serenity/Userland/Utilities/chmod.cpp

/*
 * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Format.h>
#include <AK/Optional.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

/* the new mode will be computed using the boolean function(for each bit):

    |current mode|removal mask|applying mask|result |
    |      0     |      0     |      0      |   0   |
    |      0     |      0     |      1      |   1   |
    |      0     |      1     |      0      |   0   |
    |      0     |      1     |      1      |   1   | ---> find the CNF --> find the minimal CNF
    |      1     |      0     |      0      |   1   |
    |      1     |      0     |      1      |   1   |
    |      1     |      1     |      0      |   0   |
    |      1     |      1     |      1      |   1   |
*/

class Mask {
private:
    mode_t removal_mask;  // the bits that will be removed
    mode_t applying_mask; // the bits that will be set

public:
    Mask()
        : removal_mask(0)
        , applying_mask(0)
    {
    }

    Mask& operator|=(const Mask& other)
    {
        removal_mask |= other.removal_mask;
        applying_mask |= other.applying_mask;

        return *this;
    }

    mode_t& get_removal_mask() { return removal_mask; }
    mode_t& get_applying_mask() { return applying_mask; }
};

Optional<Mask> string_to_mode(char access_scope, const char*& access_string);
Optional<Mask> apply_permission(char access_scope, char permission, char operation);

int main(int argc, char** argv)
{
    if (pledge("stdio rpath fattr", nullptr) < 0) {
        perror("pledge");
        return 1;
    }

    if (argc < 3) {
        warnln("usage: chmod <octal-mode> <path...>");
        warnln("       chmod [[ugoa][+-=][rwx...],...] <path...>");
        return 1;
    }

    Mask mask;

    /* compute a mask */
    if (argv[1][0] >= '0' && argv[1][0] <= '7') {
        if (sscanf(argv[1], "%ho", &mask.get_applying_mask()) != 1) {
            perror("sscanf");
            return 1;
        }
        mask.get_removal_mask() = ~mask.get_applying_mask();
    } else {
        const char* access_string = argv[1];

        while (*access_string != '\0') {
            Optional<Mask> tmp_mask;
            switch (*access_string) {
            case 'u':
                tmp_mask = string_to_mode('u', ++access_string);
                break;
            case 'g':
                tmp_mask = string_to_mode('g', ++access_string);
                break;
            case 'o':
                tmp_mask = string_to_mode('o', ++access_string);
                break;
            case 'a':
                tmp_mask = string_to_mode('a', ++access_string);
                break;
            case '=':
            case '+':
            case '-':
                tmp_mask = string_to_mode('a', access_string);
                break;
            case ',':
                ++access_string;
                continue;
            }
            if (!tmp_mask.has_value()) {
                warnln("chmod: invalid mode: {}", argv[1]);
                return 1;
            }
            mask |= tmp_mask.value();
        }
    }

    /* set the mask for each file's permissions */
    struct stat current_access;
    int i = 2;
    while (i < argc) {
        if (stat(argv[i], &current_access) != 0) {
            perror("stat");
            return 1;
        }
        /* found the minimal CNF by The Quine–McCluskey algorithm and use it */
        mode_t mode = mask.get_applying_mask()
            | (current_access.st_mode & ~mask.get_removal_mask());
        if (chmod(argv[i++], mode) != 0) {
            perror("chmod");
        }
    }

    return 0;
}

Optional<Mask> string_to_mode(char access_scope, const char*& access_string)
{
    char operation = *access_string;

    if (operation != '+' && operation != '-' && operation != '=') {
        return {};
    }

    Mask mask;
    if (operation == '=') {
        switch (access_scope) {
        case 'u':
            mask.get_removal_mask() = (S_IRUSR | S_IWUSR | S_IXUSR);
            break;
        case 'g':
            mask.get_removal_mask() = (S_IRGRP | S_IWGRP | S_IXGRP);
            break;
        case 'o':
            mask.get_removal_mask() = (S_IROTH | S_IWOTH | S_IXOTH);
            break;
        case 'a':
            mask.get_removal_mask() = (S_IRUSR | S_IWUSR | S_IXUSR
                | S_IRGRP | S_IWGRP | S_IXGRP
                | S_IROTH | S_IWOTH | S_IXOTH);
            break;
        }
        operation = '+';
    }

    access_string++;
    while (*access_string != '\0' && *access_string != ',') {
        Optional<Mask> tmp_mask;
        tmp_mask = apply_permission(access_scope, *access_string, operation);
        if (!tmp_mask.has_value()) {
            return {};
        }
        mask |= tmp_mask.value();
        access_string++;
    }

    return mask;
}

Optional<Mask> apply_permission(char access_scope, char permission, char operation)
{
    if (permission != 'r' && permission != 'w' && permission != 'x') {
        return {};
    }

    Mask mask;
    mode_t tmp_mask = 0;
    switch (access_scope) {
    case 'u':
        switch (permission) {
        case 'r':
            tmp_mask = S_IRUSR;
            break;
        case 'w':
            tmp_mask = S_IWUSR;
            break;
        case 'x':
            tmp_mask = S_IXUSR;
            break;
        }
        break;
    case 'g':
        switch (permission) {
        case 'r':
            tmp_mask = S_IRGRP;
            break;
        case 'w':
            tmp_mask = S_IWGRP;
            break;
        case 'x':
            tmp_mask = S_IXGRP;
            break;
        }
        break;
    case 'o':
        switch (permission) {
        case 'r':
            tmp_mask = S_IROTH;
            break;
        case 'w':
            tmp_mask = S_IWOTH;
            break;
        case 'x':
            tmp_mask = S_IXOTH;
            break;
        }
        break;
    case 'a':
        mask |= apply_permission('u', permission, operation).value();
        mask |= apply_permission('g', permission, operation).value();
        mask |= apply_permission('o', permission, operation).value();
        break;
    }

    if (operation == '+') {
        mask.get_applying_mask() |= tmp_mask;
    } else {
        mask.get_removal_mask() |= tmp_mask;
    }

    return mask;
}