check_path_exists.c

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#include "config.h"
#include "check_path_exists.h"

#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define perm_owner(m) ((m & S_IRWXU) >> 6)
#define perm_group(m) ((m & S_IRWXG) >> 3)
#define perm_other(m) (m & S_IRWXO)
#define check_mode(m, c) (((m) | (c)) == (m) ? 1 : 0)
#define checkpermission_root(m, c) check_mode(perm_owner(m) | perm_group(m) | perm_other(m), c)

int read_all_groups(gid_t **g);
int checkpermission(struct stat *st, mode_t mode);

int read_all_groups(gid_t **g)
{
        int ngroups = getgroups(0, NULL);
        gid_t *groups = (gid_t *)realloc(*g, ngroups * sizeof(gid_t));
        if(getgroups(ngroups, groups) < 0)
        {
                free(groups);
                groups = NULL;
                ngroups = 0;
        }
        *g = groups;
        return ngroups;
}

int checkpermission(struct stat *st, mode_t mode)
{
        uid_t uid = geteuid();
        if(uid == 0) /* if the program runs as root, file permissions can be (bitwise) sumerized. */
                return checkpermission_root(st->st_mode, mode);
        else if(uid == st->st_uid) /* the program and the file have the same owner, check the owner permissions. */
                return check_mode(perm_owner(st->st_mode), mode);

        /* the uid of the program is not 0 and does not match the uid of the file, now check for a machting group. */
        gid_t *gid = NULL;
        int i, ngid = read_all_groups(&gid); /* get all groups the program is in. */
        for(i = 0; i < ngid; i++)
        {
                if(gid[i] == st->st_gid)
                { /* there is a match between the program and file group */
                        free(gid); /* free the memory, because we will return. */
                        gid = NULL;
                        /* check the group permissions. */
                        return check_mode(perm_group(st->st_mode), mode);
                }
        }
        free(gid);
        gid = NULL;

        /* there is no match with the owner and group of the file, check the permissions for others */
        return check_mode(perm_other(st->st_mode), mode);
}

int check_path_exists(const char *path, mode_t type, mode_t mode)
{
        struct stat st;

        if(stat(path, &st) != 0) /* cannot get file attributes, assume that the file does not exists */
                return -1;

        /* check the file type. */
        if(!check_mode(st.st_mode & S_IFMT, type))
                return -1;

        /* check the file permission, only if 'mode' is not 0. */
        if(mode != 0 && !checkpermission(&st, mode))
                return -1;

        return 0;
}