OOM: `ReadBytes` Attempting To Read Byte Slice Which Is Too Large

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Introduction

When dealing with large datasets, it's not uncommon to encounter errors related to out-of-memory (OOM) conditions. However, in some cases, monitoring tools may not show a spike in memory usage, leading to confusion about the root cause of the issue. In this article, we'll explore a specific scenario where the ReadBytes function in the Kaitai Struct Go runtime library attempts to read a byte slice that is too large, resulting in an OOM error.

Error Analysis

The error message indicates that the ReadBytes function is attempting to read a byte slice, but the length of the slice is too large. This is evident from the stacktrace, which shows that the error occurs in the ReadBytes function:

fatal error: runtime: out of memory
github.com/kaitai-io/kaitai_struct_go_runtime/kaitai.(*Stream).ReadBytes(0x298cc20, 0x44cea269)

Upon further investigation, it appears that the issue is related to the length of the byte slice being passed to the ReadBytes function. Specifically, the length is greater than 1 GB, which is a significant amount of memory.

Possible Causes

There are two possible causes for this issue:

  1. Invalid length in the message header: It's possible that the length of the byte slice was passed incorrectly in the message header. This could be due to a mistake in the parser or a bug in the code that generates the message.
  2. Parser has a wrong value: Another possibility is that the parser has a wrong value for the length of the byte slice. This could be due to a bug in the parser or a misunderstanding of the data format.

Previous Changes

In a previous iteration, the CalcValue() function was edited to address a similar issue. However, it appears that the changes did not take into account a specific scenario where value3 is negative. Specifically, when value3 is 0xFF (signed -1), the calculation int32(-1) << 16 results in -65536, which is then cast to a uint via int(...), resulting in 4294901760 (0xFFF0000).

Solution

To resolve this issue, the parser for the Kaitai Struct needs to be edited to account for the scenario where value3 is negative. This can be achieved by adding a check to ensure that the length of the byte slice is not too large before passing it to the ReadBytes function.

Code Changes

Here's an example of the code changes that can be made to the parser:

func (k *KaitaiStruct) CalcValue() (uint, error) {
    // ...
    value3 := k.ReadU1()
    if value3 < 0 {
        // Handle negative value3
        value3 = uint(value3)
    }
    // ...
    length := int32(value3) << 16
    if length > 1<<30 { // 1 GB
        return 0, errors.New("length too large")
    }
    // ...
}

By adding this check, the parser can prevent the ReadBytes function from attempting to read a byte slice that is too large, thereby avoiding the OOM error.

Conclusion

Q&A

Q: What is the cause of the OOM error in the ReadBytes function? A: The OOM error is caused by the length of the byte slice being too large. This can occur when the parser passes an invalid length to the ReadBytes function.

Q: Why didn't the monitoring tools show a spike in memory usage? A: The monitoring tools may not show a spike in memory usage because the program is not actually using that much memory. Instead, the program is attempting to allocate a large amount of memory, which is causing the OOM error.

Q: What is the significance of the length being greater than 1 GB? A: The length being greater than 1 GB is significant because it is a large amount of memory. The ReadBytes function is attempting to read a byte slice of this size, which is causing the OOM error.

Q: What are the possible causes of the invalid length in the message header? A: The possible causes of the invalid length in the message header are:

  • A mistake in the parser
  • A bug in the code that generates the message
  • A misunderstanding of the data format

Q: What is the impact of the parser having a wrong value for the length of the byte slice? A: The impact of the parser having a wrong value for the length of the byte slice is that the ReadBytes function will attempt to read a byte slice of the wrong size, which can cause an OOM error.

Q: What changes were made to the CalcValue() function previously? A: The CalcValue() function was edited to address a similar issue. However, it appears that the changes did not take into account a specific scenario where value3 is negative.

Q: What is the scenario where value3 is negative? A: The scenario where value3 is negative is when value3 is 0xFF (signed -1). In this case, the calculation int32(-1) << 16 results in -65536, which is then cast to a uint via int(...), resulting in 4294901760 (0xFFF0000).

Q: How can the parser be edited to account for the scenario where value3 is negative? A: The parser can be edited to account for the scenario where value3 is negative by adding a check to ensure that the length of the byte slice is not too large before passing it to the ReadBytes function.

Q: What is the code change that can be made to the parser? A: The code change that can be made to the parser is to add a check to ensure that the length of the byte slice is not too large before passing it to the ReadBytes function. This can be achieved by adding the following code:

func (k *KaitaiStruct) CalcValue() (uint, error) {
    // ...
    value3 := k.ReadU1()
    if value3 < 0 {
        // Handle negative value3
        value3 = uint(value3)
    }
    // ...
    length := int32(value3) << 16
    if length > 1<<30 { // 1 GB
        return 0, errors.New("length too large")
    }
    // ...
}

Q: What is the benefit of making this code change? A: The benefit of making this code change is that it prevents the ReadBytes function from attempting to read a byte slice that is too large, thereby avoiding the OOM error.

Conclusion

In conclusion, the OOM error in the ReadBytes function is likely due to the length of the byte slice being too large. By adding a check to the parser to account for negative values of value3, we can prevent this issue from occurring. This change can be made by editing the parser for the Kaitai Struct, as shown in the code example above.