segmentation fault core dumped c - c

Question

I'm new to C, I got an error today which is:
segmentation fault core dumped
I used gdb to track the code, I found that the error occurs in this line:
if (!strcmp(user_pass, passwddata->passwd))
Where user_pass is a char array, and passwddata is a struct, passwd is a member of the struct, which is also a type of char array, I tried to change the code to
if (!strcmp(user_pass, "ttt"))
The error didn't occur, so I guess the error occurs on that struct, if more code is needed, I can add it, here I want to understand under what condition can such kind of error occur on a struct?
Here is the code:
int main(int argc, char *argv[]) {
mypwent *passwddata; /* this has to be redefined in step 2 */
/* see pwent.h */
char important[LENGTH] = "***IMPORTANT***";
char user[LENGTH];
//char *c_pass; //you might want to use this variable later...
char prompt[] = "password: ";
char *user_pass;
sighandler();
while (TRUE) {
/* check what important variable contains - do not remove, part of buffer overflow test */
printf("Value of variable 'important' before input of login name: %s\n",
important);
printf("login: ");
fflush(NULL); /* Flush all output buffers */
__fpurge(stdin); /* Purge any data in stdin buffer */
if (gets(user) == NULL) /* gets() is vulnerable to buffer */
{
exit(0); /* overflow attacks. */
}
printf("******************* %s\n",user);
/* check to see if important variable is intact after input of login name - do not remove */
printf("Value of variable 'important' after input of login name: %*.*s\n",
LENGTH - 1, LENGTH - 1, important);
user_pass = getpass(prompt);
passwddata = getpwnam(user);
printf("^^^^^^^^^^^^^^^^^^^^^^^^^^ %s\n", user_pass);
if (passwddata != NULL) {
/* You have to encrypt user_pass for this to work */
/* Don't forget to include the salt */
if (!strcmp(user_pass, "ttt")) {
printf(" You're in !\n");
/* check UID, see setuid(2) */
/* start a shell, use execve(2) */
}
}
printf("Login Incorrect \n");
}
return 0;
}

Answer 1

Most likely passwd or passwddata is NULL, in the latter case the -> is attempting to deference the NULL pointer and thus it's crashing.
By changing your code to:
if (!strcmp(user_pass, "ttt"))
You isolated the first part, so you know user_pass is OK. You can use a debugger or some checks and printf's to get the values of passwddata and passwd to close on what the problem is.
Now that you've updated the code you know the problem is with passwd. You start with an empty pointer:
mypwent *passwddata;
Later you set the pointer to the return of getpwnam, presumably this is a pointer to a structure of type mypwent that you allocated some memory for:
passwddata = getpwnam(user);
You have a check to make sure passwddata isn't null:
if (passwddata != NULL) {
if (!strcmp(user_pass, "ttt")) {
So now you've checked everything except passwd, passing a null into strcmp() will cause it to crash with that message, so I'm guessing you only allocated memory for the structure and not for the char array within the structure.

Answer 2

My guess would be that passwddata is NULL.
Or the buffer passwddata->passwd points to
shorter than the string in user_pass
and
not null-terminated
Together those two make strcmp access beyond the limits of the second argument's buffer and hence cause the error.