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// Copyright 2019 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 user contains methods for resolving filesystem paths based on the
// user and their environment.
package user
import (
"bufio"
"fmt"
"io"
"strconv"
"strings"
"gvisor.dev/gvisor/pkg/abi/linux"
"gvisor.dev/gvisor/pkg/context"
"gvisor.dev/gvisor/pkg/fspath"
"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
"gvisor.dev/gvisor/pkg/sentry/vfs"
"gvisor.dev/gvisor/pkg/usermem"
)
type fileReader struct {
ctx context.Context
fd *vfs.FileDescription
}
func (r *fileReader) Read(buf []byte) (int, error) {
n, err := r.fd.Read(r.ctx, usermem.BytesIOSequence(buf), vfs.ReadOptions{})
return int(n), err
}
func getExecUserHome(ctx context.Context, mns *vfs.MountNamespace, uid auth.KUID) (string, error) {
const defaultHome = "/"
root := mns.Root()
root.IncRef()
defer root.DecRef(ctx)
creds := auth.CredentialsFromContext(ctx)
target := &vfs.PathOperation{
Root: root,
Start: root,
Path: fspath.Parse("/etc/passwd"),
}
stat, err := root.Mount().Filesystem().VirtualFilesystem().StatAt(ctx, creds, target, &vfs.StatOptions{Mask: linux.STATX_TYPE})
if err != nil {
return defaultHome, nil
}
if stat.Mask&linux.STATX_TYPE == 0 || stat.Mode&linux.FileTypeMask != linux.ModeRegular {
return defaultHome, nil
}
opts := &vfs.OpenOptions{
Flags: linux.O_RDONLY,
}
fd, err := root.Mount().Filesystem().VirtualFilesystem().OpenAt(ctx, creds, target, opts)
if err != nil {
return defaultHome, nil
}
defer fd.DecRef(ctx)
r := &fileReader{
ctx: ctx,
fd: fd,
}
homeDir, err := findHomeInPasswd(uint32(uid), r, defaultHome)
if err != nil {
return "", err
}
return homeDir, nil
}
// MaybeAddExecUserHome returns a new slice with the HOME environment
// variable set if the slice does not already contain it, otherwise it returns
// the original slice unmodified.
func MaybeAddExecUserHome(ctx context.Context, vmns *vfs.MountNamespace, uid auth.KUID, envv []string) ([]string, error) {
// Check if the envv already contains HOME.
for _, env := range envv {
if strings.HasPrefix(env, "HOME=") {
// We have it. Return the original slice unmodified.
return envv, nil
}
}
// Read /etc/passwd for the user's HOME directory and set the HOME
// environment variable as required by POSIX if it is not overridden by
// the user.
homeDir, err := getExecUserHome(ctx, vmns, uid)
if err != nil {
return nil, fmt.Errorf("error reading exec user: %v", err)
}
return append(envv, "HOME="+homeDir), nil
}
// findHomeInPasswd parses a passwd file and returns the given user's home
// directory. This function does it's best to replicate the runc's behavior.
func findHomeInPasswd(uid uint32, passwd io.Reader, defaultHome string) (string, error) {
s := bufio.NewScanner(passwd)
for s.Scan() {
if err := s.Err(); err != nil {
return "", err
}
line := strings.TrimSpace(s.Text())
if line == "" {
continue
}
// Pull out part of passwd entry. Loosely parse the passwd entry as some
// passwd files could be poorly written and for compatibility with runc.
//
// Per 'man 5 passwd'
// /etc/passwd contains one line for each user account, with seven
// fields delimited by colons (“:”). These fields are:
//
// - login name
// - optional encrypted password
// - numerical user ID
// - numerical group ID
// - user name or comment field
// - user home directory
// - optional user command interpreter
parts := strings.Split(line, ":")
found := false
homeDir := ""
for i, p := range parts {
switch i {
case 2:
parsedUID, err := strconv.ParseUint(p, 10, 32)
if err == nil && parsedUID == uint64(uid) {
found = true
}
case 5:
homeDir = p
}
}
if found {
// NOTE: If the uid is present but the home directory is not
// present in the /etc/passwd entry we return an empty string. This
// is, for better or worse, what runc does.
return homeDir, nil
}
}
return defaultHome, nil
}
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