operate with file and crash

This is a discussion on operate with file and crash within the C Programming forums, part of the General Programming Boards category; I wrote a test file to grab all the TCP rule which has source port as "any" and destination port ...

  1. #1
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    operate with file and crash

    I wrote a test file to grab all the TCP rule which has source port as "any" and destination port as "any", save them into a temp file.

    the rule file example are as follow (the 4th parameter is source and the the 6th is destination port):

    Code:
    alert tcp any any -> any any ( content:"Volume Serial Number"; sid:1292;)
    alert tcp any 80 -> any any ( content:"Command completed"; sid:494;)
    alert tcp any 80 -> any any ( content:"Bad command or filename"; sid:495;)
    alert tcp any any -> any any ( content:"1 file|28|s|29| copied"; sid:497;)
    alert tcp any 80 -> any any ( content:"Invalid URL"; sid:1200;)
    alert tcp any 80 -> any any ( content:"Index of /cgi-bin/"; sid:1666;)
    alert tcp any any -> any any ( content:"HTTP/1.1 403"; sid:1201;)
    alert ip any any -> any any ( content:"uid=0|28|root|29|"; sid:498;)
    My code is:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    #define MAXLINECHAR 2048
    
    void DivideRule ();
    
    FILE *ruleset, *ruletemp;
    
    int main (void){
    
    	ruleset = fopen ("ruleset.rules", "r");
    	ruletemp = fopen ("ruletemp.rules", "w+");
        
    	DivideRule ();
    
    	
    	fclose (ruletemp);	
    	fclose (ruleset);
        
    	return 0;
    }
    
    
    void DivideRule (){
    
    	char F1[10], F2[10], F3[200], F4[10], F5[10], F6[10], F7[10];
    	
    	char *prerule, *temprule;
    	char bufc;
    	
        	
    	while ((fgetc (ruleset)) != EOF){ //get a character and see whether it is the end of the file
    		
    		fseek (ruleset, -1, 1); //go back to the start of the line
    		prerule = (char *)malloc (MAXLINECHAR*sizeof(char)); //allocate a bunch of memory so as to store the rule
    		fgets (prerule, MAXLINECHAR, ruleset); //grab a rule
    		fseek (ruleset, -strlen(prerule), 1); //set the internal FILE pointer back to start of the line
    		
    		fscanf(ruleset, "%s %s %s %s %s %s %s", F1, F2, F3, F4, F5, F6, F7); //scan from the start to grab each parameter
    		    
    		if (F2[0] == 't' && F2[1] == 'c' && F2[2] == 'p'){ // if the protocol in the rule is TCP, go ahead
    		    if (atoi (F4) == 0  && atoi (F7) == 0){  //port is 0 (source port is any/range and destination port is any/range), which means this rule should be insert into the Generic Set
    			
    				fprintf(ruletemp, "%s %s %s %s %s %s %s", F1, F2, F3, F4, F5, F6, F7);
    	    	    temprule = (char *)malloc (MAXLINECHAR*sizeof(char)); //create a temp buffer to store the rest of characters in the line
    		        fgets (temprule, MAXLINECHAR, ruleset);
    				fprintf (ruletemp, "%s", temprule);
    			    free (temprule);
    			}
    		}
    		else{
    		        temprule = (char *)malloc (MAXLINECHAR*sizeof(char));
    		        fgets (temprule, MAXLINECHAR, ruleset);
    			    free (temprule);
    		}
            
    		free (prerule);
        }
    	
    }
    When I compile the file, there is no error. However, when I run it in cygwin, it crashed. The error information are as follow:

    "$ ./a.exe
    10 [main] a 620_cygtls::handle_exceptions: Error while dumping state (probably corrupted stack)
    Segmentation fault (core dumped)"

    Please help me what's wrong with the code. Thanks a lot!

  2. #2
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    Why not add some error checking every time the file is read, for example whenever an fgets() or fscanf() operation is performed?
    Code:
    		//scan from the start to grab each parameter
    		if (fscanf(ruleset, "&#37;s %s %s %s %s %s %s", F1, F2, F3, F4, F5, F6, F7) != 7)
    		{
    			printf("Error reading from file.\n");
    		}

  3. #3
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    I got it. Add an "else" after "if (atoi (F4) == 0 && atoi (F7) == 0)" and works fine.

    Thanks a lot!

  4. #4
    and the hat of wrongness Salem's Avatar
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    Or even
    Code:
    char buff[MAXLINECHAR];
    while ( fgets( buff, sizeof buff, ruleset ) != NULL ) {
      if ( sscanf(ruleset, "&#37;s %s %s %s %s %s %s", F1, F2, F3, F4, F5, F6, F7) == 7 ) {
        // do ya thang!
      } else {
        // bad format
      }
    }
    A malloc/free call per line is a waste of effort, as is your apparent attempt to measure the length of each line only to do a max length line anyway with
    prerule = (char *)malloc (MAXLINECHAR*sizeof(char));

    Oh, and also see the FAQ on casting malloc in C.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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