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POC Buffer Overflow - Why is there a constant number of bytes that trips a segfault? (Mac OS 10.8 64-bit, clang)
 

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  1. POC Buffer Overflow - Why is there a constant number of bytes that trips a segfault? (Mac OS 10.8 64-bit, clang)
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    <p>I was experimenting with buffer overflow in C, and found an interesting quirk:</p> <p>For any given array size, there seems to be a set number of overflow bytes that can be written to memory before a SIGABRT crash. For example, in the code below the 10 byte array can be overflowed to 26 bytes before crashing at 27. Similarly, an array of 20 <code>char</code>s can be overflowed to 40 <code>char</code>s before it aborts on the 41st.</p> <p>Can anyone explain why this is? Also, is the SIGABRT the same as (or caused by) a "segmentation fault"?</p> <p>Mac OS 10.8 - Xcode 4.6, clang and lldb. Thanks!</p> <pre><code>#include &lt;stdio.h&gt; int main(int argc, const char * argv[]) { char aString[ 10 ]; char aLetter = 'a'; printf("The size of one array slot sizeof( aString[0] ) is %zu\n", sizeof(aString[0])); printf("The size of one letter sizeof( aLetter ) is %zu\n", sizeof(aLetter)); // Overflow the aString array of chars // lldb claims aString is initialized with values \0 or NULL at all locations // Substitute i&lt;27 and this code will crash regularly for (int i=0; i&lt;26; i++) { aString[i]= aLetter; } return 0; } </code></pre> <p>EDIT - I've stepped through it in disassembly and found this protection just after the for-loop:</p> <pre><code>0x100000f27: movq 226(%rip), %rax ; (void *)0x00007fff793f24b0: __stack_chk_guard 0x100000f2e: movq (%rax), %rax 0x100000f31: movq -8(%rbp), %rcx 0x100000f35: cmpq %rcx, %rax 0x100000f38: jne 0x100000f49 ; main + 121 at main.c:26 . . . 0x100000f49: callq 0x100000f4e ; symbol stub for: __stack_chk_fail </code></pre>
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    1. COI assume it's some sort of buffer overflow protection, I just would like to know what it is I'm bumping up against. Also, this is purely in the interest of self-education (nothing malicious)!
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    2. COCommenting on close votes: question has example code, it is IMO clear what is being asked (why there's memory protection at this amount of bytes, which seems very low to me), and this is relevant to clang on Mac in general. No need to close.
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    3. COTo understand 0x90's answer below, I needed to read up on stack alignment at: http://stackoverflow.com/questions/4175281/what-does-it-mean-to-align-the-stack
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