Libxlsxwriter: Infinite Loop Bug With Chart Scales

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Hey guys! Today, we're diving deep into a tricky bug found in libxlsxwriter that can cause your program to hang indefinitely. This issue revolves around using invalid chart scale parameters, specifically when inserting charts with infinite or extremely large scale values. Let's break it down so you can understand what's happening and how to avoid it.

Description of the Infinite Loop Vulnerability

The core of the problem lies in an infinite loop vulnerability within libxlsxwriter. This nasty bug surfaces when workbook_close() is called after you've inserted a chart with crazy high or infinite scale values in the lxw_chart_options. Imagine your program just hanging there, doing absolutely nothing – that's what this bug does, and it's super frustrating. This situation arises particularly when dealing with chart scale parameters that haven't been properly validated, leading to unexpected behavior during the workbook closing process. Understanding the nuances of infinite loop vulnerabilities and how they manifest in libraries like libxlsxwriter is critical for ensuring the stability of applications that rely on them. The root cause often involves unchecked inputs or calculations that result in non-terminating conditions, emphasizing the importance of robust input validation and error handling within the library's codebase.

Steps to Reproduce the Infinite Loop

To really get our hands dirty and understand this bug, let's go through the steps to reproduce it. I'll walk you through the environment setup, the compilation commands, and the minimal test case that triggers the loop.

Environment Setup

First things first, you'll need the right environment. Here’s what we used:

  • OS: Linux (tested on Ubuntu with Docker)
  • libxlsxwriter version: Current git master (the latest version)
  • Compiler: clang with AddressSanitizer (to help us catch memory issues too!)

Having a consistent testing environment is key to accurately reproduce and diagnose issues like the infinite loop we're tackling today. The choice of Linux, specifically Ubuntu within a Docker container, provides a stable and isolated environment that minimizes external factors that could interfere with the testing process. Using the current git master ensures that we're testing the most up-to-date version of the library, which includes the latest features and bug fixes (or in this case, the bug we're trying to trigger). The combination of clang and AddressSanitizer is particularly powerful for catching not only the infinite loop but also potential memory-related issues that might be lurking beneath the surface. This comprehensive approach to environment setup maximizes our chances of accurately reproducing the bug and understanding its underlying cause.

Compilation Command

Next up, compiling the test case. Here’s the command we used:

clang test_infinite_loop.c -o test_infinite_loop \
    -fsanitize=address \
    -I/path/to/libxlsxwriter/include \
    /path/to/libxlsxwriter.a \
    -lz -lm

Make sure you replace /path/to/libxlsxwriter with the actual path to your libxlsxwriter installation. This command essentially compiles your C code, links it with the libxlsxwriter library, and turns it into an executable program. The compilation process is a crucial step in verifying and reproducing software bugs, as it translates the high-level source code into machine-executable instructions. The use of clang as the compiler, along with flags like -fsanitize=address, plays a significant role in detecting memory-related issues and other runtime errors that might contribute to or be a consequence of the bug being investigated. By explicitly specifying the include path (-I) and linking the libxlsxwriter library (/path/to/libxlsxwriter.a), we ensure that the compiler has access to the necessary headers and compiled code required to build the test program. The additional flags -lz and -lm link the zlib and math libraries, respectively, which are dependencies of libxlsxwriter. A successful compilation is a prerequisite for executing the test case and observing the bug in action.

Minimal Test Case: The Code That Breaks It

This is the fun part – the code that actually triggers the bug! Copy and paste this into a file named test_infinite_loop.c:

#include <xlsxwriter.h>
#include <math.h>
#include <stdio.h>

int main() {
    // Create workbook
    lxw_workbook *workbook = workbook_new("/tmp/test.xlsx");
    if (!workbook) return 1;

    // Add worksheet
    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
    if (!worksheet) {
        workbook_close(workbook);
        return 1;
    }

    // Add chart
    lxw_chart *chart = workbook_add_chart(workbook, LXW_CHART_LINE);
    if (!chart) {
        workbook_close(workbook);
        return 1;
    }

    // Add data series
    chart_add_series(chart, NULL, "=Sheet1!$A$1:$A$5");

    // Insert chart with infinite x_scale - triggers infinite loop
    lxw_chart_options chart_opts = {0};
    chart_opts.x_scale = INFINITY;  // This causes the bug
    chart_opts.y_scale = 1.0;

    worksheet_insert_chart_opt(worksheet, 0, 0, chart, &chart_opts);

    // This call hangs indefinitely
    printf("Calling workbook_close...\n");
    workbook_close(workbook);  // Program hangs here
    printf("Should never reach here\n");

    return 0;
}

This minimal test case is a concise C program designed to specifically trigger the infinite loop bug in libxlsxwriter. By focusing on the essential steps required to reproduce the issue, it helps isolate the problem and makes it easier to understand the root cause. The program begins by creating a new Excel workbook and adding a worksheet, which are standard operations when working with libxlsxwriter. It then proceeds to create a chart object and add a data series to it, simulating a typical scenario where charts are generated in an Excel file. The key part of the test case is the setting of chart_opts.x_scale to INFINITY, which is the direct cause of the infinite loop. By inserting the chart with this invalid scale parameter using worksheet_insert_chart_opt, we set the stage for the bug to manifest. The call to workbook_close is where the program hangs indefinitely, confirming the presence of the infinite loop. The printf statements before and after the workbook_close call serve as markers to indicate the program's execution flow and help pinpoint the exact location where the hang occurs. This carefully crafted test case provides a clear and reproducible way to demonstrate the bug and facilitate its investigation.

Expected vs. Actual Behavior

  • Expected Behavior: The library should either validate the x_scale and y_scale parameters and return an error for invalid values (INFINITY, NAN, negative values) or handle extreme values gracefully without hanging.
  • Actual Behavior: The program hangs indefinitely in workbook_close() and must be forcefully terminated.

This discrepancy between the expected behavior and the actual behavior is a clear indication of a bug in the software. Ideally, a robust library like libxlsxwriter should include mechanisms to prevent invalid input from causing catastrophic failures such as infinite loops. Input validation is a fundamental principle of software development, where the program checks whether the data it receives is within acceptable ranges and formats. In this case, the library should have validated the x_scale and y_scale parameters to ensure they are finite and within reasonable bounds. If invalid values are detected, the library should return an error message or handle the situation gracefully, perhaps by clamping the values to a valid range or skipping the chart insertion altogether. The actual behavior, where the program hangs indefinitely, is unacceptable because it can lead to data loss, system instability, and a poor user experience. This highlights the importance of thorough testing and bug fixing to ensure that the software behaves as expected under various conditions, including when presented with invalid input.

Verifying the Bug

Run the compiled program like this:

timeout 5 ./test_infinite_loop

The timeout 5 command will kill the program after 5 seconds if it hangs, which is exactly what we expect to happen. You’ll see “Calling workbook_close…” printed, and then the program will be killed.

The use of the timeout command is a practical technique for verifying the presence of an infinite loop in a program. By setting a time limit, we can ensure that the test case does not run indefinitely and potentially consume excessive resources or hang the system. In this scenario, the timeout 5 command instructs the operating system to terminate the ./test_infinite_loop process if it does not complete within 5 seconds. This is particularly useful when dealing with bugs that cause programs to enter infinite loops or long-running computations. The expected behavior when running the test case is that the program will print the message "Calling workbook_close..." and then hang indefinitely within the workbook_close function, as we've already identified. The timeout command will then intervene and kill the process after 5 seconds, confirming that the infinite loop is indeed occurring. This verification step provides concrete evidence of the bug's existence and its impact on program execution.

Root Cause: Diving into the Code

Okay, let's get technical! The infinite loop happens in src/worksheet.c:3010 within the _worksheet_position_object_pixels() function. This is where things get interesting.

The root cause of the infinite loop can be traced back to a specific section of code within the libxlsxwriter library, located in the src/worksheet.c file at line 3010. Understanding the root cause is essential for developing an effective fix and preventing similar issues from occurring in the future. The function _worksheet_position_object_pixels() is responsible for calculating the position of objects, such as charts, within the worksheet in terms of pixels. This calculation involves converting the object's size and position from Excel's internal units (EMUs) to pixels, taking into account the column widths and row heights of the worksheet. The infinite loop arises due to a flawed algorithm for determining the end column of the object's bounding box. Specifically, the loop iteratively subtracts the width of each column from the object's total width until the remaining width is less than the current column's width. However, when the object's width is set to INFINITY, as in our test case, this subtraction never results in a finite value, causing the loop to continue indefinitely. This highlights the importance of carefully considering the behavior of algorithms when dealing with extreme or invalid input values, especially when performing arithmetic operations that can lead to infinite loops or other unexpected outcomes. A thorough understanding of the code's logic and the potential pitfalls of floating-point arithmetic is crucial for identifying and resolving such issues.

The Culprit Code Snippet

Here’s the problematic code:

/* Subtract the underlying cell widths to find the end cell. */
while (width >= _worksheet_size_col(self, col_end, anchor)) {
    width -= _worksheet_size_col(self, col_end, anchor);
    col_end++;
}

Why This Code Loops Forever

When chart_opts.x_scale is set to INFINITY:

  1. The computed width becomes INFINITY.
  2. The loop condition width >= _worksheet_size_col(...) is always true because infinity is greater than or equal to any finite value.
  3. Subtracting from infinity still leaves infinity: INFINITY - finite_value = INFINITY.
  4. The loop never terminates – boom, infinite loop!

This step-by-step breakdown clearly illustrates why the infinite loop occurs when chart_opts.x_scale is set to INFINITY. The core issue is the combination of an infinite width value and the loop's condition, which relies on decrementing the width until it becomes smaller than the column width. However, subtracting a finite value (the column width) from infinity still results in infinity, thus the loop condition remains perpetually true. This fundamental property of infinity in mathematical operations is at the heart of the bug. The loop's logic is flawed in that it does not account for the possibility of infinite values, leading to a non-terminating condition. This highlights the importance of considering edge cases and boundary conditions when designing algorithms, especially those involving numerical computations. A robust algorithm should include checks for invalid or extreme input values and handle them appropriately, either by returning an error, clamping the values to a valid range, or using alternative calculation methods that avoid the pitfalls of infinite loops.

Call Stack: Tracing the Execution

Here’s the call stack that leads to the infinite loop:

#4  _worksheet_size_col at worksheet.c:2815
#5  _worksheet_position_object_pixels at worksheet.c:3011
#6  _worksheet_position_object_emus at worksheet.c:3050
#7  lxw_worksheet_prepare_chart at worksheet.c:3674
#8  _prepare_drawings at workbook.c:1238
#9  workbook_close at workbook.c:2272

By examining the call stack, we can trace the sequence of function calls that lead to the infinite loop, providing valuable context for understanding the bug's propagation and impact. The call stack represents the active functions at a particular point in the program's execution, with the most recently called function at the top and the initial function call at the bottom. In this case, the infinite loop occurs within the _worksheet_position_object_pixels function (frame #5), as we've already identified. The functions above it in the stack represent the callers of this function, providing insights into the higher-level operations that trigger the bug. The workbook_close function (frame #9) is the entry point where the issue manifests, as it initiates the process of preparing and writing the workbook data. The _prepare_drawings function (frame #8) is responsible for handling the drawing objects, including charts. The lxw_worksheet_prepare_chart function (frame #7) prepares the chart for insertion into the worksheet. The _worksheet_position_object_emus function (frame #6) converts the object's position from EMUs to pixels. Finally, the _worksheet_size_col function (frame #4) is called within the infinite loop to determine the width of the columns. This analysis of the call stack helps us understand the bug's context and identify potential areas for fixing the issue, such as input validation or algorithmic improvements in the relevant functions.

Additional Information

This bug was actually discovered through fuzzing, which is a cool technique for finding bugs by feeding a program lots of random inputs. It’s like throwing spaghetti at the wall to see what sticks!

How to Fix It (or Avoid It for Now)

The best way to avoid this right now is to make sure you're not using INFINITY, NAN, or negative values for your chart scale parameters. If you're feeling adventurous and want to contribute to libxlsxwriter, you could help fix the bug by:

  1. Validating the input: Add checks in the code to ensure that x_scale and y_scale are valid numbers.
  2. Handling extreme values gracefully: If the values are too large, clamp them to a reasonable range or return an error.

Conclusion

So, there you have it! An infinite loop bug in libxlsxwriter caused by invalid chart scale parameters. Understanding bugs like this helps us become better developers and appreciate the importance of input validation and robust error handling. Keep your eyes peeled for updates to libxlsxwriter, and until then, be careful with those chart scales! Thanks for reading, and happy coding!