Question

1. Assume the following struct is declared for the permission list of a file in Linux.
Each permission (u or g or o) is represented as an octal. For example, u=7 means rwx, u=5 means r-x.

Note: "unsigned char" means a byte, not a character or a letter or a string.

typedef struct {
unsigned int uid; // owner id
unsigned int gid; // group id
unsigned char u; // owner's permission
unsigned char g; // group's permission
unsigned char o; // other's permission
} Permission;

The permission check procedure is
(1) A user requests an operation p on a file f.
(2) If the user is the owner of the file, the operation will be checked against the owner's permission of the file. The result is either grant or deny.
(3) Otherwise, if the user is not the owner but in the group of the file, the operation will be checked against the group's permission of the file. The result is either grant or deny.
(4) Otherwise, if the user is neither the owner nor in the group of the file, the operation will be checked against the other's permission of the file. The result is either grant or deny.

Write a C/C++ function "int accesscheck(unsigned int uid, unsigned int gid, unsigned int p, int f)" to enforce access control in Linux.
The arguments of the function accesscheck are explained below:
1) uid and gid are the user id and the group id of the user who requests to take an operation on the file.
2) f is the file id.
3) p is the requested operation. For example, p=7 means three operations rwx, p=6 means two operations rw-, p=1 means one operation --x.
The function return 1 if access is permitted, otherwise 0.

Request will be granted only if p is contained by the permission set of the file.
Assume "Permission getPermission(int f)" can get the permission of the file f.

For example, f's permission is rwxr-xr-x 1000(uid) 2000(gid). Then, accesscheck(1000, 1000, 6, f) returns 1, but accesscheck(2000, 2000, 6, f) returns 0.

Copy and paste your code in report and explain each line of code of your function in comments.

Answer 1

Answer:

#include <stdio.h>

typedef struct {
unsigned int uid; // owner id
unsigned int gid; // group id
unsigned char u; // owner's permission
unsigned char g; // group's permission
unsigned char o; // other's permission
}Permission;

int accesscheck(unsigned int uid, unsigned int gid, unsigned int p, int f)
{
    Permission cPerm = getPermission(f);//copies permissions of current file to a structer
    if(cPerm.uid == uid)//checking for user id of passed and current(opened with file id)
    {
        if(cPerm.p >= p)//checking for current process is valid for the current id (group or user)
        {
            return 1;
        }
        else
        {
            return 0;
        }
    }
    else if(cPerm.gid == gid)//checking for group id of passed and current(opened with file id)
    {
        if(cPerm.p >= p)//checking for current process is valid for the current id (group or user)
        {
            return 1;
        }
        else
        {
            return 0;
        }
    }
    else if(cPerm.o == gid)//checking for passed id and current's others id(opened with file id)
    {
        if(cPerm.p >= p)//checking for current process is valid for the current id (group or user)
        {
            return 1;
        }
        else
        {
            return 0;
        }
    }
  
}
int main()
{

    return 0;
}

Thanks and kindly upvote.