Accelerated Linux Core Dump Analysis/

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Detect NULL Pointer Exceptions Due to Data

Detect NULL Pointer Exceptions Due to Data

Gain knowledge on identifying NULL pointer exceptions caused by data errors and discovering problematic threads and instructions.

We'll cover the following...

Application source code

We have created a multi-threaded application to determine if a NULL pointer error in our data is causing the segmentation fault.

Note: It’s alright if we are unable to identify the cause of the fault.

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// Build:
// gcc main.c -pthread -static -o App2D
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
void procA()
{
    int *p = NULL;
    *p = 1;
}
void procB()
{
    sleep(1);
    void (*pf)() = NULL;
    
    pf();
}
#define THREAD_DECLARE(num,func) void bar_##num() \
{ \
    func; \
} \
\
void foo_##num() \
{ \
    bar_##num(); \
} \
\
void * thread_##num (void *arg) \
{ \
    foo_##num(); \
    \
    return 0; \
}
THREAD_DECLARE(one,sleep(-1))
THREAD_DECLARE(two,procA())
THREAD_DECLARE(three,sleep(-1))
THREAD_DECLARE(four,procB())
THREAD_DECLARE(five,sleep(-1))
#define THREAD_CREATE(num) {pthread_t threadID_##num; pthread_create (&threadID_##num, NULL, thread_##num, NULL);}
int main(int argc, const char * argv[])
{
    THREAD_CREATE(one)
    THREAD_CREATE(two)
    THREAD_CREATE(three)
    THREAD_CREATE(four)
    THREAD_CREATE(five)    
    
    sleep(3);
    return 0;
}

Loading the core dump

The first thing we are going to do is load the dump file that we have included with the course. 

gdb -c core.App2D -se App2D

As discussed in the “Fundamentals of Core Dump Analysis,” this command:

  • Loads the core dump core.App2D file.

  • Loads the symbol table from the App2D application binary.

  • Treats App2D as containing the code that is executed during debugging, whenever needed.

Listing all threads

From this output, we notice that there was a segmentation fault that terminated the program. The next step in our investigation should be to list all threads of our program to identify which thread caused the segmentation fault. We list ...

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