net: use libSystem bindings for DNS resolution on macos if cgo is unavailable

This change adds directives to link the res_search function in libSystem.
The corresponding Go function is then used in `lookup_darwin.go` for
resolution when cgo is disabled. This makes DNS resolution logic more
reliable as macOS has some unique quirks such as the `/etc/resolver/`
directory for specifying nameservers.

Fixes #12524

Change-Id: I367263c4951383965b3ef6491196152f78e614b1
GitHub-Last-Rev: 3c3ff6bfa7e4811f206f3b119a867c841a016e10
GitHub-Pull-Request: golang/go#30686
Reviewed-on: https://go-review.googlesource.com/c/go/+/166297
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/net/cgo_darwin_stub.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 !netgo,!cgo
+// +build darwin
+
+package net
+
+import (
+	"context"
+	"errors"
+	"sync"
+
+	"golang.org/x/net/dns/dnsmessage"
+)
+
+type addrinfoErrno int
+
+func (eai addrinfoErrno) Error() string   { return "<nil>" }
+func (eai addrinfoErrno) Temporary() bool { return false }
+func (eai addrinfoErrno) Timeout() bool   { return false }
+
+func cgoLookupHost(ctx context.Context, name string) (addrs []string, err error, completed bool) {
+	resources, err := resolverGetResources(ctx, name, int32(dnsmessage.TypeALL), int32(dnsmessage.ClassINET))
+	if err != nil {
+		return
+	}
+	addrs, err = parseHostsFromResources(resources)
+	if err != nil {
+		return
+	}
+	return addrs, nil, true
+}
+
+func cgoLookupPort(ctx context.Context, network, service string) (port int, err error, completed bool) {
+	port, err = goLookupPort(network, service) // we can just use netgo lookup
+	return port, err, err == nil
+}
+
+func cgoLookupIP(ctx context.Context, network, name string) (addrs []IPAddr, err error, completed bool) {
+
+	var resources []dnsmessage.Resource
+	switch ipVersion(network) {
+	case '4':
+		resources, err = resolverGetResources(ctx, name, int32(dnsmessage.TypeA), int32(dnsmessage.ClassINET))
+	case '6':
+		resources, err = resolverGetResources(ctx, name, int32(dnsmessage.TypeAAAA), int32(dnsmessage.ClassINET))
+	default:
+		resources, err = resolverGetResources(ctx, name, int32(dnsmessage.TypeALL), int32(dnsmessage.ClassINET))
+	}
+	if err != nil {
+		return
+	}
+
+	addrs, err = parseIPsFromResources(resources)
+	if err != nil {
+		return
+	}
+
+	return addrs, nil, true
+}
+
+func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error, completed bool) {
+	resources, err := resolverGetResources(ctx, name, int32(dnsmessage.TypeCNAME), int32(dnsmessage.ClassINET))
+	if err != nil {
+		return
+	}
+	cname, err = parseCNAMEFromResources(resources)
+	if err != nil {
+		return "", err, false
+	}
+	return cname, nil, true
+}
+
+func cgoLookupPTR(ctx context.Context, addr string) (ptrs []string, err error, completed bool) {
+	resources, err := resolverGetResources(ctx, addr, int32(dnsmessage.TypePTR), int32(dnsmessage.ClassINET))
+	if err != nil {
+		return
+	}
+	ptrs, err = parsePTRsFromResources(resources)
+	if err != nil {
+		return
+	}
+	return ptrs, nil, true
+}
+
+var (
+	resInitOnce sync.Once
+	errCode     int32
+)
+
+// resolverGetResources will make a call to the 'res_search' routine in libSystem
+// and parse the output as a slice of resource resources which can then be parsed
+func resolverGetResources(ctx context.Context, hostname string, rtype, class int32) ([]dnsmessage.Resource, error) {
+
+	resInitOnce.Do(func() {
+		errCode = res_init()
+	})
+	if errCode < 0 {
+		return nil, errors.New("could not initialize name resolver data")
+	}
+
+	var byteHostname = []byte(hostname)
+	var responseBuffer [512]byte
+	var size int32
+
+	size, errCode = res_search(&byteHostname[0], class, rtype, &responseBuffer[0], int32(len(responseBuffer)))
+	if errCode != 0 {
+		return nil, errors.New("could not complete domain resolution return code " + string(errCode))
+	}
+	if size == 0 {
+		return nil, errors.New("received empty response")
+	}
+
+	var msg dnsmessage.Message
+	err := msg.Unpack(responseBuffer[:])
+	if err != nil {
+		return nil, err
+	}
+
+	var dnsParser dnsmessage.Parser
+	if _, err := dnsParser.Start(responseBuffer[:]); err != nil {
+		return nil, err
+	}
+
+	var resources []dnsmessage.Resource
+	for {
+		r, err := dnsParser.Answer()
+		if err == dnsmessage.ErrSectionDone {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+		resources = append(resources, r)
+	}
+	return resources, nil
+}
+
+func parseHostsFromResources(resources []dnsmessage.Resource) ([]string, error) {
+	var answers []string
+
+	for i := range resources {
+		switch resources[i].Header.Type {
+		case dnsmessage.TypeA:
+			b := resources[i].Body.(*dnsmessage.AResource)
+			answers = append(answers, string(b.A[:]))
+		case dnsmessage.TypeAAAA:
+			b := resources[i].Body.(*dnsmessage.AAAAResource)
+			answers = append(answers, string(b.AAAA[:]))
+		default:
+			return nil, errors.New("could not parse an A or AAAA response from message buffer")
+		}
+	}
+	return answers, nil
+}
+
+func parseIPsFromResources(resources []dnsmessage.Resource) ([]IPAddr, error) {
+	var answers []IPAddr
+
+	for i := range resources {
+		switch resources[i].Header.Type {
+		case dnsmessage.TypeA:
+			b := resources[i].Body.(*dnsmessage.AResource)
+			ip := parseIPv4(string(b.A[:]))
+			answers = append(answers, IPAddr{IP: ip})
+		case dnsmessage.TypeAAAA:
+			b := resources[i].Body.(*dnsmessage.AAAAResource)
+			ip, zone := parseIPv6Zone(string(b.AAAA[:]))
+			answers = append(answers, IPAddr{IP: ip, Zone: zone})
+		default:
+			return nil, errors.New("could not parse an A or AAAA response from message buffer")
+		}
+	}
+	return answers, nil
+}
+
+func parseCNAMEFromResources(resources []dnsmessage.Resource) (string, error) {
+	if len(resources) == 0 {
+		return "", errors.New("no CNAME record received")
+	}
+	c, ok := resources[0].Body.(*dnsmessage.CNAMEResource)
+	if !ok {
+		return "", errors.New("could not parse CNAME record")
+	}
+	return c.CNAME.String(), nil
+}
+
+func parsePTRsFromResources(resources []dnsmessage.Resource) ([]string, error) {
+	var answers []string
+	for i := range resources {
+		switch resources[i].Header.Type {
+		case dnsmessage.TypePTR:
+			p := resources[0].Body.(*dnsmessage.PTRResource)
+			answers = append(answers, p.PTR.String())
+		default:
+			return nil, errors.New("could not parse a PTR response from message buffer")
+
+		}
+	}
+	return answers, nil
+}
+
+// res_init and res_search are defined in runtime/lookup_darwin.go
+
+func res_init() int32
+
+func res_search(dname *byte, class int32, rtype int32, answer *byte, anslen int32) (int32, int32)

src/net/cgo_stub.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build !cgo netgo
+// +build !darwin
 
 package net
 

src/net/conf.go

@@ -70,6 +70,11 @@ func initConfVal() {
 	// their own DNS requests. So always use cgo instead, which
 	// avoids that.
 	if runtime.GOOS == "darwin" {
+		// Normally we force netGo to be true if building without cgo enabled.
+		// On Darwin, we can call libc even if cgo is not enabled, so only set netGo to true
+		// if explicitly requested.
+		confVal.netGo = dnsMode == "go"
+
 		confVal.forceCgoLookupHost = true
 		return
 	}

src/runtime/lookup_darwin.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.
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:linkname res_init net.res_init
+//go:nosplit
+//go:cgo_unsafe_args
+func res_init() int32 {
+	return libcCall(unsafe.Pointer(funcPC(res_init_trampoline)), nil)
+}
+func res_init_trampoline()
+
+//go:linkname res_search net.res_search
+//go:nosplit
+//go:cgo_unsafe_args
+func res_search(dname *byte, class int32, rtype int32, answer *byte, anslen int32) (int32, int32) {
+	args := struct {
+		dname                   *byte
+		class, rtype            int32
+		answer                  *byte
+		anslen, retSize, retErr int32
+	}{dname, class, rtype, answer, anslen, 0, 0}
+	libcCall(unsafe.Pointer(funcPC(res_search_trampoline)), unsafe.Pointer(&args))
+	return args.retSize, args.retErr
+}
+func res_search_trampoline()
+
+//go:cgo_import_dynamic libc_res_search res_search "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_res_init res_init "/usr/lib/libSystem.B.dylib"

src/runtime/lookup_darwin_386.s

+// 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.
+
+#include "go_asm.h"
+#include "go_tls.h"
+#include "textflag.h"
+
+TEXT runtime·res_init_trampoline(SB),NOSPLIT,$0
+    PUSHL   BP
+    MOVL    SP, BP
+    SUBL    $8, SP
+    CALL    libc_res_init(SB)
+    CMPL    AX, $-1
+    JNE ok
+    CALL    libc_error(SB)
+ok:
+    MOVL    BP, SP
+    POPL    BP
+    RET
+
+TEXT runtime·res_search_trampoline(SB),NOSPLIT,$0
+    PUSHL   BP
+    MOVL    SP, BP
+    SUBL    $24, SP
+    MOVL    32(SP), CX
+    MOVL    16(CX), AX      // arg 5 anslen
+    MOVL    AX, 16(SP)
+    MOVL    12(CX), AX      // arg 4 answer
+    MOVL    AX, 12(SP)
+    MOVL    8(CX), AX       // arg 3 type
+    MOVL    AX, 8(SP)
+    MOVL    4(CX), AX       // arg 2 class
+    MOVL    AX, 4(SP)
+    MOVL    0(CX), AX       // arg 1 name
+    MOVL    AX, 0(SP)
+    CALL    libc_res_search(SB)
+    XORL    DX, DX
+    CMPL    AX, $-1
+    JNE ok
+    CALL    libc_error(SB)
+    MOVL    (AX), DX
+    XORL    AX, AX
+ok:
+    MOVL    32(SP), CX
+    MOVL    AX, 20(CX)
+    MOVL    DX, 24(CX)
+    MOVL    BP, SP
+    POPL    BP
+    RET

src/runtime/lookup_darwin_amd64.s

+// 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.
+
+#include "go_asm.h"
+#include "go_tls.h"
+#include "textflag.h"
+
+TEXT runtime·res_init_trampoline(SB),NOSPLIT,$0
+    PUSHQ    BP
+    MOVQ     SP, BP
+    CALL     libc_res_init(SB)
+    CMPQ     AX, $-1
+    JNE ok
+    CALL     libc_error(SB)
+ok:
+    POPQ    BP
+    RET
+
+TEXT runtime·res_search_trampoline(SB),NOSPLIT,$0
+    PUSHQ    BP
+    MOVQ     SP, BP
+    MOVQ     DI, BX   // move DI into BX to preserve struct addr
+    MOVL     24(BX), R8  // arg 5 anslen
+    MOVQ     16(BX), CX  // arg 4 answer
+    MOVL     12(BX), DX  // arg 3 type
+    MOVL     8(BX), SI   // arg 2 class
+    MOVQ     0(BX), DI   // arg 1 name
+    CALL     libc_res_search(SB)
+    XORL     DX, DX
+    CMPQ     AX, $-1
+    JNE ok
+    CALL     libc_error(SB)
+    MOVLQSX  (AX), DX             // move return from libc_error into DX
+    XORL     AX, AX               // size on error is 0
+ok:
+    MOVQ    AX, 28(BX) // size
+    MOVQ    DX, 32(BX) // error code
+    POPQ    BP
+    RET

src/runtime/lookup_darwin_arm.s

+// Copyright 2015 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.
+
+// System calls and other sys.stuff for ARM64, Darwin
+// System calls are implemented in libSystem, this file contains
+// trampolines that convert from Go to C calling convention.
+
+#include "go_asm.h"
+#include "go_tls.h"
+#include "textflag.h"
+
+// On darwin/arm, the runtime always uses runtime/cgo
+// for resolution. This will just exit with a nominal
+// exit code.
+
+TEXT runtime·res_search_trampoline(SB),NOSPLIT,$0
+    MOVW    $90, R0
+    BL    libc_exit(SB)
+    RET
+
+TEXT runtime·res_init_trampoline(SB),NOSPLIT,$0
+    MOVW    $91, R0
+    BL    libc_exit(SB)
+    RET

src/runtime/lookup_darwin_arm64.s

+// 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.
+
+#include "go_asm.h"
+#include "go_tls.h"
+#include "textflag.h"
+
+// On darwin/arm, the runtime always uses runtime/cgo
+// for resolution. This will just exit with a nominal
+// exit code.
+
+TEXT runtime·res_search_trampoline(SB),NOSPLIT,$0
+    MOVW    $90, R0
+    BL    libc_exit(SB)
+    RET
+
+TEXT runtime·res_init_trampoline(SB),NOSPLIT,$0
+    MOVW    $91, R0
+    BL    libc_exit(SB)
+    RET