os: Add support for long path names on unix RemoveAll

On unix systems, long enough path names will fail on doing syscall like
`Lstat`. The current RemoveAll uses several of these syscalls, and so
will fail for long paths. This can be risky, as it can let users "hide"
files from the system or otherwise make long enough paths for programs
to fail. By using `Unlinkat` and `Openat` syscalls instead, RemoveAll is
safer on unix systems. Initially implemented for linux and darwin.

Fixes golang#27029

Co-authored-by: Giuseppe Capizzi <[email protected]>
Co-authored-by: Julia Nedialkova <[email protected]>

Change-Id: I9d8a634bb7a479e5ef80fc346aeff478bde1f607

Author: ostenbom

src/internal/syscall/unix/link_sysnum_darwin.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap = uintptr(472)
+const openatTrap = uintptr(463)
+
+const AT_REMOVEDIR = 0x80

src/internal/syscall/unix/link_sysnum_linux_386.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap uintptr = 301
+const openatTrap uintptr = 295
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_amd64.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap uintptr = 263
+const openatTrap uintptr = 257
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_arm.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap = uintptr(328)
+const openatTrap = uintptr(322)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_arm64.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap = uintptr(35)
+const openatTrap = uintptr(56)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_mips64x.go

@@ -0,0 +1,12 @@
+// Copyright 2018 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.
+
+// +build mips64 mips64le
+
+package unix
+
+const unlinkatTrap = uintptr(5253)
+const openatTrap = uintptr(5247)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_mipsx.go

@@ -0,0 +1,12 @@
+// Copyright 2018 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.
+
+// +build mips mipsle
+
+package unix
+
+const unlinkatTrap = uintptr(4294)
+const openatTrap = uintptr(4288)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_ppc64x.go

@@ -0,0 +1,12 @@
+// Copyright 2018 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.
+
+// +build ppc64 ppc64le
+
+package unix
+
+const unlinkatTrap = uintptr(292)
+const openatTrap = uintptr(286)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/link_sysnum_linux_s390x.go

@@ -0,0 +1,10 @@
+// Copyright 2018 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 unix
+
+const unlinkatTrap = uintptr(294)
+const openatTrap = uintptr(288)
+
+const AT_REMOVEDIR = 0x200

src/internal/syscall/unix/openat_unix.go

@@ -0,0 +1,28 @@
+// Copyright 2018 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.
+
+// +build linux darwin dragonfly freebsd netbsd openbsd
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func Openat(fdat int, path string, flags int, perm uint32) (int, error) {
+	var pathBytePointer *byte
+	pathBytePointer, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+
+	fdPointer, _, errNo := syscall.Syscall6(openatTrap, uintptr(fdat), uintptr(unsafe.Pointer(pathBytePointer)), uintptr(flags), uintptr(perm), 0, 0)
+	fd := int(fdPointer)
+	if errNo != 0 {
+		return 0, errNo
+	}
+
+	return fd, nil
+}