syscall: fix a number of exec bugs on Plan 9

1. Readdirnames was erroneously returning an
   empty slice on every invocation.

2. The logic for determining which files to
   close before exec was incorrect.  If the
   set of files to be kept open (provided by
   the caller) did not include the files
   opened at startup, those files would be
   accidentally closed.

I also cleaned up readdupdevice while I was
in the vicinity.

R=golang-dev, seed, rsc
CC=golang-dev
https://golang.org/cl/6016044

src/pkg/syscall/exec_plan9.go

@@ -86,66 +86,60 @@ func gstring(b []byte) (string, []byte) {
 
 // readdirnames returns the names of files inside the directory represented by dirfd.
 func readdirnames(dirfd int) (names []string, err error) {
-	result := make([]string, 0, 100)
+	names = make([]string, 0, 100)
 	var buf [STATMAX]byte
 
 	for {
 		n, e := Read(dirfd, buf[:])
 		if e != nil {
-			return []string{}, e
+			return nil, e
 		}
 		if n == 0 {
 			break
 		}
-
 		for i := 0; i < n; {
 			m, _ := gbit16(buf[i:])
 			m += 2
 
 			if m < STATFIXLEN {
-				return []string{}, NewError("malformed stat buffer")
+				return nil, NewError("malformed stat buffer")
 			}
 
-			name, _ := gstring(buf[i+41:])
-			result = append(result, name)
-
+			s, _ := gstring(buf[i+41:])
+			names = append(names, s)
 			i += int(m)
 		}
 	}
-	return []string{}, nil
+	return
 }
 
 // readdupdevice returns a list of currently opened fds (excluding stdin, stdout, stderr) from the dup device #d.
 // ForkLock should be write locked before calling, so that no new fds would be created while the fd list is being read.
 func readdupdevice() (fds []int, err error) {
 	dupdevfd, err := Open("#d", O_RDONLY)
-
 	if err != nil {
 		return
 	}
 	defer Close(dupdevfd)
 
-	fileNames, err := readdirnames(dupdevfd)
+	names, err := readdirnames(dupdevfd)
 	if err != nil {
 		return
 	}
 
-	fds = make([]int, 0, len(fileNames)>>1)
-	for _, fdstr := range fileNames {
-		if l := len(fdstr); l > 2 && fdstr[l-3] == 'c' && fdstr[l-2] == 't' && fdstr[l-1] == 'l' {
+	fds = make([]int, 0, len(names)/2)
+	for _, name := range names {
+		if n := len(name); n > 3 && name[n-3:n] == "ctl" {
 			continue
 		}
-
-		fd := int(atoi([]byte(fdstr)))
-
-		if fd == 0 || fd == 1 || fd == 2 || fd == dupdevfd {
+		fd := int(atoi([]byte(name)))
+		switch fd {
+		case 0, 1, 2, dupdevfd:
 			continue
 		}
-
 		fds = append(fds, fd)
 	}
-
-	return fds[0:len(fds)], nil
+	return
 }
 
 var startupFds []int
@@ -282,14 +276,13 @@ func forkAndExecInChild(argv0 *byte, argv []*byte, envv []envItem, dir *byte, at
 		if fd[i] == int(i) {
 			continue
 		}
-
 		r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(i), 0)
 		if int(r1) == -1 {
 			goto childerror
 		}
 	}
 
-	// Pass 3: close fds that were dup-ed
+	// Pass 3: close fd[i] if it was moved in the previous pass.
 	for i = 0; i < len(fd); i++ {
 		if fd[i] >= 0 && fd[i] != int(i) {
 			RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
@@ -406,39 +399,32 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 	// get a list of open fds, excluding stdin,stdout and stderr that need to be closed in the child.
 	// no new fds can be created while we hold the ForkLock for writing.
 	openFds, e := readdupdevice()
-
 	if e != nil {
 		ForkLock.Unlock()
 		return 0, e
 	}
 
 	fdsToClose := make([]int, 0, len(openFds))
-	// exclude fds opened from startup from the list of fds to be closed.
 	for _, fd := range openFds {
-		isReserved := false
-		for _, reservedFd := range startupFds {
-			if fd == reservedFd {
-				isReserved = true
+		doClose := true
+
+		// exclude files opened at startup.
+		for _, sfd := range startupFds {
+			if fd == sfd {
+				doClose = false
 				break
 			}
 		}
 
-		if !isReserved {
-			fdsToClose = append(fdsToClose, fd)
-		}
-	}
-
-	// exclude fds requested by the caller from the list of fds to be closed.
-	for _, fd := range openFds {
-		isReserved := false
-		for _, reservedFd := range attr.Files {
-			if fd == int(reservedFd) {
-				isReserved = true
+		// exclude files explicitly requested by the caller.
+		for _, rfd := range attr.Files {
+			if fd == int(rfd) {
+				doClose = false
 				break
 			}
 		}
 
-		if !isReserved {
+		if doClose {
 			fdsToClose = append(fdsToClose, fd)
 		}
 	}