<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/56718>56718</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Stack protector] Unnecessary stack cookie checking
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          JianxiaoLuIntel
      </td>
    </tr>
</table>

<pre>
    The flag **-fstack-protector** inserts a guard variable (aka stack cookie) onto the stack frame for each vulnerable function.  For example, if a function will write a buffer, after the buffer is written, the stack cookie need to be checked to make sure the buffer overflow didn't happen.

Consider the following test case:
```C++
#include <cstring>
#include <cstdlib>
#include <cstdio>
#pragma warning(disable : 4996)   // for strcpy use

// Vulnerable function
__attribute__((noinline)) int vulnerable(const char* str , int a) {
        if (a % 2 == 0)
        {
                // Write buffer
                char buffer[10];
                strcpy(buffer, str);
                return buffer[0];
        }
        return 0;
}

int main() {
        // declare buffer that is bigger than expected
        const char* str = "This string is longer than 10 characters!!";
        printf("%d\n", vulnerable(str, std::rand()));
}
```

The function is vulnerable because it may write the buffer, so there will be a stack cookie on the stack frame. If it enter the branch which write the buffer, the stack cookie need to be checked to avoid buffer overflow.
**If it enter another branch which won't write any buffer, ideally, it's unnecessary to check the cookie. However, we found it will be checked in fact.**

Here is the assembly code:
On Windows:

On Ubuntu:

**By the way, I wonder whether we can inline the __security_check_cookie on Windows.**


</pre>
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