syscall: avoid race in plan9 while syncing Chdir across goroutines

Because each M in Plan 9 runs in a separate OS process with its
own current working directory, a Chdir call in one goroutine needs
to be propagated to other goroutines before a subsequent syscall
with a local pathname (see #9428). This is done by function
syscall.Fixwd, but there is still a race if a goroutine is
preempted and rescheduled on a different M between calling Fixwd
and executing the syscall which it protects. By locking the
goroutine to its OS thread from the start of Fixwd to the end of
the protected syscall, this race can be prevented.

Fixes #58802.

Change-Id: I89c0e43ef4544b5bfb5db7d2158f13f24b42e1f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/474055
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/syscall/pwd_plan9.go

@@ -11,7 +11,10 @@
 
 package syscall
 
-import "sync"
+import (
+	"runtime"
+	"sync"
+)
 
 var (
 	wdmu  sync.Mutex // guards following
@@ -19,6 +22,12 @@ var (
 	wdStr string
 )
 
+// Ensure current working directory seen by this goroutine matches
+// the most recent Chdir called in any goroutine. It's called internally
+// before executing any syscall which uses a relative pathname. Must
+// be called with the goroutine locked to the OS thread, to prevent
+// rescheduling on a different thread (potentially with a different
+// working directory) before the syscall is executed.
 func Fixwd() {
 	wdmu.Lock()
 	defer wdmu.Unlock()
@@ -39,13 +48,17 @@ func fixwdLocked() {
 	}
 }
 
-func fixwd(paths ...string) {
+// If any of the paths is relative, call Fixwd and return true
+// (locked to OS thread). Otherwise return false.
+func fixwd(paths ...string) bool {
 	for _, path := range paths {
 		if path != "" && path[0] != '/' && path[0] != '#' {
+			runtime.LockOSThread()
 			Fixwd()
-			return
+			return true
 		}
 	}
+	return false
 }
 
 // goroutine-specific getwd
@@ -75,10 +88,15 @@ func Getwd() (wd string, err error) {
 }
 
 func Chdir(path string) error {
-	fixwd(path)
+	// If Chdir is to a relative path, sync working dir first
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	wdmu.Lock()
 	defer wdmu.Unlock()
 
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
 	if err := chdir(path); err != nil {
 		return err
 	}

src/syscall/syscall_plan9.go

@@ -13,6 +13,7 @@ package syscall
 
 import (
 	"internal/oserror"
+	"runtime"
 	"unsafe"
 )
 
@@ -301,7 +302,9 @@ func Await(w *Waitmsg) (err error) {
 }
 
 func Unmount(name, old string) (err error) {
-	fixwd(name, old)
+	if fixwd(name, old) {
+		defer runtime.UnlockOSThread()
+	}
 	oldp, err := BytePtrFromString(old)
 	if err != nil {
 		return err
@@ -384,49 +387,63 @@ func Getgroups() (gids []int, err error) {
 //sys	open(path string, mode int) (fd int, err error)
 
 func Open(path string, mode int) (fd int, err error) {
-	fixwd(path)
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	return open(path, mode)
 }
 
 //sys	create(path string, mode int, perm uint32) (fd int, err error)
 
 func Create(path string, mode int, perm uint32) (fd int, err error) {
-	fixwd(path)
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	return create(path, mode, perm)
 }
 
 //sys	remove(path string) (err error)
 
 func Remove(path string) error {
-	fixwd(path)
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	return remove(path)
 }
 
 //sys	stat(path string, edir []byte) (n int, err error)
 
 func Stat(path string, edir []byte) (n int, err error) {
-	fixwd(path)
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	return stat(path, edir)
 }
 
 //sys	bind(name string, old string, flag int) (err error)
 
 func Bind(name string, old string, flag int) (err error) {
-	fixwd(name, old)
+	if fixwd(name, old) {
+		defer runtime.UnlockOSThread()
+	}
 	return bind(name, old, flag)
 }
 
 //sys	mount(fd int, afd int, old string, flag int, aname string) (err error)
 
 func Mount(fd int, afd int, old string, flag int, aname string) (err error) {
-	fixwd(old)
+	if fixwd(old) {
+		defer runtime.UnlockOSThread()
+	}
 	return mount(fd, afd, old, flag, aname)
 }
 
 //sys	wstat(path string, edir []byte) (err error)
 
 func Wstat(path string, edir []byte) (err error) {
-	fixwd(path)
+	if fixwd(path) {
+		defer runtime.UnlockOSThread()
+	}
 	return wstat(path, edir)
 }