cmd/go: check if destination is a regular file

builder.copyFile ensures that the destination is an object file.  This
wouldn't be true if we are not writing to a regular file and the copy
fails.  Check if the destination is an object file only if we are
writing to a regular file.  While removing the file, ensure that it is a
regular file so that device files and such aren't removed when running
as a user with suggicient privileges.

Fixes #12407

Change-Id: Ie86ce9770fa59aa56fc486a5962287859b69db3d
Reviewed-on: https://go-review.googlesource.com/16585Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -1669,7 +1669,7 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode, force b
 		if fi.IsDir() {
 			return fmt.Errorf("build output %q already exists and is a directory", dst)
 		}
-		if !force && !isObject(dst) {
+		if !force && fi.Mode().IsRegular() && !isObject(dst) {
 			return fmt.Errorf("build output %q already exists and is not an object file", dst)
 		}
 	}
@@ -1681,7 +1681,7 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode, force b
 		}
 	}
 
-	os.Remove(dst)
+	mayberemovefile(dst)
 	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
 	if err != nil && toolIsWindows {
 		// Windows does not allow deletion of a binary file
@@ -1700,7 +1700,7 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode, force b
 	_, err = io.Copy(df, sf)
 	df.Close()
 	if err != nil {
-		os.Remove(dst)
+		mayberemovefile(dst)
 		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
 	}
 	return nil
@@ -1765,6 +1765,16 @@ func isObject(s string) bool {
 	return false
 }
 
+// mayberemovefile removes a file only if it is a regular file
+// When running as a user with sufficient privileges, we may delete
+// even device files, for example, which is not intended.
+func mayberemovefile(s string) {
+	if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
+		return
+	}
+	os.Remove(s)
+}
+
 // fmtcmd formats a command in the manner of fmt.Sprintf but also:
 //
 //	If dir is non-empty and the script is not in dir right now,

src/cmd/go/go_test.go

@@ -1165,6 +1165,14 @@ func TestInstallIntoGOPATH(t *testing.T) {
 	tg.wantExecutable("testdata/bin/go-cmd-test"+exeSuffix, "go install go-cmd-test did not write to testdata/bin/go-cmd-test")
 }
 
+// Issue 12407
+func TestBuildOutputToDevNull(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.run("build", "-o", os.DevNull, "go-cmd-test")
+}
+
 func TestPackageMainTestImportsArchiveNotBinary(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()