|
| 1 | +package main |
| 2 | + |
| 3 | +import ( |
| 4 | + "encoding/json" |
| 5 | + "errors" |
| 6 | + "fmt" |
| 7 | + "os" |
| 8 | + "path/filepath" |
| 9 | + "syscall" |
| 10 | + |
| 11 | + "github.com/urfave/cli" |
| 12 | + |
| 13 | + "github.com/opencontainers/runtime-spec/specs-go" |
| 14 | +) |
| 15 | + |
| 16 | +const usage = `contrib/cmd/remap-rootfs |
| 17 | +
|
| 18 | +remap-rootfs is a helper tool to remap the root filesystem of a Open Container |
| 19 | +Initiative bundle using user namespaces such that the file owners are remapped |
| 20 | +from "host" mappings to the user namespace's mappings. |
| 21 | +
|
| 22 | +Effectively, this is a slightly more complicated 'chown -R', and is primarily |
| 23 | +used within runc's integration tests to remap the test filesystem to match the |
| 24 | +test user namespace. Note that calling remap-rootfs multiple times, or changing |
| 25 | +the mapping and then calling remap-rootfs will likely produce incorrect results |
| 26 | +because we do not "un-map" any pre-applied mappings from previous remap-rootfs |
| 27 | +calls. |
| 28 | +
|
| 29 | +Note that the bundle is assumed to be produced by a trusted source, and thus |
| 30 | +malicious configuration files will likely not be handled safely. |
| 31 | +
|
| 32 | +To use remap-rootfs, simply pass it the path to an OCI bundle (a directory |
| 33 | +containing a config.json): |
| 34 | +
|
| 35 | + $ sudo remap-rootfs ./bundle |
| 36 | +` |
| 37 | + |
| 38 | +func toHostID(mappings []specs.LinuxIDMapping, id uint32) (int, bool) { |
| 39 | + for _, m := range mappings { |
| 40 | + if m.ContainerID <= id && id < m.ContainerID+m.Size { |
| 41 | + return int(m.HostID + id), true |
| 42 | + } |
| 43 | + } |
| 44 | + return -1, false |
| 45 | +} |
| 46 | + |
| 47 | +type inodeID struct { |
| 48 | + Dev, Ino uint64 |
| 49 | +} |
| 50 | + |
| 51 | +func toInodeID(st *syscall.Stat_t) inodeID { |
| 52 | + return inodeID{Dev: st.Dev, Ino: st.Ino} |
| 53 | +} |
| 54 | + |
| 55 | +func remapRootfs(root string, uidMap, gidMap []specs.LinuxIDMapping) error { |
| 56 | + seenInodes := make(map[inodeID]struct{}) |
| 57 | + return filepath.Walk(root, func(path string, info os.FileInfo, err error) error { |
| 58 | + if err != nil { |
| 59 | + return err |
| 60 | + } |
| 61 | + |
| 62 | + mode := info.Mode() |
| 63 | + st := info.Sys().(*syscall.Stat_t) |
| 64 | + |
| 65 | + // Skip symlinks. |
| 66 | + if mode.Type() == os.ModeSymlink { |
| 67 | + return nil |
| 68 | + } |
| 69 | + // Skip hard-links to files we've already remapped. |
| 70 | + id := toInodeID(st) |
| 71 | + if _, seen := seenInodes[id]; seen { |
| 72 | + return nil |
| 73 | + } |
| 74 | + seenInodes[id] = struct{}{} |
| 75 | + |
| 76 | + // Calculate the new uid:gid. |
| 77 | + uid := st.Uid |
| 78 | + newUID, ok1 := toHostID(uidMap, uid) |
| 79 | + gid := st.Gid |
| 80 | + newGID, ok2 := toHostID(gidMap, gid) |
| 81 | + |
| 82 | + // Skip files that cannot be mapped. |
| 83 | + if !ok1 || !ok2 { |
| 84 | + niceName := path |
| 85 | + if relName, err := filepath.Rel(root, path); err == nil { |
| 86 | + niceName = "/" + relName |
| 87 | + } |
| 88 | + fmt.Printf("skipping file %s: cannot remap user %d:%d -> %d:%d\n", niceName, uid, gid, newUID, newGID) |
| 89 | + return nil |
| 90 | + } |
| 91 | + if err := os.Lchown(path, newUID, newGID); err != nil { |
| 92 | + return err |
| 93 | + } |
| 94 | + // Re-apply any setid bits that would be cleared due to chown(2). |
| 95 | + return os.Chmod(path, mode) |
| 96 | + }) |
| 97 | +} |
| 98 | + |
| 99 | +func main() { |
| 100 | + app := cli.NewApp() |
| 101 | + app.Name = "remap-rootfs" |
| 102 | + app.Usage = usage |
| 103 | + |
| 104 | + app.Action = func(ctx *cli.Context) error { |
| 105 | + args := ctx.Args() |
| 106 | + if len(args) != 1 { |
| 107 | + return errors.New("exactly one bundle argument must be provided") |
| 108 | + } |
| 109 | + bundle := args[0] |
| 110 | + |
| 111 | + configFile, err := os.Open(filepath.Join(bundle, "config.json")) |
| 112 | + if err != nil { |
| 113 | + return err |
| 114 | + } |
| 115 | + defer configFile.Close() |
| 116 | + |
| 117 | + var spec specs.Spec |
| 118 | + if err := json.NewDecoder(configFile).Decode(&spec); err != nil { |
| 119 | + return fmt.Errorf("parsing config.json: %w", err) |
| 120 | + } |
| 121 | + |
| 122 | + if spec.Root == nil { |
| 123 | + return errors.New("invalid config.json: root section is null") |
| 124 | + } |
| 125 | + rootfs := filepath.Join(bundle, spec.Root.Path) |
| 126 | + |
| 127 | + if spec.Linux == nil { |
| 128 | + return errors.New("invalid config.json: linux section is null") |
| 129 | + } |
| 130 | + uidMap := spec.Linux.UIDMappings |
| 131 | + gidMap := spec.Linux.GIDMappings |
| 132 | + if len(uidMap) == 0 && len(gidMap) == 0 { |
| 133 | + fmt.Println("skipping remapping -- no userns mappings specified") |
| 134 | + return nil |
| 135 | + } |
| 136 | + |
| 137 | + return remapRootfs(rootfs, uidMap, gidMap) |
| 138 | + } |
| 139 | + if err := app.Run(os.Args); err != nil { |
| 140 | + fmt.Fprintln(os.Stderr, "error:", err) |
| 141 | + os.Exit(1) |
| 142 | + } |
| 143 | +} |
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