cmd/link: mmap output file

Use mmap for writing most of the output file content,
specifically, the sections and segments. After layout, we
already know the sizes and file offsets for the sections and
segments. So we can just write the bytes by copying to a mmap'd
backing store.

The writing of the output file is split into two parts. The first
part writes the sections and segments to the mmap'd region. The
second part writes some extra content, for which we don't know
the size, so we use direct file IO.

This is in preparation for mmap'ing input files read-only.

Change-Id: I9f3b4616a9f96bfd5c940d74c50aacd6d330f7d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/170738
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/link/internal/amd64/asm.go

@@ -704,7 +704,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	machlink := int64(0)
 	if ctxt.HeadType == objabi.Hdarwin {
 		machlink = ld.Domacholink(ctxt)

src/cmd/link/internal/amd64/obj.go

@@ -54,6 +54,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/arm/asm.go

@@ -800,7 +800,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	machlink := uint32(0)
 	if ctxt.HeadType == objabi.Hdarwin {
 		machlink = uint32(ld.Domacholink(ctxt))

src/cmd/link/internal/arm/obj.go

@@ -52,6 +52,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archrelocvariant: archrelocvariant,
 		Trampoline:       trampoline,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/arm64/asm.go

@@ -426,7 +426,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	machlink := uint32(0)
 	if ctxt.HeadType == objabi.Hdarwin {
 		machlink = uint32(ld.Domacholink(ctxt))

src/cmd/link/internal/arm64/obj.go

@@ -51,6 +51,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/ld/data.go

@@ -792,6 +792,7 @@ func Datblk(ctxt *Link, addr int64, size int64) {
 	writeDatblkToOutBuf(ctxt, ctxt.Out, addr, size)
 }
 
+// Used only on Wasm for now.
 func DatblkBytes(ctxt *Link, addr int64, size int64) []byte {
 	buf := bytes.NewBuffer(make([]byte, 0, size))
 	out := &OutBuf{w: bufio.NewWriter(buf)}
@@ -2319,7 +2320,8 @@ func (ctxt *Link) address() []*sym.Segment {
 }
 
 // layout assigns file offsets and lengths to the segments in order.
-func (ctxt *Link) layout(order []*sym.Segment) {
+// Returns the file size containing all the segments.
+func (ctxt *Link) layout(order []*sym.Segment) uint64 {
 	var prev *sym.Segment
 	for _, seg := range order {
 		if prev == nil {
@@ -2348,7 +2350,7 @@ func (ctxt *Link) layout(order []*sym.Segment) {
 		}
 		prev = seg
 	}
-
+	return prev.Fileoff + prev.Filelen
 }
 
 // add a trampoline with symbol s (to be laid down after the current function)

src/cmd/link/internal/ld/lib.go

@@ -128,7 +128,14 @@ type Arch struct {
 	Archrelocvariant func(link *Link, rel *sym.Reloc, sym *sym.Symbol,
 		offset int64) (relocatedOffset int64)
 	Trampoline func(*Link, *sym.Reloc, *sym.Symbol)
+
+	// Asmb and Asmb2 are arch-specific routines that write the output
+	// file. Typically, Asmb writes most of the content (sections and
+	// segments), for which we have computed the size and offset. Asmb2
+	// writes the rest.
 	Asmb  func(*Link)
+	Asmb2 func(*Link)
+
 	Elfreloc1   func(*Link, *sym.Reloc, int64) bool
 	Elfsetupplt func(*Link)
 	Gentext     func(*Link)
@@ -261,7 +268,7 @@ func libinit(ctxt *Link) {
 	Lflag(ctxt, filepath.Join(objabi.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", objabi.GOOS, objabi.GOARCH, suffixsep, suffix)))
 
 	mayberemoveoutfile()
-	f, err := os.OpenFile(*flagOutfile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
+	f, err := os.OpenFile(*flagOutfile, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0775)
 	if err != nil {
 		Exitf("cannot create %s: %v", *flagOutfile, err)
 	}
@@ -1014,7 +1021,7 @@ func hostlinksetup(ctxt *Link) {
 
 	p := filepath.Join(*flagTmpdir, "go.o")
 	var err error
-	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
+	f, err := os.OpenFile(p, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0775)
 	if err != nil {
 		Exitf("cannot create %s: %v", p, err)
 	}

src/cmd/link/internal/ld/main.go

@@ -241,8 +241,29 @@ func Main(arch *sys.Arch, theArch Arch) {
 	order := ctxt.address()
 	ctxt.reloc()
 	dwarfcompress(ctxt)
-	ctxt.layout(order)
+	filesize := ctxt.layout(order)
+
+	// Write out the output file.
+	// It is split into two parts (Asmb and Asmb2). The first
+	// part writes most of the content (sections and segments),
+	// for which we have computed the size and offset, in a
+	// mmap'd region. The second part writes more content, for
+	// which we don't know the size.
+	var outputMmapped bool
+	if ctxt.Arch.Family != sys.Wasm {
+		// Don't mmap if we're building for Wasm. Wasm file
+		// layout is very different so filesize is meaningless.
+		err := ctxt.Out.Mmap(filesize)
+		outputMmapped = err == nil
+	}
+	if outputMmapped {
+		thearch.Asmb(ctxt)
+		ctxt.Out.Munmap()
+	} else {
 		thearch.Asmb(ctxt)
+	}
+	thearch.Asmb2(ctxt)
+
 	ctxt.undef()
 	ctxt.hostlink()
 	ctxt.archive()

src/cmd/link/internal/ld/outbuf.go

@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"cmd/internal/sys"
 	"encoding/binary"
+	"log"
 	"os"
 )
 
@@ -20,10 +21,18 @@ import (
 //
 // Second, it provides a very cheap offset counter that doesn't require
 // any system calls to read the value.
+//
+// It also mmaps the output file (if available). The intended usage is:
+// - Mmap the output file
+// - Write the content
+// - possibly apply any edits in the output buffer
+// - Munmap the output file
+// - possibly write more content to the file, which will not be edited later.
 type OutBuf struct {
 	arch   *sys.Arch
 	off    int64
 	w      *bufio.Writer
+	buf    []byte // backing store of mmap'd output file
 	f      *os.File
 	encbuf [8]byte // temp buffer used by WriteN methods
 }
@@ -32,10 +41,12 @@ func (out *OutBuf) SeekSet(p int64) {
 	if p == out.off {
 		return
 	}
+	if out.buf == nil {
 		out.Flush()
 		if _, err := out.f.Seek(p, 0); err != nil {
 			Exitf("seeking to %d in %s: %v", p, out.f.Name(), err)
 		}
+	}
 	out.off = p
 }
 
@@ -49,12 +60,22 @@ func (out *OutBuf) Offset() int64 {
 // to explicitly handle the returned error as long as Flush is
 // eventually called.
 func (out *OutBuf) Write(v []byte) (int, error) {
+	if out.buf != nil {
+		n := copy(out.buf[out.off:], v)
+		out.off += int64(n)
+		return n, nil
+	}
 	n, err := out.w.Write(v)
 	out.off += int64(n)
 	return n, err
 }
 
 func (out *OutBuf) Write8(v uint8) {
+	if out.buf != nil {
+		out.buf[out.off] = v
+		out.off++
+		return
+	}
 	if err := out.w.WriteByte(v); err == nil {
 		out.off++
 	}
@@ -92,6 +113,14 @@ func (out *OutBuf) Write64b(v uint64) {
 }
 
 func (out *OutBuf) WriteString(s string) {
+	if out.buf != nil {
+		n := copy(out.buf[out.off:], s)
+		if n != len(s) {
+			log.Fatalf("WriteString truncated. buffer size: %d, offset: %d, len(s)=%d", len(out.buf), out.off, len(s))
+		}
+		out.off += int64(n)
+		return
+	}
 	n, _ := out.w.WriteString(s)
 	out.off += int64(n)
 }
@@ -120,7 +149,13 @@ func (out *OutBuf) WriteStringPad(s string, n int, pad []byte) {
 }
 
 func (out *OutBuf) Flush() {
-	if err := out.w.Flush(); err != nil {
+	var err error
+	if out.buf != nil {
+		err = out.Msync()
+	} else {
+		err = out.w.Flush()
+	}
+	if err != nil {
 		Exitf("flushing %s: %v", out.f.Name(), err)
 	}
 }

src/cmd/link/internal/ld/outbuf_mmap.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux openbsd
+
+package ld
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func (out *OutBuf) Mmap(filesize uint64) error {
+	err := out.f.Truncate(int64(filesize))
+	if err != nil {
+		Exitf("resize output file failed: %v", err)
+	}
+	out.buf, err = syscall.Mmap(int(out.f.Fd()), 0, int(filesize), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED|syscall.MAP_FILE)
+	return err
+}
+
+func (out *OutBuf) Munmap() {
+	err := out.Msync()
+	if err != nil {
+		Exitf("msync output file failed: %v", err)
+	}
+	syscall.Munmap(out.buf)
+	out.buf = nil
+	_, err = out.f.Seek(out.off, 0)
+	if err != nil {
+		Exitf("seek output file failed: %v", err)
+	}
+}
+
+func (out *OutBuf) Msync() error {
+	// TODO: netbsd supports mmap and msync, but the syscall package doesn't define MSYNC.
+	// It is excluded from the build tag for now.
+	_, _, errno := syscall.Syscall(syscall.SYS_MSYNC, uintptr(unsafe.Pointer(&out.buf[0])), uintptr(len(out.buf)), syscall.MS_SYNC)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/cmd/link/internal/ld/outbuf_nommap.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !darwin,!dragonfly,!freebsd,!linux,!openbsd
+
+package ld
+
+import "errors"
+
+var errNotSupported = errors.New("mmap not supported")
+
+func (out *OutBuf) Mmap(filesize uint64) error { return errNotSupported }
+func (out *OutBuf) Munmap()                    { panic("unreachable") }
+func (out *OutBuf) Msync() error               { panic("unreachable") }

src/cmd/link/internal/mips/asm.go

@@ -197,7 +197,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	/* output symbol table */
 	ld.Symsize = 0
 

src/cmd/link/internal/mips/obj.go

@@ -54,6 +54,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/mips64/asm.go

@@ -209,7 +209,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	/* output symbol table */
 	ld.Symsize = 0
 

src/cmd/link/internal/mips64/obj.go

@@ -53,6 +53,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/ppc64/asm.go

@@ -1103,7 +1103,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	/* output symbol table */
 	ld.Symsize = 0
 

src/cmd/link/internal/ppc64/obj.go

@@ -54,6 +54,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/s390x/asm.go

@@ -540,7 +540,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	/* output symbol table */
 	ld.Symsize = 0
 

src/cmd/link/internal/s390x/obj.go

@@ -51,6 +51,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,  // in asm.go
+		Asmb2:            asmb2, // in asm.go
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,

src/cmd/link/internal/wasm/asm.go

@@ -92,9 +92,11 @@ func assignAddress(ctxt *ld.Link, sect *sym.Section, n int, s *sym.Symbol, va ui
 	return sect, n, va
 }
 
+func asmb(ctxt *ld.Link) {} // dummy
+
 // asmb writes the final WebAssembly module binary.
 // Spec: https://webassembly.github.io/spec/core/binary/modules.html
-func asmb(ctxt *ld.Link) {
+func asmb2(ctxt *ld.Link) {
 	if ctxt.Debugvlog != 0 {
 		ctxt.Logf("%5.2f asmb\n", ld.Cputime())
 	}

src/cmd/link/internal/wasm/obj.go

@@ -18,6 +18,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archinit:      archinit,
 		AssignAddress: assignAddress,
 		Asmb:          asmb,
+		Asmb2:         asmb2,
 		Gentext:       gentext,
 	}
 

src/cmd/link/internal/x86/asm.go

@@ -662,7 +662,9 @@ func asmb(ctxt *ld.Link) {
 
 	ctxt.Out.SeekSet(int64(ld.Segdwarf.Fileoff))
 	ld.Dwarfblk(ctxt, int64(ld.Segdwarf.Vaddr), int64(ld.Segdwarf.Filelen))
+}
 
+func asmb2(ctxt *ld.Link) {
 	machlink := uint32(0)
 	if ctxt.HeadType == objabi.Hdarwin {
 		machlink = uint32(ld.Domacholink(ctxt))

src/cmd/link/internal/x86/obj.go

@@ -51,6 +51,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Archreloc:        archreloc,
 		Archrelocvariant: archrelocvariant,
 		Asmb:             asmb,
+		Asmb2:            asmb2,
 		Elfreloc1:        elfreloc1,
 		Elfsetupplt:      elfsetupplt,
 		Gentext:          gentext,