os: avoid symlink races in RemoveAll on Windows

Make the openat-using version of RemoveAll use the appropriate
Windows equivalent, via new portable (but internal) functions
added for os.Root.

We could reimplement everything in terms of os.Root,
but this is a bit simpler and keeps the existing code structure.

Fixes #52745

Change-Id: I0eba0286398b351f2ee9abaa60e1675173988787
Reviewed-on: https://go-review.googlesource.com/c/go/+/661575
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: neild

Diff for: src/internal/syscall/unix/constants.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build unix
+//go:build unix || wasip1
 
 package unix
 

Diff for: src/internal/syscall/unix/nofollow_posix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build unix && !dragonfly && !freebsd && !netbsd
+//go:build (unix && !dragonfly && !freebsd && !netbsd) || wasip1
 
 package unix
 

Diff for: src/internal/syscall/windows/at_windows.go

@@ -188,7 +188,7 @@ func Mkdirat(dirfd syscall.Handle, name string, mode uint32) error {
 	return nil
 }
 
-func Deleteat(dirfd syscall.Handle, name string) error {
+func Deleteat(dirfd syscall.Handle, name string, options uint32) error {
 	objAttrs := &OBJECT_ATTRIBUTES{}
 	if err := objAttrs.init(dirfd, name); err != nil {
 		return err
@@ -200,7 +200,7 @@ func Deleteat(dirfd syscall.Handle, name string) error {
 		objAttrs,
 		&IO_STATUS_BLOCK{},
 		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
-		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT,
+		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT|options,
 	)
 	if err != nil {
 		return ntCreateFileError(err, 0)

Diff for: src/os/path_windows.go

@@ -21,6 +21,16 @@ func IsPathSeparator(c uint8) bool {
 	return c == '\\' || c == '/'
 }
 
+// splitPath returns the base name and parent directory.
+func splitPath(path string) (string, string) {
+	dirname, basename := filepathlite.Split(path)
+	volnamelen := filepathlite.VolumeNameLen(dirname)
+	for len(dirname) > volnamelen && IsPathSeparator(dirname[len(dirname)-1]) {
+		dirname = dirname[:len(dirname)-1]
+	}
+	return dirname, basename
+}
+
 func dirname(path string) string {
 	vol := filepathlite.VolumeName(path)
 	i := len(path) - 1

Diff for: src/os/removeall_at.go

@@ -2,12 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build unix
+//go:build unix || wasip1 || windows
 
 package os
 
 import (
-	"internal/syscall/unix"
 	"io"
 	"syscall"
 )
@@ -56,11 +55,10 @@ func removeAll(path string) error {
 }
 
 func removeAllFrom(parent *File, base string) error {
-	parentFd := int(parent.Fd())
+	parentFd := sysfdType(parent.Fd())
+
 	// Simple case: if Unlink (aka remove) works, we're done.
-	err := ignoringEINTR(func() error {
-		return unix.Unlinkat(parentFd, base, 0)
-	})
+	err := removefileat(parentFd, base)
 	if err == nil || IsNotExist(err) {
 		return nil
 	}
@@ -82,13 +80,13 @@ func removeAllFrom(parent *File, base string) error {
 		const reqSize = 1024
 		var respSize int
 
-		// Open the directory to recurse into
+		// Open the directory to recurse into.
 		file, err := openDirAt(parentFd, base)
 		if err != nil {
 			if IsNotExist(err) {
 				return nil
 			}
-			if err == syscall.ENOTDIR || err == unix.NoFollowErrno {
+			if err == syscall.ENOTDIR || isErrNoFollow(err) {
 				// Not a directory; return the error from the unix.Unlinkat.
 				return &PathError{Op: "unlinkat", Path: base, Err: uErr}
 			}
@@ -144,9 +142,7 @@ func removeAllFrom(parent *File, base string) error {
 	}
 
 	// Remove the directory itself.
-	unlinkError := ignoringEINTR(func() error {
-		return unix.Unlinkat(parentFd, base, unix.AT_REMOVEDIR)
-	})
+	unlinkError := removedirat(parentFd, base)
 	if unlinkError == nil || IsNotExist(unlinkError) {
 		return nil
 	}
@@ -165,18 +161,10 @@ func removeAllFrom(parent *File, base string) error {
 // This acts like openFileNolog rather than OpenFile because
 // we are going to (try to) remove the file.
 // The contents of this file are not relevant for test caching.
-func openDirAt(dirfd int, name string) (*File, error) {
-	r, err := ignoringEINTR2(func() (int, error) {
-		return unix.Openat(dirfd, name, O_RDONLY|syscall.O_CLOEXEC|syscall.O_DIRECTORY|syscall.O_NOFOLLOW, 0)
-	})
+func openDirAt(dirfd sysfdType, name string) (*File, error) {
+	fd, err := rootOpenDir(dirfd, name)
 	if err != nil {
 		return nil, err
 	}
-
-	if !supportsCloseOnExec {
-		syscall.CloseOnExec(r)
-	}
-
-	// We use kindNoPoll because we know that this is a directory.
-	return newFile(r, name, kindNoPoll, false), nil
+	return newDirFile(fd, name)
 }

Diff for: src/os/removeall_noat.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !unix
+//go:build (js && wasm) || plan9
 
 package os
 

Diff for: src/os/removeall_unix.go

@@ -0,0 +1,20 @@
+// Copyright 2025 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.
+
+//go:build unix || wasip1
+
+package os
+
+import (
+	"internal/syscall/unix"
+)
+
+func isErrNoFollow(err error) bool {
+	return err == unix.NoFollowErrno
+}
+
+func newDirFile(fd int, name string) (*File, error) {
+	// We use kindNoPoll because we know that this is a directory.
+	return newFile(fd, name, kindNoPoll, false), nil
+}

Diff for: src/os/removeall_windows.go

@@ -0,0 +1,17 @@
+// Copyright 2025 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.
+
+//go:build windows
+
+package os
+
+import "syscall"
+
+func isErrNoFollow(err error) bool {
+	return err == syscall.ELOOP
+}
+
+func newDirFile(fd syscall.Handle, name string) (*File, error) {
+	return newFile(fd, name, "file"), nil
+}

Diff for: src/os/root_unix.go

@@ -219,6 +219,18 @@ func removeat(fd int, name string) error {
 	return e
 }
 
+func removefileat(fd int, name string) error {
+	return ignoringEINTR(func() error {
+		return unix.Unlinkat(fd, name, 0)
+	})
+}
+
+func removedirat(fd int, name string) error {
+	return ignoringEINTR(func() error {
+		return unix.Unlinkat(fd, name, unix.AT_REMOVEDIR)
+	})
+}
+
 func renameat(oldfd int, oldname string, newfd int, newname string) error {
 	return unix.Renameat(oldfd, oldname, newfd, newname)
 }

Diff for: src/os/root_windows.go

@@ -336,7 +336,15 @@ func mkdirat(dirfd syscall.Handle, name string, perm FileMode) error {
 }
 
 func removeat(dirfd syscall.Handle, name string) error {
-	return windows.Deleteat(dirfd, name)
+	return windows.Deleteat(dirfd, name, 0)
+}
+
+func removefileat(dirfd syscall.Handle, name string) error {
+	return windows.Deleteat(dirfd, name, windows.FILE_NON_DIRECTORY_FILE)
+}
+
+func removedirat(dirfd syscall.Handle, name string) error {
+	return windows.Deleteat(dirfd, name, windows.FILE_DIRECTORY_FILE)
 }
 
 func chtimesat(dirfd syscall.Handle, name string, atime time.Time, mtime time.Time) error {