linux execve, segmentation fault (strcmp_sse42)

Question

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

void main(){
   int x,y,status, i;
   int cnt = 0;
   int flag = 0;
   char buf[50];
   char str[50];
   char * argv[10];
   char * ptr;


   for(i=0; i<10; i++){
    printf("$");
    gets(buf);
    strcpy(str, buf);

    ptr = strtok(buf, " ");

    while(ptr != NULL){
      argv[cnt] = ptr;
      cnt++;
      ptr = strtok(NULL," ");
    }



    if(!strcmp(argv[cnt-1], "&")) {
      argv[cnt-1] = 0;
      flag = 1;
    }
    else {
        argv[cnt] = 0;
    }



    if(!strcmp(argv[cnt-1], "exit")) exit(0); 

    x=fork();

    if (x==0){
        sleep(1);
        printf("I am child to execute %s\n", str);
        y=execve(argv[0], argv, 0);

        if (y<0){
           perror("exec failed");
           exit(1);
        }

    }
    else {
      if(flag == 0) { 
          wait(&status); 
      }
    }


    flag = 0;
    cnt = 0;
   }
}

run this code in linux then, segement falut (core dump)

also when using gdb,

==========================================================

Program received signal SIGSEGV, Segmentation fault. 0x0000003b6572fa96 in __strcmp_sse42 () from /lib64/libc.so.6

==========================================================

why it is not working?

if I type /bin/ls -al (anything without '&') good working

buf type /bin/ls -al & error

OT: Do not use `gets(buf)`! Use `fgets(buf, 50, stdin)` instead. — alk, May 13 '18 at 12:44
regardless of what visual studio will allow, the only valid return type from `main()` is `int`, not `void` — user3629249, Jun 01 '18 at 05:19

benjarobin · Accepted Answer · 2018-05-13T12:18:01.977

2

If you enter an &, the variable argv[cnt-1] will be set to NULL, therefore in if(!strcmp(argv[cnt-1], "exit")) the first argument of the function strcmp will be NULL, which will crash the application...

Furthermore your code does not check any buffer overflow, index out of bound, ... This code is quite "dangerous"

This implies, that:

you should not use gets, see https://stackoverflow.com/a/1694042/808101
Check that argv (note: this variable name is maybe not a good one, argv is very often the arguments of the program itself) array is big enough when inserting element in it

edited May 13 '18 at 12:18

answered May 13 '18 at 12:12

benjarobin

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@JongSeokKim I improved my answer, if my answer did resolve your problem, please validate it. – benjarobin May 13 '18 at 12:19

linux execve, segmentation fault (strcmp_sse42)

1 Answers1