Unimplemented syscall counter: Pre-allocate 1-element slices of field values.

This is necessary in order to avoid a heap allocation when calling
`unimplementedSyscallCounter.Increment`. Prior to this CL, this code actually
allocated a 1-element slice, because the `Increment` method takes in a
variadic number of arguments.

Somehow this had not tripped up the `+checkescape` check at submission time,
but running `nogo` manually confirms the problem. This CL fixes it.

PiperOrigin-RevId: 539737956

pkg/sentry/kernel/syscalls.go

@@ -43,7 +43,7 @@ const (
 
 // outOfRangeSyscallNumber is used to represent a syscall number that is out of the
 // range [0, maxSyscallNum] in monitoring.
-var outOfRangeSyscallNumber = metric.FieldValue{"-1"}
+var outOfRangeSyscallNumber = []*metric.FieldValue{&metric.FieldValue{"-1"}}
 
 // SyscallSupportLevel is a syscall support levels.
 type SyscallSupportLevel int
@@ -359,7 +359,10 @@ var (
 
 	// unimplementedSyscallNumbers maps syscall numbers to their string representation.
 	// Used such that incrementing unimplementedSyscallCounter does not require allocating memory.
-	unimplementedSyscallNumbers map[uintptr]*metric.FieldValue
+	// Each element in the mapped slices are of length 1, as there is only one field for the
+	// unimplemented syscall counter metric. Allocating a slice is necessary as it is passed as a
+	// variadic argument to the metric library.
+	unimplementedSyscallNumbers map[uintptr][]*metric.FieldValue
 
 	// unimplementedSyscallCounter tracks the number of times each unimplemented syscall has been
 	// called by the sandboxed application.
@@ -392,13 +395,13 @@ func RegisterSyscallTable(s *SyscallTable) {
 	allSyscallTables = append(allSyscallTables, s)
 	unimplementedSyscallCounterInit.Do(func() {
 		allowedValues := make([]*metric.FieldValue, maxSyscallNum+2)
-		unimplementedSyscallNumbers = make(map[uintptr]*metric.FieldValue, len(allowedValues))
+		unimplementedSyscallNumbers = make(map[uintptr][]*metric.FieldValue, len(allowedValues))
 		for i := uintptr(0); i <= maxSyscallNum; i++ {
 			s := &metric.FieldValue{strconv.Itoa(int(i))}
 			allowedValues[i] = s
-			unimplementedSyscallNumbers[i] = s
+			unimplementedSyscallNumbers[i] = []*metric.FieldValue{s}
 		}
-		allowedValues[len(allowedValues)-1] = &outOfRangeSyscallNumber
+		allowedValues[len(allowedValues)-1] = outOfRangeSyscallNumber[0]
 		unimplementedSyscallCounter = metric.MustCreateNewUint64Metric("unimplemented_syscalls", true, "Number of times the application tried to call an unimplemented syscall, broken down by syscall number", metric.NewField("sysno", allowedValues...))
 	})
 	s.Init()
@@ -501,7 +504,7 @@ type SyscallInfo struct {
 func IncrementUnimplementedSyscallCounter(sysno uintptr) {
 	s, found := unimplementedSyscallNumbers[sysno]
 	if !found {
-		s = &outOfRangeSyscallNumber
+		s = outOfRangeSyscallNumber
 	}
-	unimplementedSyscallCounter.Increment(s)
+	unimplementedSyscallCounter.Increment(s...)
 }