/*
 * read-access-check.c - check if the user has read permission to a given
 *                       file. If not, check where the problem lies.
 *
 * Background: In order to read from a file, we must have execute permission
 *             in all directories leading to this file, as well as read access
 *             to the file itself. For example, to read from file
 *             "/etc/config/blabla.conf", we must have X permission on '/etc'
 *             and on '/etc/config', as well as R permission on
 *             "/etc/config/blabla.conf" itself.
 *
 * Algorithm: split the file path to its directories, make sure the directory
 *            exists and that the user has execute permission to all
 *            directories. Finally check that the file exists, and that the
 *            user has read permission to the file itself.
 */

#include <stdio.h>              /* standard input/output routines.    */
#include <unistd.h>             /* access(), R_OK etc.                */
#include <string.h>             /* strchr(), etc.                     */
#include <malloc.h>             /* malloc(), etc.                     */

/*
 * function: main. 
 * input:    full path to a file.
 * output:   'OK' if permission is granted, or 'ACCESS DENIED' and a
 *           detailed explanation if some problem was found.
 */
void
main(int argc, char* argv[])
{
    char* file_path;	        /* full path to file.           */
    char* dir_path;	        /* full path to a directory.    */
    char* p_slash;	        /* location of next '/' in path. */

    /* read command line arguments */
    if (argc != 2 || !argv[1]) {
	fprintf(stderr, "Usage: %s <full file path>\n", argv[0]);
	exit(1);
    }
    file_path = argv[1];
    dir_path = (char*)malloc(strlen(file_path)+1);
    if (!dir_path) {
	fprintf(stderr, "out of memory\n");
	exit(1);
    }

    /* scan all directories in the path and check each of them. */
    p_slash = file_path;
    while ( (p_slash = strchr(p_slash, '/')) != NULL) {
	/* copy directory path (including trailing slash) into dir_path. */
	strncpy(dir_path, file_path, p_slash-file_path+1);
        dir_path[p_slash-file_path+1] = '\0';

	/* check existance and execute permission for this directory. */
	if (access(dir_path, F_OK) == -1) {
	    printf("%s: Directory '%s' in the path does not exist.\n",
                   "ACCESS DENIED", dir_path);
	    exit(2);
	}
	if (access(dir_path, X_OK) == -1) {
	    printf("%s: Directory '%s' in the path - no 'X' permission.\n",
		   "ACCESS DENIED", dir_path);
	    exit(2);
	}
	/* skip the current slash, so the next strchr() call will find */
	/* the next slash in the file path.                            */
	p_slash++;
    }

    /* all directories in the path exist and are executable.   */
    /* now check existance and read access to the file itself. */
    if (access(file_path, F_OK) == -1) {
        printf("%s: File does not exist.\n", "ACCESS DENIED");
        exit(2);
    }
    if (access(file_path, R_OK) == -1) {
        printf("%s: no read access to file.\n", "ACCESS DENIED");
        exit(2);
    }

    /* all tests passed. */
    printf("OK\n");
}