Question

i have written module which reads and write on /proc file and is working fine but want to use permissions with it when i make the function for permissions shown below its gives me error (basically i want every one could read the file but only root could write in it).

int my_permission(struct inode *inode, int op)
{
if(op == 4||(op == 2 && current->euid = 0))  //euid is not a member of task_struct
return 0;
return -EACCES;
}
const  struct inode_operations my_iops = {
.permission = my_permission,
};

but its giving me error as follow:

/home/karan/practice/procf/testproc1.c: In function ‘my_permission’:
/home/karan/practice/procf/testproc1.c:50:32: error: ‘struct task_struct’ has no member named ‘euid’

I think there is other member in task_struct which points to user id . I am interested in solution as well as description of task_struct members field used for.

regards
karan

Answer 1

See include/linux/cred.h:

    #define current_euid()          (current_cred_xxx(euid))  
    #define current_cred_xxx(xxx)                   \
    ({                                              \
            current_cred()->xxx;                    \
    })

and so current_euid() falls back on current_cred():

    /**
     * current_cred - Access the current task's subjective credentials
     *
     * Access the subjective credentials of the current task.  RCU-safe,
     * since nobody else can modify it.
     */
    #define current_cred() \
            rcu_dereference_protected(current->cred, 1)

So for you problem, to do effective UID comparison, take a look at /fs/exec.c:

    if (current_euid() == current_uid() && current_egid() == current_gid())
    bprm->cred->euid = current_euid();
            bprm->cred->euid = inode->i_uid

contrast with your program:

    if (current_euid() == 0)

which means even if the user is not logined with UID=0, it will be treated effecively as UID==0? Sounds dangerous