key.c

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/* Key to pathname encoder
 *
 * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells ([email protected])
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/slab.h>
#include "internal.h"

static const char cachefiles_charmap[64] =
    "0123456789"            /* 0 - 9 */
    "abcdefghijklmnopqrstuvwxyz"    /* 10 - 35 */
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"    /* 36 - 61 */
    "_-"                /* 62 - 63 */
    ;

static const char cachefiles_filecharmap[256] = {
    /* we skip space and tab and control chars */
    [33 ... 46] = 1,        /* '!' -> '.' */
    /* we skip '/' as it's significant to pathwalk */
    [48 ... 127] = 1,       /* '0' -> '~' */
};

/*
 * turn the raw key into something cooked
 * - the raw key should include the length in the two bytes at the front
 * - the key may be up to 514 bytes in length (including the length word)
 *   - "base64" encode the strange keys, mapping 3 bytes of raw to four of
 *     cooked
 *   - need to cut the cooked key into 252 char lengths (189 raw bytes)
 */
char *cachefiles_cook_key(const u8 *raw, int keylen, uint8_t type)
{
    unsigned char csum, ch;
    unsigned int acc;
    char *key;
    int loop, len, max, seg, mark, print;

    _enter(",%d", keylen);

    BUG_ON(keylen < 2 || keylen > 514);

    csum = raw[0] + raw[1];
    print = 1;
    for (loop = 2; loop < keylen; loop++) {
        ch = raw[loop];
        csum += ch;
        print &= cachefiles_filecharmap[ch];
    }

    if (print) {
        /* if the path is usable ASCII, then we render it directly */
        max = keylen - 2;
        max += 2;   /* two base64'd length chars on the front */
        max += 5;   /* @checksum/M */
        max += 3 * 2;   /* maximum number of segment dividers (".../M")
                 * is ((514 + 251) / 252) = 3
                 */
        max += 1;   /* NUL on end */
    } else {
        /* calculate the maximum length of the cooked key */
        keylen = (keylen + 2) / 3;

        max = keylen * 4;
        max += 5;   /* @checksum/M */
        max += 3 * 2;   /* maximum number of segment dividers (".../M")
                 * is ((514 + 188) / 189) = 3
                 */
        max += 1;   /* NUL on end */
    }

    max += 1;   /* 2nd NUL on end */

    _debug("max: %d", max);

    key = kmalloc(max, GFP_KERNEL);
    if (!key)
        return NULL;

    len = 0;

    /* build the cooked key */
    sprintf(key, "@%02x%c+", (unsigned) csum, 0);
    len = 5;
    mark = len - 1;

    if (print) {
        acc = *(uint16_t *) raw;
        raw += 2;

        key[len + 1] = cachefiles_charmap[acc & 63];
        acc >>= 6;
        key[len] = cachefiles_charmap[acc & 63];
        len += 2;

        seg = 250;
        for (loop = keylen; loop > 0; loop--) {
            if (seg <= 0) {
                key[len++] = '\0';
                mark = len;
                key[len++] = '+';
                seg = 252;
            }

            key[len++] = *raw++;
            ASSERT(len < max);
        }

        switch (type) {
        case FSCACHE_COOKIE_TYPE_INDEX:     type = 'I'; break;
        case FSCACHE_COOKIE_TYPE_DATAFILE:  type = 'D'; break;
        default:                type = 'S'; break;
        }
    } else {
        seg = 252;
        for (loop = keylen; loop > 0; loop--) {
            if (seg <= 0) {
                key[len++] = '\0';
                mark = len;
                key[len++] = '+';
                seg = 252;
            }

            acc = *raw++;
            acc |= *raw++ << 8;
            acc |= *raw++ << 16;

            _debug("acc: %06x", acc);

            key[len++] = cachefiles_charmap[acc & 63];
            acc >>= 6;
            key[len++] = cachefiles_charmap[acc & 63];
            acc >>= 6;
            key[len++] = cachefiles_charmap[acc & 63];
            acc >>= 6;
            key[len++] = cachefiles_charmap[acc & 63];

            ASSERT(len < max);
        }

        switch (type) {
        case FSCACHE_COOKIE_TYPE_INDEX:     type = 'J'; break;
        case FSCACHE_COOKIE_TYPE_DATAFILE:  type = 'E'; break;
        default:                type = 'T'; break;
        }
    }

    key[mark] = type;
    key[len++] = 0;
    key[len] = 0;

    _leave(" = %p %d", key, len);
    return key;
}