1// SPDX-License-Identifier: MIT
2
3package vm
4
5import (
6 "context"
7 "crypto/sha256"
8 "encoding/hex"
9 "fmt"
10 "io"
11 "os"
12 "path/filepath"
13
14 "bnema.dev/ermite-runner/internal/cmdrun"
15 "bnema.dev/ermite-runner/internal/sandbox"
16)
17
18// ensureExecDisk builds (or reuses) the small read-only ext4 disk that carries
19// the runner's own executable: the guest root during boot, and /opt/ermite
20// afterwards. It is named after the executable's hash, so a runner upgrade
21// builds a new one and no image rebuild is needed; older ones are removed.
22// mkfs.ext4 -d needs no root.
23func ensureExecDisk(ctx context.Context, run cmdrun.Runner, exe, stateDir string) (string, error) {
24 sum, size, err := hashFile(exe)
25 if err != nil {
26 return "", fmt.Errorf("hash executable: %w", err)
27 }
28 cache := filepath.Join(stateDir, "cache")
29 if err := os.MkdirAll(cache, 0o755); err != nil {
30 return "", err
31 }
32 name := "execdisk-" + sum[:16] + ".ext4"
33 dest := filepath.Join(cache, name)
34 if _, err := os.Stat(dest); err == nil {
35 return dest, nil
36 }
37 // Drop disks of other executable versions.
38 if olds, _ := filepath.Glob(filepath.Join(cache, "execdisk-*")); len(olds) > 0 {
39 for _, o := range olds {
40 _ = os.Remove(o)
41 }
42 }
43
44 tmp, err := os.MkdirTemp(cache, "execroot-")
45 if err != nil {
46 return "", err
47 }
48 defer func() { _ = os.RemoveAll(tmp) }()
49 for _, d := range []string{"proc", "sys", "dev", "mnt/base", "mnt/scratch", "mnt/newroot"} {
50 if err := os.MkdirAll(filepath.Join(tmp, d), 0o755); err != nil {
51 return "", err
52 }
53 }
54 if err := copyFile(exe, filepath.Join(tmp, sandbox.GuestExecName)); err != nil {
55 return "", fmt.Errorf("copy executable: %w", err)
56 }
57 if err := os.Chmod(filepath.Join(tmp, sandbox.GuestExecName), 0o755); err != nil {
58 return "", err
59 }
60
61 part := dest + ".part"
62 _ = os.Remove(part)
63 f, err := os.OpenFile(part, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
64 if err != nil {
65 return "", err
66 }
67 // The executable, ext4 overhead and some room; the disk is read-only.
68 mib := size/(1<<20)*115/100 + 16
69 if err := f.Truncate(mib << 20); err != nil {
70 _ = f.Close()
71 return "", err
72 }
73 _ = f.Close()
74 if _, err := run.Run(ctx, nil, "mkfs.ext4", "-q", "-F", "-L", "ermite-exec",
75 "-O", "^has_journal", "-E", "root_owner=0:0", "-d", tmp, part); err != nil {
76 _ = os.Remove(part)
77 return "", fmt.Errorf("build exec disk: %w", err)
78 }
79 if err := os.Rename(part, dest); err != nil {
80 return "", err
81 }
82 return dest, nil
83}
84
85func hashFile(path string) (sum string, size int64, err error) {
86 f, err := os.Open(path)
87 if err != nil {
88 return "", 0, err
89 }
90 defer func() { _ = f.Close() }()
91 h := sha256.New()
92 n, err := io.Copy(h, f)
93 if err != nil {
94 return "", 0, err
95 }
96 return hex.EncodeToString(h.Sum(nil)), n, nil
97}