Add serial number example for linux

This is how collection of serial could look on Linux
(and this lib also supports Windows).

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

posture/serial_linux.go

@@ -0,0 +1,82 @@
+//go:build linux
+
+package posture
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/digitalocean/go-smbios"
+)
+
+// GetByte retrieves a 8-bit unsigned integer at the given offset.
+func GetByte(s *smbios.Structure, offset int) uint8 {
+	// the `Formatted` byte slice is missing the first 4 bytes of the structure that are stripped out as header info.
+	// so we need to subtract 4 from the offset mentioned in the SMBIOS documentation to get the right value.
+	index := offset - 4
+	if index >= len(s.Formatted) {
+		return 0
+	}
+
+	return s.Formatted[index]
+}
+
+// GetStringOrEmpty retrieves a string at the given offset.
+// Returns an empty string if no string was present.
+func GetStringOrEmpty(s *smbios.Structure, offset int) (string, error) {
+	index := GetByte(s, offset)
+
+	if index == 0 || int(index) > len(s.Strings) {
+		return errors.New("offset does not exist in smbios structure")
+	}
+
+	str := s.Strings[index-1]
+	trimmed := strings.TrimSpace(str)
+
+	// Convert to lowercase to address multiple formats:
+	//   - "To Be Filled By O.E.M."
+	//   - "To be filled by O.E.M."
+	if strings.ToLower(trimmed) == "to be filled by o.e.m." {
+		return errors.New("data is not provided by O.E.M.")
+	}
+
+	return trimmed
+}
+
+// System Information (Type 1) structure
+// https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.1.1.pdf
+// Page 34
+const (
+	sysInfoHeaderType  = 1
+	serialNumberOffset = 0x07
+)
+
+func getSerialNumber() (string, error) {
+	// Find SMBIOS data in operating system-specific location.
+	rc, _, err := smbios.Stream()
+	if err != nil {
+		return "", fmt.Errorf("failed to open dmi/smbios stream: %w", err)
+	}
+	defer rc.Close()
+
+	// Decode SMBIOS structures from the stream.
+	d := smbios.NewDecoder(rc)
+	ss, err := d.Decode()
+	if err != nil {
+		return "", fmt.Errorf("failed to decode dmi/smbios structures: %w", err)
+	}
+
+	for _, s := range ss {
+		if s.Header.Type == sysInfoHeaderType {
+			serial, err := GetStringFromSmbiosStructure(s, serialNumberOffset)
+			if err != nil {
+				return "", fmt.Errorf("could not read serial from dmi/smbios structure: %w", err)
+			}
+
+			return serial, nil
+		}
+	}
+
+	return "", fmt.Errorf("could not read serial from dmi/smbios structure: no data found")
+}