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fix: prevent overflow on counter adding #1474

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fix: prevent overflow on counter adding #1474

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d222f97
demo: overflow case for counter
xieyuschen Mar 18, 2024
3332118
fix: fix the possible overflow
xieyuschen Mar 18, 2024
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19 changes: 15 additions & 4 deletions prometheus/counter.go
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Original file line number Diff line number Diff line change
Expand Up @@ -129,11 +129,22 @@ func (c *counter) Add(v float64) {
}

ival := uint64(v)
if float64(ival) == v {
atomic.AddUint64(&c.valInt, ival)
return
}

// if the v is an unsigned integer
// and the precise doesn't lose during uint cast and cast it back to float
for v == float64(ival) {
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@xieyuschen xieyuschen Mar 19, 2024 •
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one thing to note here is that the casting here varies in different platform. for example, in linux, the v won't be equal with float64(uint64(v)). but in macos m1, it equals. It means the checking here is needed and cannot be skipped

The verification code is:

func TestVerifyRounding(t *testing.T) {
	var f float64 = 1<<64 - 1
	if f == float64(uint64(f)) {
		t.Fatal("values are the same after casting")
	} else {
		t.Fatal("values are different")
	}
}

LInux output could be found in the pipeline here.
image

and the macos outputs:
image

oldUint := atomic.LoadUint64(&c.valInt)
leftNum := math.MaxUint64 - oldUint

if leftNum < ival {
// overflow happens, we need to handle as a float number
break
}
if atomic.CompareAndSwapUint64(&c.valInt, oldUint, oldUint+ival) {
return
}

}
for {
oldBits := atomic.LoadUint64(&c.valBits)
newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
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44 changes: 44 additions & 0 deletions prometheus/counter_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,50 @@ import (
"google.golang.org/protobuf/types/known/timestamppb"
)

func TestCounterAddExcess(t *testing.T) {
now := time.Now()
counter := NewCounter(CounterOpts{
Name: "test",
Help: "test help",
ConstLabels: Labels{"a": "1", "b": "2"},
now: func() time.Time { return now },
}).(*counter)

counter.Add(1 << 63)
if expected, got := uint64(1<<63), counter.valInt; expected != got {
t.Errorf("Expected %d, got %d.", expected, got)
}

counter.Add(1 << 63)
if expected, got := uint64(0), counter.valInt; expected == got {
t.Errorf("oops! overflow")
}

m := &dto.Metric{}
counter.Write(m)

expected := &dto.Metric{
Label: []*dto.LabelPair{
{Name: proto.String("a"), Value: proto.String("1")},
{Name: proto.String("b"), Value: proto.String("2")},
},
Counter: &dto.Counter{
Value: proto.Float64(1 << 64),
CreatedTimestamp: timestamppb.New(now),
},
}

if !proto.Equal(expected, m) {
t.Errorf("failed because the internal overflow")
}

// verify the overflow case
expected.Counter.Value = proto.Float64(0)
if proto.Equal(expected, m) {
t.Errorf("verify the overflow case")
}
}

func TestCounterAdd(t *testing.T) {
now := time.Now()

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