all: introduce and use internal/execabs

Introduces a wrapper around os/exec, internal/execabs, for use in
all commands. This wrapper prevents exec.LookPath and exec.Command from
running executables in the current directory.

All imports of os/exec in non-test files in cmd/ are replaced with
imports of internal/execabs.

This issue was reported by RyotaK.

Fixes CVE-2021-3115
Fixes #43783

Change-Id: I0423451a6e27ec1e1d6f3fe929ab1ef69145c08f
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/955304
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/284783
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>

40 files changed, 220 insertions, 38 deletions

diff --git a/src/cmd/api/goapi.go b/src/cmd/api/goapi.go
index ba42812fa6..efc2696f8f 100644
--- a/src/cmd/api/goapi.go
+++ b/src/cmd/api/goapi.go
@@ -16,10 +16,10 @@ import (
 	"go/parser"
 	"go/token"
 	"go/types"
+	exec "internal/execabs"
 	"io"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"runtime"
diff --git a/src/cmd/api/run.go b/src/cmd/api/run.go
index a36f1179c1..ecb1d0f81a 100644
--- a/src/cmd/api/run.go
+++ b/src/cmd/api/run.go
@@ -10,9 +10,9 @@ package main
 
 import (
 	"fmt"
+	exec "internal/execabs"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index fa6f0efbbe..8e83f02202 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -14,10 +14,10 @@ import (
 	"go/ast"
 	"go/printer"
 	"go/token"
+	exec "internal/execabs"
 	"internal/xcoff"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"sort"
diff --git a/src/cmd/cgo/util.go b/src/cmd/cgo/util.go
index 779f7be225..00d931b98a 100644
--- a/src/cmd/cgo/util.go
+++ b/src/cmd/cgo/util.go
@@ -8,9 +8,9 @@ import (
 	"bytes"
 	"fmt"
 	"go/token"
+	exec "internal/execabs"
 	"io/ioutil"
 	"os"
-	"os/exec"
 )
 
 // run runs the command argv, feeding in stdin on standard input.
diff --git a/src/cmd/compile/internal/ssa/html.go b/src/cmd/compile/internal/ssa/html.go
index a9d52fa4ee..c06b5808e1 100644
--- a/src/cmd/compile/internal/ssa/html.go
+++ b/src/cmd/compile/internal/ssa/html.go
@@ -9,9 +9,9 @@ import (
 	"cmd/internal/src"
 	"fmt"
 	"html"
+	exec "internal/execabs"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strconv"
 	"strings"
diff --git a/src/cmd/cover/func.go b/src/cmd/cover/func.go
index 988c4caebf..ce7c771ac9 100644
--- a/src/cmd/cover/func.go
+++ b/src/cmd/cover/func.go
@@ -15,9 +15,9 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
+	exec "internal/execabs"
 	"io"
 	"os"
-	"os/exec"
 	"path"
 	"path/filepath"
 	"runtime"
diff --git a/src/cmd/cover/testdata/toolexec.go b/src/cmd/cover/testdata/toolexec.go
index 1769efedbe..386de79038 100644
--- a/src/cmd/cover/testdata/toolexec.go
+++ b/src/cmd/cover/testdata/toolexec.go
@@ -16,7 +16,7 @@ package main
 
 import (
 	"os"
-	"os/exec"
+	exec "internal/execabs"
 	"strings"
 )
 
diff --git a/src/cmd/dist/buildtool.go b/src/cmd/dist/buildtool.go
index e7bedfb84e..cf85f2ac8e 100644
--- a/src/cmd/dist/buildtool.go
+++ b/src/cmd/dist/buildtool.go
@@ -305,8 +305,10 @@ func bootstrapFixImports(srcFile string) string {
 			continue
 		}
 		if strings.HasPrefix(line, `import "`) || strings.HasPrefix(line, `import . "`) ||
-			inBlock && (strings.HasPrefix(line, "\t\"") || strings.HasPrefix(line, "\t. \"")) {
+			inBlock && (strings.HasPrefix(line, "\t\"") || strings.HasPrefix(line, "\t. \"") || strings.HasPrefix(line, "\texec \"")) {
 			line = strings.Replace(line, `"cmd/`, `"bootstrap/cmd/`, -1)
+			// During bootstrap, must use plain os/exec.
+			line = strings.Replace(line, `exec "internal/execabs"`, `"os/exec"`, -1)
 			for _, dir := range bootstrapDirs {
 				if strings.HasPrefix(dir, "cmd/") {
 					continue
diff --git a/src/cmd/doc/dirs.go b/src/cmd/doc/dirs.go
index 38cbe7fa02..661624cfe4 100644
--- a/src/cmd/doc/dirs.go
+++ b/src/cmd/doc/dirs.go
@@ -7,9 +7,9 @@ package main
 import (
 	"bytes"
 	"fmt"
+	exec "internal/execabs"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"
diff --git a/src/cmd/fix/typecheck.go b/src/cmd/fix/typecheck.go
index 40b2287f26..39a53785b7 100644
--- a/src/cmd/fix/typecheck.go
+++ b/src/cmd/fix/typecheck.go
@@ -9,8 +9,8 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"reflect"
 	"runtime"
diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
index 004588c732..954ce47a98 100644
--- a/src/cmd/go/internal/base/base.go
+++ b/src/cmd/go/internal/base/base.go
@@ -10,9 +10,9 @@ import (
 	"context"
 	"flag"
 	"fmt"
+	exec "internal/execabs"
 	"log"
 	"os"
-	"os/exec"
 	"strings"
 	"sync"
 
diff --git a/src/cmd/go/internal/bug/bug.go b/src/cmd/go/internal/bug/bug.go
index 1085feaaee..4aa08b4ff6 100644
--- a/src/cmd/go/internal/bug/bug.go
+++ b/src/cmd/go/internal/bug/bug.go
@@ -9,10 +9,10 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	exec "internal/execabs"
 	"io"
 	urlpkg "net/url"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"runtime"
diff --git a/src/cmd/go/internal/generate/generate.go b/src/cmd/go/internal/generate/generate.go
index b1e001c800..a48311d51b 100644
--- a/src/cmd/go/internal/generate/generate.go
+++ b/src/cmd/go/internal/generate/generate.go
@@ -12,10 +12,10 @@ import (
 	"fmt"
 	"go/parser"
 	"go/token"
+	exec "internal/execabs"
 	"io"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strconv"
diff --git a/src/cmd/go/internal/modfetch/codehost/codehost.go b/src/cmd/go/internal/modfetch/codehost/codehost.go
index 86c1c14d4a..378fbae34f 100644
--- a/src/cmd/go/internal/modfetch/codehost/codehost.go
+++ b/src/cmd/go/internal/modfetch/codehost/codehost.go
@@ -10,10 +10,10 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"fmt"
+	exec "internal/execabs"
 	"io"
 	"io/fs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 	"sync"
diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index 8abc039e7f..72005e27d5 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -8,11 +8,11 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	exec "internal/execabs"
 	"io"
 	"io/fs"
 	"net/url"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"sort"
 	"strconv"
diff --git a/src/cmd/go/internal/test/genflags.go b/src/cmd/go/internal/test/genflags.go
index 5e83d53980..30334b0f30 100644
--- a/src/cmd/go/internal/test/genflags.go
+++ b/src/cmd/go/internal/test/genflags.go
@@ -9,9 +9,9 @@ package main
 import (
 	"bytes"
 	"flag"
+	exec "internal/execabs"
 	"log"
 	"os"
-	"os/exec"
 	"strings"
 	"testing"
 	"text/template"
diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
index 50fe2dbf39..7fc9e8fbdc 100644
--- a/src/cmd/go/internal/test/test.go
+++ b/src/cmd/go/internal/test/test.go
@@ -11,10 +11,10 @@ import (
 	"errors"
 	"fmt"
 	"go/build"
+	exec "internal/execabs"
 	"io"
 	"io/fs"
 	"os"
-	"os/exec"
 	"path"
 	"path/filepath"
 	"regexp"
diff --git a/src/cmd/go/internal/tool/tool.go b/src/cmd/go/internal/tool/tool.go
index 6a755bc436..95c90ea7c8 100644
--- a/src/cmd/go/internal/tool/tool.go
+++ b/src/cmd/go/internal/tool/tool.go
@@ -8,8 +8,8 @@ package tool
 import (
 	"context"
 	"fmt"
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"os/signal"
 	"sort"
 	"strings"
diff --git a/src/cmd/go/internal/vcs/vcs.go b/src/cmd/go/internal/vcs/vcs.go
index 327ea7cc86..9feffe0765 100644
--- a/src/cmd/go/internal/vcs/vcs.go
+++ b/src/cmd/go/internal/vcs/vcs.go
@@ -8,13 +8,13 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	exec "internal/execabs"
 	"internal/lazyregexp"
 	"internal/singleflight"
 	"io/fs"
 	"log"
 	urlpkg "net/url"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"
diff --git a/src/cmd/go/internal/vet/vetflag.go b/src/cmd/go/internal/vet/vetflag.go
index ef995ef835..5bf5cf4446 100644
--- a/src/cmd/go/internal/vet/vetflag.go
+++ b/src/cmd/go/internal/vet/vetflag.go
@@ -10,9 +10,9 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	exec "internal/execabs"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 873d85de4e..780d639c5d 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -9,9 +9,9 @@ import (
 	"errors"
 	"fmt"
 	"go/build"
+	exec "internal/execabs"
 	"internal/goroot"
 	"os"
-	"os/exec"
 	"path"
 	"path/filepath"
 	"runtime"
diff --git a/src/cmd/go/internal/work/buildid.go b/src/cmd/go/internal/work/buildid.go
index d76988145b..c555d4a9f1 100644
--- a/src/cmd/go/internal/work/buildid.go
+++ b/src/cmd/go/internal/work/buildid.go
@@ -7,8 +7,8 @@ package work
 import (
 	"bytes"
 	"fmt"
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"strings"
 
 	"cmd/go/internal/base"
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index e750904266..16a4ebaa0f 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -13,13 +13,13 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	exec "internal/execabs"
 	"internal/lazyregexp"
 	"io"
 	"io/fs"
 	"log"
 	"math/rand"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"runtime"
diff --git a/src/cmd/go/internal/work/gccgo.go b/src/cmd/go/internal/work/gccgo.go
index 45ff7c9838..b58c8aa885 100644
--- a/src/cmd/go/internal/work/gccgo.go
+++ b/src/cmd/go/internal/work/gccgo.go
@@ -6,8 +6,8 @@ package work
 
 import (
 	"fmt"
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 	"sync"
diff --git a/src/cmd/go/testdata/addmod.go b/src/cmd/go/testdata/addmod.go
index 58376b7ed4..09fc8e713b 100644
--- a/src/cmd/go/testdata/addmod.go
+++ b/src/cmd/go/testdata/addmod.go
@@ -25,7 +25,7 @@ import (
 	"io/fs"
 	"log"
 	"os"
-	"os/exec"
+	exec "internal/execabs"
 	"path/filepath"
 	"strings"
 
diff --git a/src/cmd/internal/browser/browser.go b/src/cmd/internal/browser/browser.go
index 6867c85d23..577d31789f 100644
--- a/src/cmd/internal/browser/browser.go
+++ b/src/cmd/internal/browser/browser.go
@@ -6,8 +6,8 @@
 package browser
 
 import (
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"runtime"
 	"time"
 )
diff --git a/src/cmd/internal/diff/diff.go b/src/cmd/internal/diff/diff.go
index e9d2c23780..c0ca2f3106 100644
--- a/src/cmd/internal/diff/diff.go
+++ b/src/cmd/internal/diff/diff.go
@@ -7,9 +7,9 @@
 package diff
 
 import (
+	exec "internal/execabs"
 	"io/ioutil"
 	"os"
-	"os/exec"
 	"runtime"
 )
 
diff --git a/src/cmd/internal/dwarf/dwarf.go b/src/cmd/internal/dwarf/dwarf.go
index e1a70ef853..8de4096f06 100644
--- a/src/cmd/internal/dwarf/dwarf.go
+++ b/src/cmd/internal/dwarf/dwarf.go
@@ -12,7 +12,7 @@ import (
 	"cmd/internal/objabi"
 	"errors"
 	"fmt"
-	"os/exec"
+	exec "internal/execabs"
 	"sort"
 	"strconv"
 	"strings"
diff --git a/src/cmd/internal/pkgpath/pkgpath.go b/src/cmd/internal/pkgpath/pkgpath.go
index 40a040a81a..72e3bdb631 100644
--- a/src/cmd/internal/pkgpath/pkgpath.go
+++ b/src/cmd/internal/pkgpath/pkgpath.go
@@ -10,9 +10,9 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	exec "internal/execabs"
 	"io/ioutil"
 	"os"
-	"os/exec"
 	"strings"
 )
 
diff --git a/src/cmd/link/internal/ld/execarchive.go b/src/cmd/link/internal/ld/execarchive.go
index fe5cc40865..4687c624de 100644
--- a/src/cmd/link/internal/ld/execarchive.go
+++ b/src/cmd/link/internal/ld/execarchive.go
@@ -7,8 +7,8 @@
 package ld
 
 import (
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"syscall"
 )
diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index 014969664b..17d5040827 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -49,11 +49,11 @@ import (
 	"encoding/base64"
 	"encoding/binary"
 	"fmt"
+	exec "internal/execabs"
 	"io"
 	"io/ioutil"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"runtime"
 	"sort"
diff --git a/src/cmd/test2json/main.go b/src/cmd/test2json/main.go
index e40881ab3f..fdf681a8ff 100644
--- a/src/cmd/test2json/main.go
+++ b/src/cmd/test2json/main.go
@@ -86,9 +86,9 @@ package main
 import (
 	"flag"
 	"fmt"
+	exec "internal/execabs"
 	"io"
 	"os"
-	"os/exec"
 
 	"cmd/internal/test2json"
 )
diff --git a/src/cmd/trace/pprof.go b/src/cmd/trace/pprof.go
index a73ff5336a..c4d3742820 100644
--- a/src/cmd/trace/pprof.go
+++ b/src/cmd/trace/pprof.go
@@ -9,11 +9,11 @@ package main
 import (
 	"bufio"
 	"fmt"
+	exec "internal/execabs"
 	"internal/trace"
 	"io"
 	"net/http"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"runtime"
 	"sort"
diff --git a/src/go/build/build.go b/src/go/build/build.go
index 72311c7d2c..217fadf5bd 100644
--- a/src/go/build/build.go
+++ b/src/go/build/build.go
@@ -11,13 +11,13 @@ import (
 	"go/ast"
 	"go/doc"
 	"go/token"
+	exec "internal/execabs"
 	"internal/goroot"
 	"internal/goversion"
 	"io"
 	"io/fs"
 	"io/ioutil"
 	"os"
-	"os/exec"
 	pathpkg "path"
 	"path/filepath"
 	"runtime"
diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
index 99cd59e5b5..c97c668cc4 100644
--- a/src/go/build/deps_test.go
+++ b/src/go/build/deps_test.go
@@ -178,7 +178,7 @@ var depsRules = `
 	reflect !< OS;
 
 	OS
-	< golang.org/x/sys/cpu, internal/goroot;
+	< golang.org/x/sys/cpu;
 
 	# FMT is OS (which includes string routines) plus reflect and fmt.
 	# It does not include package log, which should be avoided in core packages.
@@ -194,6 +194,12 @@ var depsRules = `
 
 	log !< FMT;
 
+	OS, FMT
+	< internal/execabs;
+
+	OS, internal/execabs
+	< internal/goroot;
+
 	# Misc packages needing only FMT.
 	FMT
 	< flag,
diff --git a/src/go/internal/gccgoimporter/gccgoinstallation.go b/src/go/internal/gccgoimporter/gccgoinstallation.go
index 8fc7ce3232..e90a3cc0b0 100644
--- a/src/go/internal/gccgoimporter/gccgoinstallation.go
+++ b/src/go/internal/gccgoimporter/gccgoinstallation.go
@@ -7,8 +7,8 @@ package gccgoimporter
 import (
 	"bufio"
 	"go/types"
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 )
diff --git a/src/go/internal/srcimporter/srcimporter.go b/src/go/internal/srcimporter/srcimporter.go
index c4d501dcd9..438ae0ff2e 100644
--- a/src/go/internal/srcimporter/srcimporter.go
+++ b/src/go/internal/srcimporter/srcimporter.go
@@ -13,9 +13,9 @@ import (
 	"go/parser"
 	"go/token"
 	"go/types"
+	exec "internal/execabs"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 	"sync"
diff --git a/src/internal/execabs/execabs.go b/src/internal/execabs/execabs.go
new file mode 100644
index 0000000000..9a05d971da
--- /dev/null
+++ b/src/internal/execabs/execabs.go
@@ -0,0 +1,70 @@
+// Copyright 2021 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 execabs is a drop-in replacement for os/exec
+// that requires PATH lookups to find absolute paths.
+// That is, execabs.Command("cmd") runs the same PATH lookup
+// as exec.Command("cmd"), but if the result is a path
+// which is relative, the Run and Start methods will report
+// an error instead of running the executable.
+package execabs
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"unsafe"
+)
+
+var ErrNotFound = exec.ErrNotFound
+
+type (
+	Cmd       = exec.Cmd
+	Error     = exec.Error
+	ExitError = exec.ExitError
+)
+
+func relError(file, path string) error {
+	return fmt.Errorf("%s resolves to executable relative to current directory (.%c%s)", file, filepath.Separator, path)
+}
+
+func LookPath(file string) (string, error) {
+	path, err := exec.LookPath(file)
+	if err != nil {
+		return "", err
+	}
+	if filepath.Base(file) == file && !filepath.IsAbs(path) {
+		return "", relError(file, path)
+	}
+	return path, nil
+}
+
+func fixCmd(name string, cmd *exec.Cmd) {
+	if filepath.Base(name) == name && !filepath.IsAbs(cmd.Path) {
+		// exec.Command was called with a bare binary name and
+		// exec.LookPath returned a path which is not absolute.
+		// Set cmd.lookPathErr and clear cmd.Path so that it
+		// cannot be run.
+		lookPathErr := (*error)(unsafe.Pointer(reflect.ValueOf(cmd).Elem().FieldByName("lookPathErr").Addr().Pointer()))
+		if *lookPathErr == nil {
+			*lookPathErr = relError(name, cmd.Path)
+		}
+		cmd.Path = ""
+	}
+}
+
+func CommandContext(ctx context.Context, name string, arg ...string) *exec.Cmd {
+	cmd := exec.CommandContext(ctx, name, arg...)
+	fixCmd(name, cmd)
+	return cmd
+
+}
+
+func Command(name string, arg ...string) *exec.Cmd {
+	cmd := exec.Command(name, arg...)
+	fixCmd(name, cmd)
+	return cmd
+}
diff --git a/src/internal/execabs/execabs_test.go b/src/internal/execabs/execabs_test.go
new file mode 100644
index 0000000000..b71458587c
--- /dev/null
+++ b/src/internal/execabs/execabs_test.go
@@ -0,0 +1,104 @@
+// Copyright 2020 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 execabs
+
+import (
+	"context"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+func TestFixCmd(t *testing.T) {
+	cmd := &exec.Cmd{Path: "hello"}
+	fixCmd("hello", cmd)
+	if cmd.Path != "" {
+		t.Error("fixCmd didn't clear cmd.Path")
+	}
+	expectedErr := fmt.Sprintf("hello resolves to executable relative to current directory (.%chello)", filepath.Separator)
+	if err := cmd.Run(); err == nil {
+		t.Fatal("Command.Run didn't fail")
+	} else if err.Error() != expectedErr {
+		t.Fatalf("Command.Run returned unexpected error: want %q, got %q", expectedErr, err.Error())
+	}
+}
+
+func TestCommand(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	for _, cmd := range []func(string) *Cmd{
+		func(s string) *Cmd { return Command(s) },
+		func(s string) *Cmd { return CommandContext(context.Background(), s) },
+	} {
+		tmpDir := t.TempDir()
+		executable := "execabs-test"
+		if runtime.GOOS == "windows" {
+			executable += ".exe"
+		}
+		if err := ioutil.WriteFile(filepath.Join(tmpDir, executable), []byte{1, 2, 3}, 0111); err != nil {
+			t.Fatalf("ioutil.WriteFile failed: %s", err)
+		}
+		cwd, err := os.Getwd()
+		if err != nil {
+			t.Fatalf("os.Getwd failed: %s", err)
+		}
+		defer os.Chdir(cwd)
+		if err = os.Chdir(tmpDir); err != nil {
+			t.Fatalf("os.Chdir failed: %s", err)
+		}
+		if runtime.GOOS != "windows" {
+			// add "." to PATH so that exec.LookPath looks in the current directory on
+			// non-windows platforms as well
+			origPath := os.Getenv("PATH")
+			defer os.Setenv("PATH", origPath)
+			os.Setenv("PATH", fmt.Sprintf(".:%s", origPath))
+		}
+		expectedErr := fmt.Sprintf("execabs-test resolves to executable relative to current directory (.%c%s)", filepath.Separator, executable)
+		if err = cmd("execabs-test").Run(); err == nil {
+			t.Fatalf("Command.Run didn't fail when exec.LookPath returned a relative path")
+		} else if err.Error() != expectedErr {
+			t.Errorf("Command.Run returned unexpected error: want %q, got %q", expectedErr, err.Error())
+		}
+	}
+}
+
+func TestLookPath(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	tmpDir := t.TempDir()
+	executable := "execabs-test"
+	if runtime.GOOS == "windows" {
+		executable += ".exe"
+	}
+	if err := ioutil.WriteFile(filepath.Join(tmpDir, executable), []byte{1, 2, 3}, 0111); err != nil {
+		t.Fatalf("ioutil.WriteFile failed: %s", err)
+	}
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("os.Getwd failed: %s", err)
+	}
+	defer os.Chdir(cwd)
+	if err = os.Chdir(tmpDir); err != nil {
+		t.Fatalf("os.Chdir failed: %s", err)
+	}
+	if runtime.GOOS != "windows" {
+		// add "." to PATH so that exec.LookPath looks in the current directory on
+		// non-windows platforms as well
+		origPath := os.Getenv("PATH")
+		defer os.Setenv("PATH", origPath)
+		os.Setenv("PATH", fmt.Sprintf(".:%s", origPath))
+	}
+	expectedErr := fmt.Sprintf("execabs-test resolves to executable relative to current directory (.%c%s)", filepath.Separator, executable)
+	if _, err := LookPath("execabs-test"); err == nil {
+		t.Fatalf("LookPath didn't fail when finding a non-relative path")
+	} else if err.Error() != expectedErr {
+		t.Errorf("LookPath returned unexpected error: want %q, got %q", expectedErr, err.Error())
+	}
+}
diff --git a/src/internal/goroot/gc.go b/src/internal/goroot/gc.go
index 0f541d734b..ce72bc3896 100644
--- a/src/internal/goroot/gc.go
+++ b/src/internal/goroot/gc.go
@@ -7,8 +7,8 @@
 package goroot
 
 import (
+	exec "internal/execabs"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 	"sync"