program

core dumped

What is coredump

How to analyse coredump

Why program coredump

  • memory overload
  • using thread-unsafe functions on multi-thread
  • not using lock when read-write data in multi-thread
  • invalid point
  • stack overload

How to use core file

  • allow to generate core file
    • ulimit –c unlimited
    • echo /tmp/core.%e.%p > /proc/sys/kernel/core_pattern
    • gcc –g core_dump_test.c -o core_dump_test, must used -g to build.
  • get core file
    • ./test
  • debug
    • gdb ./test /tmp/core
    • bt
    • f 3
    • p buffer

Signal of core

  • man 7 signal

Examples

  • write data to protected memory

    #include <stdio.h>
int main(){
   int i=0;
   scanf("%d",i);
   printf("%d\n",i);
   return 0;
}

#include <stdio.h>
int main(){
   char *p;
   p = NULL;
   *p = 'x';
   printf("%c", *p);
   return 0;
}

  • memory overflow (array overflow, variable type different and so on)

    #include <stdio.h>
int main(){
    char test[1];
    printf("%c", test[1000000000]);
    return 0;
}

Access to a not exist address

#include <stdio.h>
 int main(){
      int b = 10;
      printf("%s\n", b);
      return 0;
}

#include <stdio.h>
#include <string.h>
  int main(){
     char c='c';
     int i=10;
     char buf[100];
     printf("%s", c); //试图把char型按照字符串格式输出，这里的字符会解释成整数，
                           //再解释成地址，所以原因同上面那个例子
     printf("%s", i); //试图把int型按照字符串输出
     memset(buf, 0, 100);
     sprintf(buf, "%s", c);  试图把char型按照字符串格式转换
     memset(buf, 0, 100);
     sprintf(buf, "%s", i);//试图把int型按照字符串转换
}
  • Other, likes:
    • Initialization when defined point, check point before using.
    • Careful array index
    • check if success when do pthread_create, if fail and do pthread_join for
      this thread will occur segment fault

Auto start gdb when core occur

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>

void dump(int signo)
{
    char buf[1024];
    char cmd[1024];
    FILE *fh;

    snprintf(buf, sizeof(buf), "/proc/%d/cmdline", getpid());
    if(!(fh = fopen(buf, "r")))
        exit(0);
    if(!fgets(buf, sizeof(buf), fh))
        exit(0);
    fclose(fh);
    if(buf[strlen(buf) - 1] == '\n')
        buf[strlen(buf) - 1] = '\0';
    snprintf(cmd, sizeof(cmd), "gdb %s %d", buf, getpid());
    system(cmd);

    exit(0);
}

void dummy_function (void)
{
    unsigned char *ptr = 0x00;
    *ptr = 0x00;
}

int main (void)
{
    signal(SIGSEGV, &dump);
    dummy_function ();

    return 0;
}

Reference