task_fds.go

// 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 proc

import (
	"bytes"
	"fmt"
	"sort"
	"strconv"

	"github.com/MerlinKodo/gvisor/pkg/abi/linux"
	"github.com/MerlinKodo/gvisor/pkg/context"
	"github.com/MerlinKodo/gvisor/pkg/errors/linuxerr"
	"github.com/MerlinKodo/gvisor/pkg/sentry/fsimpl/kernfs"
	"github.com/MerlinKodo/gvisor/pkg/sentry/kernel"
	"github.com/MerlinKodo/gvisor/pkg/sentry/kernel/auth"
	"github.com/MerlinKodo/gvisor/pkg/sentry/vfs"
)

func getTaskFD(t *kernel.Task, fd int32) (*vfs.FileDescription, kernel.FDFlags) {
	var (
		file  *vfs.FileDescription
		flags kernel.FDFlags
	)
	t.WithMuLocked(func(t *kernel.Task) {
		if fdt := t.FDTable(); fdt != nil {
			file, flags = fdt.Get(fd)
		}
	})
	return file, flags
}

func taskFDExists(ctx context.Context, fs *filesystem, t *kernel.Task, fd int32) bool {
	var exists bool
	t.WithMuLocked(func(task *kernel.Task) {
		if fdt := t.FDTable(); fdt != nil {
			exists = fdt.Exists(fd)
		}
	})
	return exists
}

// +stateify savable
type fdDir struct {
	locks vfs.FileLocks

	fs   *filesystem
	task *kernel.Task

	// When produceSymlinks is set, dirents produces for the FDs are reported
	// as symlink. Otherwise, they are reported as regular files.
	produceSymlink bool
}

// IterDirents implements kernfs.inodeDirectory.IterDirents.
func (i *fdDir) IterDirents(ctx context.Context, mnt *vfs.Mount, cb vfs.IterDirentsCallback, offset, relOffset int64) (int64, error) {
	var fds []int32
	i.task.WithMuLocked(func(t *kernel.Task) {
		if fdTable := t.FDTable(); fdTable != nil {
			fds = fdTable.GetFDs(ctx)
		}
	})

	typ := uint8(linux.DT_REG)
	if i.produceSymlink {
		typ = linux.DT_LNK
	}

	// Find the appropriate starting point.
	idx := sort.Search(len(fds), func(i int) bool { return fds[i] >= int32(relOffset) })
	if idx >= len(fds) {
		return offset, nil
	}
	for _, fd := range fds[idx:] {
		dirent := vfs.Dirent{
			Name:    strconv.FormatUint(uint64(fd), 10),
			Type:    typ,
			Ino:     i.fs.NextIno(),
			NextOff: int64(fd) + 3,
		}
		if err := cb.Handle(dirent); err != nil {
			// Getdents should iterate correctly despite mutation
			// of fds, so we return the next fd to serialize plus
			// 2 (which accounts for the "." and ".." tracked by
			// kernfs) as the offset.
			return int64(fd) + 2, err
		}
	}
	// We serialized them all.  Next offset should be higher than last
	// serialized fd.
	return int64(fds[len(fds)-1]) + 3, nil
}

// fdDirInode represents the inode for /proc/[pid]/fd directory.
//
// +stateify savable
type fdDirInode struct {
	fdDir
	fdDirInodeRefs
	implStatFS
	kernfs.InodeAlwaysValid
	kernfs.InodeAttrs
	kernfs.InodeDirectoryNoNewChildren
	kernfs.InodeNotAnonymous
	kernfs.InodeNotSymlink
	kernfs.InodeTemporary
	kernfs.InodeWatches
	kernfs.OrderedChildren
}

var _ kernfs.Inode = (*fdDirInode)(nil)

func (fs *filesystem) newFDDirInode(ctx context.Context, task *kernel.Task) kernfs.Inode {
	inode := &fdDirInode{
		fdDir: fdDir{
			fs:             fs,
			task:           task,
			produceSymlink: true,
		},
	}
	inode.InodeAttrs.Init(ctx, task.Credentials(), linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.ModeDirectory|0555)
	inode.InitRefs()
	inode.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
	return inode
}

// IterDirents implements kernfs.inodeDirectory.IterDirents.
func (i *fdDirInode) IterDirents(ctx context.Context, mnt *vfs.Mount, cb vfs.IterDirentsCallback, offset, relOffset int64) (int64, error) {
	return i.fdDir.IterDirents(ctx, mnt, cb, offset, relOffset)
}

// Lookup implements kernfs.inodeDirectory.Lookup.
func (i *fdDirInode) Lookup(ctx context.Context, name string) (kernfs.Inode, error) {
	fdInt, err := strconv.ParseInt(name, 10, 32)
	if err != nil {
		return nil, linuxerr.ENOENT
	}
	fd := int32(fdInt)
	if !taskFDExists(ctx, i.fs, i.task, fd) {
		return nil, linuxerr.ENOENT
	}
	return i.fs.newFDSymlink(ctx, i.task, fd, i.fs.NextIno()), nil
}

// Open implements kernfs.Inode.Open.
func (i *fdDirInode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), d, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
		SeekEnd: kernfs.SeekEndZero,
	})
	if err != nil {
		return nil, err
	}
	return fd.VFSFileDescription(), nil
}

// CheckPermissions implements kernfs.Inode.CheckPermissions.
//
// This is to match Linux, which uses a special permission handler to guarantee
// that a process can still access /proc/self/fd after it has executed
// setuid. See fs/proc/fd.c:proc_fd_permission.
func (i *fdDirInode) CheckPermissions(ctx context.Context, creds *auth.Credentials, ats vfs.AccessTypes) error {
	err := i.InodeAttrs.CheckPermissions(ctx, creds, ats)
	if err == nil {
		// Access granted, no extra check needed.
		return nil
	}
	if t := kernel.TaskFromContext(ctx); t != nil {
		// Allow access if the task trying to access it is in the thread group
		// corresponding to this directory.
		if i.task.ThreadGroup() == t.ThreadGroup() {
			// Access granted (overridden).
			return nil
		}
	}
	return err
}

// DecRef implements kernfs.Inode.DecRef.
func (i *fdDirInode) DecRef(ctx context.Context) {
	i.fdDirInodeRefs.DecRef(func() { i.Destroy(ctx) })
}

// fdSymlink is an symlink for the /proc/[pid]/fd/[fd] file.
//
// +stateify savable
type fdSymlink struct {
	implStatFS
	kernfs.InodeAttrs
	kernfs.InodeNoopRefCount
	kernfs.InodeNotAnonymous
	kernfs.InodeSymlink
	kernfs.InodeWatches

	fs   *filesystem
	task *kernel.Task
	fd   int32
}

var _ kernfs.Inode = (*fdSymlink)(nil)

func (fs *filesystem) newFDSymlink(ctx context.Context, task *kernel.Task, fd int32, ino uint64) kernfs.Inode {
	inode := &fdSymlink{
		fs:   fs,
		task: task,
		fd:   fd,
	}
	inode.Init(ctx, task.Credentials(), linux.UNNAMED_MAJOR, fs.devMinor, ino, linux.ModeSymlink|0777)
	return inode
}

func (s *fdSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error) {
	file, _ := getTaskFD(s.task, s.fd)
	if file == nil {
		return "", linuxerr.ENOENT
	}
	defer s.fs.SafeDecRefFD(ctx, file)
	root := vfs.RootFromContext(ctx)
	defer s.fs.SafeDecRef(ctx, root)

	// Note: it's safe to reenter kernfs from Readlink if needed to resolve path.
	return s.task.Kernel().VFS().PathnameWithDeleted(ctx, root, file.VirtualDentry())
}

func (s *fdSymlink) Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error) {
	file, _ := getTaskFD(s.task, s.fd)
	if file == nil {
		return vfs.VirtualDentry{}, "", linuxerr.ENOENT
	}
	defer s.fs.SafeDecRefFD(ctx, file)
	vd := file.VirtualDentry()
	vd.IncRef()
	return vd, "", nil
}

// Valid implements kernfs.Inode.Valid.
func (s *fdSymlink) Valid(ctx context.Context) bool {
	return taskFDExists(ctx, s.fs, s.task, s.fd)
}

// fdInfoDirInode represents the inode for /proc/[pid]/fdinfo directory.
//
// +stateify savable
type fdInfoDirInode struct {
	fdDir
	fdInfoDirInodeRefs
	implStatFS
	kernfs.InodeAlwaysValid
	kernfs.InodeAttrs
	kernfs.InodeDirectoryNoNewChildren
	kernfs.InodeNotAnonymous
	kernfs.InodeNotSymlink
	kernfs.InodeTemporary
	kernfs.InodeWatches
	kernfs.OrderedChildren
}

var _ kernfs.Inode = (*fdInfoDirInode)(nil)

func (fs *filesystem) newFDInfoDirInode(ctx context.Context, task *kernel.Task) kernfs.Inode {
	inode := &fdInfoDirInode{
		fdDir: fdDir{
			fs:   fs,
			task: task,
		},
	}
	inode.InodeAttrs.Init(ctx, task.Credentials(), linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.ModeDirectory|0555)
	inode.InitRefs()
	inode.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
	return inode
}

// Lookup implements kernfs.inodeDirectory.Lookup.
func (i *fdInfoDirInode) Lookup(ctx context.Context, name string) (kernfs.Inode, error) {
	fdInt, err := strconv.ParseInt(name, 10, 32)
	if err != nil {
		return nil, linuxerr.ENOENT
	}
	fd := int32(fdInt)
	if !taskFDExists(ctx, i.fs, i.task, fd) {
		return nil, linuxerr.ENOENT
	}
	data := &fdInfoData{
		fs:   i.fs,
		task: i.task,
		fd:   fd,
	}
	return i.fs.newTaskOwnedInode(ctx, i.task, i.fs.NextIno(), 0444, data), nil
}

// IterDirents implements Inode.IterDirents.
func (i *fdInfoDirInode) IterDirents(ctx context.Context, mnt *vfs.Mount, cb vfs.IterDirentsCallback, offset, relOffset int64) (newOffset int64, err error) {
	return i.fdDir.IterDirents(ctx, mnt, cb, offset, relOffset)
}

// Open implements kernfs.Inode.Open.
func (i *fdInfoDirInode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), d, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
		SeekEnd: kernfs.SeekEndZero,
	})
	if err != nil {
		return nil, err
	}
	return fd.VFSFileDescription(), nil
}

// DecRef implements kernfs.Inode.DecRef.
func (i *fdInfoDirInode) DecRef(ctx context.Context) {
	i.fdInfoDirInodeRefs.DecRef(func() { i.Destroy(ctx) })
}

// fdInfoData implements vfs.DynamicBytesSource for /proc/[pid]/fdinfo/[fd].
//
// +stateify savable
type fdInfoData struct {
	kernfs.DynamicBytesFile

	fs   *filesystem
	task *kernel.Task
	fd   int32
}

var _ dynamicInode = (*fdInfoData)(nil)

// Generate implements vfs.DynamicBytesSource.Generate.
func (d *fdInfoData) Generate(ctx context.Context, buf *bytes.Buffer) error {
	file, descriptorFlags := getTaskFD(d.task, d.fd)
	if file == nil {
		return linuxerr.ENOENT
	}
	defer d.fs.SafeDecRefFD(ctx, file)
	// TODO(b/121266871): Include pos, locks, and other data. For now we only
	// have flags.
	// See https://www.kernel.org/doc/Documentation/filesystems/proc.txt
	flags := uint(file.StatusFlags()) | descriptorFlags.ToLinuxFileFlags()
	fmt.Fprintf(buf, "flags:\t0%o\n", flags)
	return nil
}

// Valid implements kernfs.Inode.Valid.
func (d *fdInfoData) Valid(ctx context.Context) bool {
	return taskFDExists(ctx, d.fs, d.task, d.fd)
}