			stopOnNextComment = true
+		}
+	}
+
+	return nil, errors.New("generated build file does not specify -asmhdr when building the runtime")
+}
+
+func execBuildScript(script []byte, workDir string) error {
+	f, err := os.Create(fmt.Sprintf("%s/build.sh", workDir))
+	if err != nil {
+		return err
+	}
+
+	_, err = f.Write(script)
+	if err != nil {
+		f.Close()
+		return err
+	}
+	f.Close()
+
+	cmd := exec.Command("sh", f.Name())
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to execute build script\nMore info:\n%s", out)
+	}
+
+	return nil
+}
+
+func genAsmIncludes(workDir string) ([]byte, error) {
+	headers, err := ioutil.ReadFile(fmt.Sprintf("%s/runtime/_obj/go_asm.h", workDir))
+	if err != nil {
+		return nil, err
+	}
+
+	var includes []string
+	includes = append(includes, "; vim: set ft=nasm :\n")
+	includes = append(includes, fmt.Sprintf("; generated by tools/offsets at %v\n", time.Now()))
+
+	for _, line := range strings.Split(string(headers), "\n") {
+		line = strings.TrimPrefix(line, "#define ")
+
+		// We are only interested in the offsets for the g, m and stack structures
+		if strings.HasPrefix(line, "g_") || strings.HasPrefix(line, "m_") || strings.HasPrefix(line, "stack_") {
+			tokens := strings.Fields(line)
+			if len(tokens) != 2 {
+				continue
+			}
+
+			offset, err := strconv.ParseInt(tokens[1], 10, 32)
+			if err != nil {
+				continue
+			}
+
+			includes = append(includes,
+				fmt.Sprintf("GO_%s equ 0x%x ; %d",
+					strings.ToUpper(tokens[0]),
+					offset,
+					offset,
+				),
+			)
+		}
+	}
+
+	return []byte(strings.Join(includes, "\n")), nil
+}
+
+func runTool() error {
+	targetOS := flag.String("target-os", "", "a valid GOOS value for generating the asm offsets")
+	targetArch := flag.String("target-arch", "", "a valid GOARCH value for generating the asm offsets")
+	goBinary := flag.String("go-binary", "go", "the Go binary to use")
+	asmOutput := flag.String("out", "-", "a file to write the asm headers or - to output to STDOUT")
+	flag.Parse()
+
+	switch {
+	case *targetOS == "":
+		exit(errors.New("-target-os parameter missing"))
+	case *targetArch == "":
+		exit(errors.New("-target-arch parameter missing"))
+	}
+
+	workDir, err := ioutil.TempDir("", "offsets-tool")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(workDir)
+
+	buildScript, err := genBuildScript(*targetOS, *targetArch, *goBinary, workDir)
+	if err != nil {
+		return err
+	}
+
+	buildScript, err = patchBuildScript(buildScript, workDir, *targetOS, *targetArch, *goBinary)
+	if err != nil {
+		return err
+	}
+
+	if err = execBuildScript(buildScript, workDir); err != nil {
+		return err
+	}
+
+	asmIncludes, err := genAsmIncludes(workDir)
+	if err != nil {
+		return err
+	}
+
+	switch *asmOutput {
+	case "-":
+		fmt.Printf("%s\n", string(asmIncludes))
+	default:
+		if err = ioutil.WriteFile(*asmOutput, asmIncludes, os.ModePerm); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func main() {
+	if err := runTool(); err != nil {
+		exit(err)
+	}
+}