fs: Support possible and online knobs for cpu

Some linux commands depend on /sys/devices/system/cpu/possible, such
as 'lscpu'.

Add 2 knobs for cpu:
/sys/devices/system/cpu/possible
/sys/devices/system/cpu/online
Both the values are '0 - Kernel.ApplicationCores()-1'.

Change-Id: Iabd8a4e559cbb630ed249686b92c22b4e7120663
PiperOrigin-RevId: 209070163

pkg/sentry/fs/sys/BUILD

@@ -6,6 +6,7 @@ go_library(
     name = "sys",
     srcs = [
         "device.go",
+        "devices.go",
         "fs.go",
         "sys.go",
     ],
@@ -16,6 +17,8 @@ go_library(
         "//pkg/sentry/device",
         "//pkg/sentry/fs",
         "//pkg/sentry/fs/ramfs",
+        "//pkg/sentry/kernel",
         "//pkg/sentry/usermem",
+        "//pkg/syserror",
     ],
 )

pkg/sentry/fs/sys/devices.go

@@ -0,0 +1,76 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sys
+
+import (
+	"fmt"
+	"io"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/fs/ramfs"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+)
+
+// +stateify savable
+type cpunum struct {
+	ramfs.Entry
+}
+
+func (c *cpunum) DeprecatedPreadv(ctx context.Context, dst usermem.IOSequence, offset int64) (int64, error) {
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+
+	k := kernel.KernelFromContext(ctx)
+	if k == nil {
+		return 0, io.EOF
+	}
+
+	str := []byte(fmt.Sprintf("0-%d\n", k.ApplicationCores()-1))
+	if offset >= int64(len(str)) {
+		return 0, io.EOF
+	}
+
+	n, err := dst.CopyOut(ctx, str[offset:])
+	return int64(n), err
+}
+
+func newPossible(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
+	c := &cpunum{}
+	c.InitEntry(ctx, fs.RootOwner, fs.FilePermsFromMode(0444))
+	return newFile(c, msrc)
+}
+
+func newCPU(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
+	return newDir(ctx, msrc, map[string]*fs.Inode{
+		"possible": newPossible(ctx, msrc),
+		"online":   newPossible(ctx, msrc),
+	})
+}
+
+func newSystemDir(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
+	return newDir(ctx, msrc, map[string]*fs.Inode{
+		"cpu": newCPU(ctx, msrc),
+	})
+}
+
+func newDevicesDir(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
+	return newDir(ctx, msrc, map[string]*fs.Inode{
+		"system": newSystemDir(ctx, msrc),
+	})
+}

pkg/sentry/fs/sys/sys.go

@@ -22,13 +22,25 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
 )
 
+// sys is a root sys node.
+//
 // +stateify savable
-type dir struct {
+type sys struct {
 	ramfs.Dir
 }
 
+func newFile(node fs.InodeOperations, msrc *fs.MountSource) *fs.Inode {
+	sattr := fs.StableAttr{
+		DeviceID:  sysfsDevice.DeviceID(),
+		InodeID:   sysfsDevice.NextIno(),
+		BlockSize: usermem.PageSize,
+		Type:      fs.SpecialFile,
+	}
+	return fs.NewInode(node, msrc, sattr)
+}
+
 func newDir(ctx context.Context, msrc *fs.MountSource, contents map[string]*fs.Inode) *fs.Inode {
-	d := &dir{}
+	d := &sys{}
 	d.InitDir(ctx, contents, fs.RootOwner, fs.FilePermsFromMode(0555))
 	return fs.NewInode(d, msrc, fs.StableAttr{
 		DeviceID:  sysfsDevice.DeviceID(),
@@ -48,7 +60,7 @@ func New(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
 		"bus":      newDir(ctx, msrc, nil),
 		"class":    newDir(ctx, msrc, nil),
 		"dev":      newDir(ctx, msrc, nil),
-		"devices":  newDir(ctx, msrc, nil),
+		"devices":  newDevicesDir(ctx, msrc),
 		"firmware": newDir(ctx, msrc, nil),
 		"fs":       newDir(ctx, msrc, nil),
 		"kernel":   newDir(ctx, msrc, nil),