inlined strcmp() on ppc64le (sometimes) returns a wrong value when the strings are same

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Summary

The issue at hand is an inlined strcmp() function on ppc64le architecture that sometimes returns a wrong value when comparing two identical strings. This problem occurs when the code is compiled with -O optimization using gcc (GCC) 11.2.1 20220127. The comparison involves two variables: a local character array and a memory-allocated variable.

Root Cause

The root cause of this issue is related to the compiler optimization and the inlined strcmp() function. The disassembled code shows that the comparison is done using vector instructions, which might lead to incorrect results due to alignment issues or vectorization problems. The key factors contributing to this issue are:

  • Inlined strcmp() function: The compiler’s decision to inline the strcmp() function can lead to optimized but incorrect code.
  • Vector instructions: The use of vector instructions can cause alignment issues or vectorization problems, resulting in incorrect comparisons.
  • Compiler optimization: The -O optimization flag can lead to aggressive optimization, which might compromise the correctness of the code.

Why This Happens in Real Systems

This issue can occur in real systems due to the following reasons:

  • Complex codebases: Large and complex codebases can make it difficult to identify and reproduce issues like this.
  • Compiler optimizations: Aggressive compiler optimizations can lead to correctness issues, especially when combined with inlined functions and vector instructions.
  • Architecture-specific issues: The ppc64le architecture might have specific quirks or optimization opportunities that can lead to incorrect results.

Real-World Impact

The real-world impact of this issue can be significant, including:

  • Incorrect results: The strcmp() function returning a wrong value can lead to incorrect decisions or actions in the program.
  • Crashes or errors: The incorrect results can cause crashes or errors in the program, leading to downtime or data loss.
  • Security vulnerabilities: In some cases, incorrect comparisons can lead to security vulnerabilities, such as buffer overflows or authentication bypasses.

Example or Code

#include 
#include 

int main() {
    char local_str[] = "i_cs_bill_addr_sk";
    char* allocated_str = strdup("i_cs_bill_addr_sk");

    int result = strcmp(local_str, allocated_str);
    printf("Comparison result: %d\n", result);

    free(allocated_str);
    return 0;
}

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Disabling inlining: Using the -fno-inline flag or attribute__((noinline)) to prevent the compiler from inlining the strcmp() function.
  • Reducing optimization: Lowering the optimization level or using -O0 to reduce the aggressiveness of the compiler optimizations.
  • Using alternative comparisons: Implementing custom comparison functions or using memcmp() instead of strcmp() to avoid vectorization issues.

Why Juniors Miss It

Junior engineers might miss this issue due to:

  • Lack of experience: Limited experience with compiler optimizations, inlined functions, and vector instructions can make it difficult to identify the root cause.
  • Insufficient testing: Inadequate testing or lack of test cases can fail to reproduce the issue, making it harder to detect.
  • Unfamiliarity with architecture: Limited knowledge of the ppc64le architecture and its specific quirks can lead to incorrect assumptions or overlooking the issue.