Fix panic and OOM in repeatString for large repeat counts (#2644)

The existing check (count > 10 million) does not account for string
length. A 68-byte string repeated 35 trillion times passes the count
check but panics in strings.Repeat with "makeslice: len out of range".
Smaller counts (e.g. 10 million * 6-byte string = 60 MB) cause OOM on
memory-constrained environments like OSS-Fuzz (2560 MB limit).

Replace the count-only check with a result size check: the product of
string length and repeat count must not exceed 10 MiB. Use division
(len > limit/count) instead of multiplication (len*count > limit) to
avoid integer overflow — a large count can wrap the product to a
negative value, bypassing the guard entirely.

Fixes at least four OSS-Fuzz bugs found via Lima's FuzzEvaluateExpression:
  https://issues.oss-fuzz.com/issues/418818862 (makeslice overflow)
  https://issues.oss-fuzz.com/issues/422001683 (timeout from huge alloc)
  https://issues.oss-fuzz.com/issues/383195001 (OOM, 3 GB allocation)
  https://issues.oss-fuzz.com/issues/385180606 (OOM, 97 TB allocation)

Signed-off-by: Jan Dubois <jan@jandubois.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jandubois

pkg/yqlib/operator_multiply.go

@@ -155,8 +155,10 @@ func repeatString(lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error
 		return nil, err
 	} else if count < 0 {
 		return nil, fmt.Errorf("cannot repeat string by a negative number (%v)", count)
-	} else if count > 10000000 {
-		return nil, fmt.Errorf("cannot repeat string by more than 100 million (%v)", count)
+	}
+	maxResultLen := 10 * 1024 * 1024 // 10 MiB
+	if count > 0 && len(stringNode.Value) > maxResultLen/count {
+		return nil, fmt.Errorf("result of repeating string (%v bytes) by %v would exceed %v bytes", len(stringNode.Value), count, maxResultLen)
 	}
 	target.Value = strings.Repeat(stringNode.Value, count)
 

pkg/yqlib/operator_multiply_test.go

@@ -237,12 +237,11 @@ var multiplyOperatorScenarios = []expressionScenario{
 		expectedError: "cannot repeat string by a negative number (-4)",
 	},
 	{
-		description: "Multiply string X by more than 100 million",
-		// very large string.repeats causes a panic
+		description:   "Multiply string by count that exceeds result size limit",
 		skipDoc:       true,
 		document:      `n: 100000001`,
 		expression:    `"banana" * .n`,
-		expectedError: "cannot repeat string by more than 100 million (100000001)",
+		expectedError: "result of repeating string (6 bytes) by 100000001 would exceed 10485760 bytes",
 	},
 	{
 		description: "Multiply int node X string",
@@ -693,6 +692,27 @@ var multiplyOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!null)::null\n",
 		},
 	},
+	{
+		// Regression test for https://issues.oss-fuzz.com/issues/418818862
+		// Large repeat count with a long string must not panic.
+		skipDoc:       true,
+		expression:    `"abc" * 99999999`,
+		expectedError: "result of repeating string (3 bytes) by 99999999 would exceed 10485760 bytes",
+	},
+	{
+		// Regression test for https://issues.oss-fuzz.com/issues/383195001
+		// Product of string length * repeat count must be bounded.
+		skipDoc:       true,
+		expression:    `"x" * 99999999`,
+		expectedError: "result of repeating string (1 bytes) by 99999999 would exceed 10485760 bytes",
+	},
+	{
+		// The size guard must not overflow: len * count can wrap to
+		// a negative or small value on 64-bit, bypassing the check.
+		skipDoc:       true,
+		expression:    `"ab" * 4611686018427387904`,
+		expectedError: "result of repeating string (2 bytes) by 4611686018427387904 would exceed 10485760 bytes",
+	},
 }
 
 func TestMultiplyOperatorScenarios(t *testing.T) {