scanner.go

// Package osrelease provides an "os-release" distribution scanner.
package osrelease

import (
	"bufio"
	"bytes"
	"context"
	"fmt"
	"io"
	"runtime/trace"
	"sort"
	"strings"

	"github.com/quay/zlog"

	"github.com/quay/claircore"
	"github.com/quay/claircore/indexer"
	"github.com/quay/claircore/pkg/cpe"
	"github.com/quay/claircore/pkg/tarfs"
)

const (
	scannerName    = "os-release"
	scannerVersion = "2"
	scannerKind    = "distribution"
)

// Path and FallbackPath are the two documented locations for the os-release
// file. The latter should be consulted only if the former does not exist.
const (
	Path         = `etc/os-release`
	FallbackPath = `usr/lib/os-release`
)

var (
	_ indexer.DistributionScanner = (*Scanner)(nil)
	_ indexer.VersionedScanner    = (*Scanner)(nil)
)

// Scanner implements a scanner.DistributionScanner that examines os-release
// files, as documented at
// https://www.freedesktop.org/software/systemd/man/os-release.html
type Scanner struct{}

// Name implements scanner.VersionedScanner.
func (*Scanner) Name() string { return scannerName }

// Version implements scanner.VersionedScanner.
func (*Scanner) Version() string { return scannerVersion }

// Kind implements scanner.VersionedScanner.
func (*Scanner) Kind() string { return scannerKind }

// Scan reports any found os-release Distribution information in the provided
// layer.
//
// It's an expected outcome to return (nil, nil) when the os-release file is not
// present in the layer.
func (s *Scanner) Scan(ctx context.Context, l *claircore.Layer) ([]*claircore.Distribution, error) {
	defer trace.StartRegion(ctx, "Scanner.Scan").End()
	ctx = zlog.ContextWithValues(ctx,
		"component", "osrelease/Scanner.Scan",
		"version", s.Version(),
		"layer", l.Hash.String())
	zlog.Debug(ctx).Msg("start")
	defer zlog.Debug(ctx).Msg("done")

	r, err := l.Reader()
	if err != nil {
		return nil, fmt.Errorf("osrelease: unable to open layer: %w", err)
	}
	defer r.Close()
	sys, err := tarfs.New(r)
	if err != nil {
		return nil, fmt.Errorf("osrelease: unable to open layer: %w", err)
	}

	// Attempt to parse each os-release file encountered. On a successful parse,
	// return the distribution.
	var rd io.Reader
	for _, n := range []string{Path, FallbackPath} {
		f, err := sys.Open(n)
		if err != nil {
			zlog.Debug(ctx).
				Str("name", n).
				Err(err).
				Msg("unable to open file")
			continue
		}
		defer f.Close()
		rd = f
		break
	}
	if rd == nil {
		zlog.Debug(ctx).Msg("didn't find an os-release file")
		return nil, nil
	}
	d, err := toDist(ctx, rd)
	if err != nil {
		return nil, err
	}
	return []*claircore.Distribution{d}, nil
}

// ToDist returns the distribution information from the file contents provided on
// r.
func toDist(ctx context.Context, r io.Reader) (*claircore.Distribution, error) {
	ctx = zlog.ContextWithValues(ctx,
		"component", "osrelease/parse")
	defer trace.StartRegion(ctx, "parse").End()
	m, err := Parse(ctx, r)
	if err != nil {
		return nil, err
	}
	d := claircore.Distribution{
		Name: "Linux",
		DID:  "linux",
	}
	ks := make([]string, 0, len(m))
	for key := range m {
		ks = append(ks, key)
	}
	sort.Strings(ks)
	for _, key := range ks {
		value := m[key]
		switch key {
		case "ID":
			zlog.Debug(ctx).Msg("found ID")
			d.DID = value
		case "VERSION_ID":
			zlog.Debug(ctx).Msg("found VERSION_ID")
			d.VersionID = value
		case "BUILD_ID":
		case "VARIANT_ID":
		case "CPE_NAME":
			zlog.Debug(ctx).Msg("found CPE_NAME")
			wfn, err := cpe.Unbind(value)
			if err != nil {
				zlog.Warn(ctx).
					Err(err).
					Str("value", value).
					Msg("failed to unbind the cpe")
				break
			}
			d.CPE = wfn
		case "NAME":
			zlog.Debug(ctx).Msg("found NAME")
			d.Name = value
		case "VERSION":
			zlog.Debug(ctx).Msg("found VERSION")
			d.Version = value
		case "ID_LIKE":
		case "VERSION_CODENAME":
			zlog.Debug(ctx).Msg("found VERISON_CODENAME")
			d.VersionCodeName = value
		case "PRETTY_NAME":
			zlog.Debug(ctx).Msg("found PRETTY_NAME")
			d.PrettyName = value
		case "REDHAT_BUGZILLA_PRODUCT":
			zlog.Debug(ctx).Msg("using RHEL hack")
			// This is a dirty hack because the Red Hat OVAL database and the
			// CPE contained in the os-release file don't agree.
			d.PrettyName = value
		}
	}
	zlog.Debug(ctx).Str("name", d.Name).Msg("found dist")
	return &d, nil
}

// Parse splits the contents r into key-value pairs as described in
// os-release(5).
//
// See comments in the source for edge cases.
func Parse(ctx context.Context, r io.Reader) (map[string]string, error) {
	ctx = zlog.ContextWithValues(ctx, "component", "osrelease/Parse")
	defer trace.StartRegion(ctx, "Parse").End()
	m := make(map[string]string)
	s := bufio.NewScanner(r)
	s.Split(bufio.ScanLines)
	for s.Scan() && ctx.Err() == nil {
		b := s.Bytes()
		switch {
		case len(b) == 0:
			continue
		case b[0] == '#':
			continue
		}
		eq := bytes.IndexRune(b, '=')
		if eq == -1 {
			return nil, fmt.Errorf("osrelease: malformed line %q", s.Text())
		}
		key := strings.TrimSpace(string(b[:eq]))
		value := strings.TrimSpace(string(b[eq+1:]))

		// The value side is defined to follow shell-like quoting rules, which I
		// take to mean:
		//
		// - Within single quotes, no characters are special, and escaping is
		//   not possible. The only special case that needs to be handled is
		//   getting a single quote, which is done in shell by ending the
		//   string, escaping a single quote, then starting a new string.
		//
		// - Within double quotes, single quotes are not special, but double
		//   quotes and a handful of other characters are, and almost the entire
		//   lower-case ASCII alphabet can be escaped to produce various
		//   codepoints.
		//
		// With these in mind, the arms of the switch below implement the first
		// case and a limited version of the second.
		switch value[0] {
		case '\'':
			value = strings.TrimFunc(value, func(r rune) bool { return r == '\'' })
			value = strings.ReplaceAll(value, `'\''`, `'`)
		case '"':
			// This only implements the metacharacters that are called out in
			// the os-release documentation.
			value = strings.TrimFunc(value, func(r rune) bool { return r == '"' })
			value = dqReplacer.Replace(value)
		default:
		}

		m[key] = value
	}
	if err := s.Err(); err != nil {
		return nil, err
	}
	if err := ctx.Err(); err != nil {
		return nil, err
	}
	return m, nil
}

var dqReplacer = strings.NewReplacer(
	"\\`", "`",
	`\\`, `\`,
	`\"`, `"`,
	`\$`, `$`,
)