1 #include <stdio.h>

     2 #include <stdlib.h>

     3 #include <unistd.h>

     4 

     5 int main(int argc, char *argv[])

     6 {   

     7     FILE *f; //file pointer

     8 

     9     printf("Real UID = %d\n", getuid());

    10     printf("Effective UID = %d\n", geteuid());

    11     

    12     //read test

    13     if ((f = fopen(argv[1], "r")) == NULL) {

    14       fprintf(stderr, "%s is not readable\n", argv[1]);

    15     } else {

    16       fprintf(stderr, "%s is readable\n", argv[1]); fclose(f);

    17     }

    18 

    19     //write test

    20     if ((f = fopen(argv[1], "w")) == NULL) {

    21       fprintf(stderr, "%s is not writable\n", argv[1]);

    22     } else {

    23       fprintf(stderr, "%s is writable\n", argv[1]); fclose(f);

    24     }

    25 

    26     //lowering the access rights to the caller

    27     if (setuid(getuid())) {  

    28       fprintf(stderr, "Could not reset setuid\n"); return 1;

    29     }

    30         

    31     printf("Real UID = %d\n", getuid());

    32     printf("Effective UID = %d\n", geteuid());      

    33 

    34     //read test

    35     if ((f = fopen(argv[1], "r")) == NULL) {

    36       fprintf(stderr, "%s is not readable\n", argv[1]);

    37     } else {

    38       fprintf(stderr, "%s is readable\n", argv[1]); fclose(f);

    39     }

    40 

    41     //write test

    42     if ((f = fopen(argv[1], "w")) == NULL) {

    43       fprintf(stderr, "%s is not writable\n", argv[1]);

    44     } else {

    45       fprintf(stderr, "%s is writable\n", argv[1]); fclose(f);

    46     }

    47 

    48     return 0;

    49 }