hostinet: support /proc/net/snmp and /proc/net/dev

For hostinet, we inherit the data from host procfs. To to that, we
cache the fds for these files for later reads.

Fixes #506

Signed-off-by: Jianfeng Tan <henry.tjf@antfin.com>
Change-Id: I2f81215477455b9c59acf67e33f5b9af28ee0165

pkg/sentry/socket/hostinet/stack.go

@@ -16,8 +16,11 @@ package hostinet
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
+	"reflect"
+	"strconv"
 	"strings"
 	"syscall"
 
@@ -26,6 +29,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/inet"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/syserr"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
@@ -51,6 +55,8 @@ type Stack struct {
 	tcpRecvBufSize inet.TCPBufferSize
 	tcpSendBufSize inet.TCPBufferSize
 	tcpSACKEnabled bool
+	netDevFile     *os.File
+	netSNMPFile    *os.File
 }
 
 // NewStack returns an empty Stack containing no configuration.
@@ -98,6 +104,18 @@ func (s *Stack) Configure() error {
 		log.Warningf("Failed to read if TCP SACK if enabled, setting to true")
 	}
 
+	if f, err := os.Open("/proc/net/dev"); err != nil {
+		log.Warningf("Failed to open /proc/net/dev: %v", err)
+	} else {
+		s.netDevFile = f
+	}
+
+	if f, err := os.Open("/proc/net/snmp"); err != nil {
+		log.Warningf("Failed to open /proc/net/snmp: %v", err)
+	} else {
+		s.netSNMPFile = f
+	}
+
 	return nil
 }
 
@@ -326,9 +344,95 @@ func (s *Stack) SetTCPSACKEnabled(enabled bool) error {
 	return syserror.EACCES
 }
 
+// getLine reads one line from proc file, with specified prefix.
+// The last argument, withHeader, specifies if it contains line header.
+func getLine(f *os.File, prefix string, withHeader bool) string {
+	data := make([]byte, 4096)
+
+	if _, err := f.Seek(0, 0); err != nil {
+		return ""
+	}
+
+	if _, err := io.ReadFull(f, data); err != io.ErrUnexpectedEOF {
+		return ""
+	}
+
+	prefix = prefix + ":"
+	lines := strings.Split(string(data), "\n")
+	for _, l := range lines {
+		l = strings.TrimSpace(l)
+		if strings.HasPrefix(l, prefix) {
+			if withHeader {
+				withHeader = false
+				continue
+			}
+			return l
+		}
+	}
+	return ""
+}
+
+func toSlice(i interface{}) []uint64 {
+	v := reflect.Indirect(reflect.ValueOf(i))
+	return v.Slice(0, v.Len()).Interface().([]uint64)
+}
+
 // Statistics implements inet.Stack.Statistics.
 func (s *Stack) Statistics(stat interface{}, arg string) error {
-	return syserror.EOPNOTSUPP
+	var (
+		snmpTCP   bool
+		rawLine   string
+		sliceStat []uint64
+	)
+
+	switch stat.(type) {
+	case *inet.StatDev:
+		if s.netDevFile == nil {
+			return fmt.Errorf("/proc/net/dev is not opened for hostinet")
+		}
+		rawLine = getLine(s.netDevFile, arg, false /* with no header */)
+	case *inet.StatSNMPIP, *inet.StatSNMPICMP, *inet.StatSNMPICMPMSG, *inet.StatSNMPTCP, *inet.StatSNMPUDP, *inet.StatSNMPUDPLite:
+		if s.netSNMPFile == nil {
+			return fmt.Errorf("/proc/net/snmp is not opened for hostinet")
+		}
+		rawLine = getLine(s.netSNMPFile, arg, true)
+	default:
+		return syserr.ErrEndpointOperation.ToError()
+	}
+
+	if rawLine == "" {
+		return fmt.Errorf("Failed to get raw line")
+	}
+
+	parts := strings.SplitN(rawLine, ":", 2)
+	if len(parts) != 2 {
+		return fmt.Errorf("Failed to get prefix from: %q", rawLine)
+	}
+
+	sliceStat = toSlice(stat)
+	fields := strings.Fields(strings.TrimSpace(parts[1]))
+	if len(fields) != len(sliceStat) {
+		return fmt.Errorf("Failed to parse fields: %q", rawLine)
+	}
+	if _, ok := stat.(*inet.StatSNMPTCP); ok {
+		snmpTCP = true
+	}
+	for i := 0; i < len(sliceStat); i++ {
+		var err error
+		if snmpTCP && i == 3 {
+			var tmp int64
+			// MaxConn field is signed, RFC 2012.
+			tmp, err = strconv.ParseInt(fields[i], 10, 64)
+			sliceStat[i] = uint64(tmp) // Convert back to int before use.
+		} else {
+			sliceStat[i], err = strconv.ParseUint(fields[i], 10, 64)
+		}
+		if err != nil {
+			return fmt.Errorf("Failed to parse field %d from: %q, %v", i, rawLine, err)
+		}
+	}
+
+	return nil
 }
 
 // RouteTable implements inet.Stack.RouteTable.