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// Copyright 2020 The gVisor Authors.
//
// 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 devtmpfs provides an implementation of /dev based on tmpfs,
// analogous to Linux's devtmpfs.
package devtmpfs
import (
"fmt"
"gvisor.dev/gvisor/pkg/abi/linux"
"gvisor.dev/gvisor/pkg/context"
"gvisor.dev/gvisor/pkg/fspath"
"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
"gvisor.dev/gvisor/pkg/sentry/vfs"
"gvisor.dev/gvisor/pkg/sync"
)
// Name is the default filesystem name.
const Name = "devtmpfs"
// FilesystemType implements vfs.FilesystemType.
type FilesystemType struct {
initOnce sync.Once
initErr error
// fs is the tmpfs filesystem that backs all mounts of this FilesystemType.
// root is fs' root. fs and root are immutable.
fs *vfs.Filesystem
root *vfs.Dentry
}
// GetFilesystem implements vfs.FilesystemType.GetFilesystem.
func (fst *FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
fst.initOnce.Do(func() {
fs, root, err := tmpfs.FilesystemType{}.GetFilesystem(ctx, vfsObj, creds, "" /* source */, vfs.GetFilesystemOptions{
Data: "mode=0755", // opts from drivers/base/devtmpfs.c:devtmpfs_init()
})
if err != nil {
fst.initErr = err
return
}
fst.fs = fs
fst.root = root
})
if fst.initErr != nil {
return nil, nil, fst.initErr
}
fst.fs.IncRef()
fst.root.IncRef()
return fst.fs, fst.root, nil
}
// Accessor allows devices to create device special files in devtmpfs.
type Accessor struct {
vfsObj *vfs.VirtualFilesystem
mntns *vfs.MountNamespace
root vfs.VirtualDentry
creds *auth.Credentials
}
// NewAccessor returns an Accessor that supports creation of device special
// files in the devtmpfs instance registered with name fsTypeName in vfsObj.
func NewAccessor(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, fsTypeName string) (*Accessor, error) {
mntns, err := vfsObj.NewMountNamespace(ctx, creds, "devtmpfs" /* source */, fsTypeName, &vfs.GetFilesystemOptions{})
if err != nil {
return nil, err
}
return &Accessor{
vfsObj: vfsObj,
mntns: mntns,
root: mntns.Root(),
creds: creds,
}, nil
}
// Release must be called when a is no longer in use.
func (a *Accessor) Release() {
a.root.DecRef()
a.mntns.DecRef()
}
// accessorContext implements context.Context by extending an existing
// context.Context with an Accessor's values for VFS-relevant state.
type accessorContext struct {
context.Context
a *Accessor
}
func (a *Accessor) wrapContext(ctx context.Context) *accessorContext {
return &accessorContext{
Context: ctx,
a: a,
}
}
// Value implements context.Context.Value.
func (ac *accessorContext) Value(key interface{}) interface{} {
switch key {
case vfs.CtxMountNamespace:
ac.a.mntns.IncRef()
return ac.a.mntns
case vfs.CtxRoot:
ac.a.root.IncRef()
return ac.a.root
default:
return ac.Context.Value(key)
}
}
func (a *Accessor) pathOperationAt(pathname string) *vfs.PathOperation {
return &vfs.PathOperation{
Root: a.root,
Start: a.root,
Path: fspath.Parse(pathname),
}
}
// CreateDeviceFile creates a device special file at the given pathname in the
// devtmpfs instance accessed by the Accessor.
func (a *Accessor) CreateDeviceFile(ctx context.Context, pathname string, kind vfs.DeviceKind, major, minor uint32, perms uint16) error {
mode := (linux.FileMode)(perms)
switch kind {
case vfs.BlockDevice:
mode |= linux.S_IFBLK
case vfs.CharDevice:
mode |= linux.S_IFCHR
default:
panic(fmt.Sprintf("invalid vfs.DeviceKind: %v", kind))
}
// NOTE: Linux's devtmpfs refuses to automatically delete files it didn't
// create, which it recognizes by storing a pointer to the kdevtmpfs struct
// thread in struct inode::i_private. Accessor doesn't yet support deletion
// of files at all, and probably won't as long as we don't need to support
// kernel modules, so this is moot for now.
return a.vfsObj.MknodAt(a.wrapContext(ctx), a.creds, a.pathOperationAt(pathname), &vfs.MknodOptions{
Mode: mode,
DevMajor: major,
DevMinor: minor,
})
}
// UserspaceInit creates symbolic links and mount points in the devtmpfs
// instance accessed by the Accessor that are created by userspace in Linux. It
// does not create mounts.
func (a *Accessor) UserspaceInit(ctx context.Context) error {
actx := a.wrapContext(ctx)
// systemd: src/shared/dev-setup.c:dev_setup()
for _, symlink := range []struct {
source string
target string
}{
// /proc/kcore is not implemented.
{source: "fd", target: "/proc/self/fd"},
{source: "stdin", target: "/proc/self/fd/0"},
{source: "stdout", target: "/proc/self/fd/1"},
{source: "stderr", target: "/proc/self/fd/2"},
} {
if err := a.vfsObj.SymlinkAt(actx, a.creds, a.pathOperationAt(symlink.source), symlink.target); err != nil {
return fmt.Errorf("failed to create symlink %q => %q: %v", symlink.source, symlink.target, err)
}
}
// systemd: src/core/mount-setup.c:mount_table
for _, dir := range []string{
"shm",
"pts",
} {
if err := a.vfsObj.MkdirAt(actx, a.creds, a.pathOperationAt(dir), &vfs.MkdirOptions{
// systemd: src/core/mount-setup.c:mount_one()
Mode: 0755,
}); err != nil {
return fmt.Errorf("failed to create directory %q: %v", dir, err)
}
}
return nil
}
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