lftpd_io.c

/*
   lftpd_io.c
   Copyright (C) 2018 Jason von Nieda
*/

/* Path helpers for lftpd: canonicalize a client-supplied path against the
   current directory, resolving '.' and '..' and clamping at the root. */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <stdarg.h>
#include <stdbool.h>
#include <unistd.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <dirent.h>
#include <errno.h>
#include <sys/stat.h>

#include "private/lftpd_io.h"

char *lftpd_io_canonicalize_path(const char *base, const char *name) {
    // if either argument is null, treat it as empty
    if(base == NULL) {
        base = "";
    }
    if(name == NULL) {
        name = "";
    }

    // if name is absolute, ignore the base and use name as the
    // full path
    char *path = NULL;
    if(name[0] == '/') {
        path = strdup(name);
    }
    // otherwise append name to base with / as a separator
    else {
        size_t len = strlen(base) + 1 + strlen(name) + 1;
        path = malloc(len);
        if(path == NULL) {
            return NULL;
        }
        snprintf(path, len, "%s/%s", base, name);
    }

    if(path == NULL)
        return NULL;

    // allocate enough room for the absolute path, which can never be
    // longer than the path, plus 1 for a / and 1 for the terminator
    size_t abs_path_len = strlen(path) + 1 + 1;
    char *abs_path = malloc(abs_path_len);
    if(abs_path == NULL) {
        free(path);
        return NULL;
    }
    memset(abs_path, 0, abs_path_len);

    // run through the path a segment at a time, adding each to
    // abs_path with with a preceding / and resolving . and ..
    char *save_pointer = NULL;
    char *token = strtok_r(path, "/", &save_pointer);
    while(token) {
        if(strcmp(token, ".") == 0) {
            // ignore it
        }
        else if(strcmp(token, "..") == 0) {
            // go back one element
            char *p = strrchr(abs_path, '/');
            if(p != NULL) {
                p[0] = '\0';
            }
        }
        else {
            strcat(abs_path, "/");
            strcat(abs_path, token);
        }

        token = strtok_r(NULL, "/", &save_pointer);
    }
    free(path);

    // a path like /test/.. might have removed everything and left
    // an empty path, so detect that condition and fix it
    if(strlen(abs_path) == 0) {
        strcpy(abs_path, "/");
    }

    return abs_path;
}