LXR linux/include/linux/stringhash.h

   1/* SPDX-License-Identifier: GPL-2.0 */
   2#ifndef __LINUX_STRINGHASH_H
   3#define __LINUX_STRINGHASH_H
   4
   5#include <linux/compiler.h>     /* For __pure */
   6#include <linux/types.h>        /* For u32, u64 */
   7#include <linux/hash.h>
   8
   9/*
  10 * Routines for hashing strings of bytes to a 32-bit hash value.
  11 *
  12 * These hash functions are NOT GUARANTEED STABLE between kernel
  13 * versions, architectures, or even repeated boots of the same kernel.
  14 * (E.g. they may depend on boot-time hardware detection or be
  15 * deliberately randomized.)
  16 *
  17 * They are also not intended to be secure against collisions caused by
  18 * malicious inputs; much slower hash functions are required for that.
  19 *
  20 * They are optimized for pathname components, meaning short strings.
  21 * Even if a majority of files have longer names, the dynamic profile of
  22 * pathname components skews short due to short directory names.
  23 * (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
  24 */
  25
  26/*
  27 * Version 1: one byte at a time.  Example of use:
  28 *
  29 * unsigned long hash = init_name_hash;
  30 * while (*p)
  31 *      hash = partial_name_hash(tolower(*p++), hash);
  32 * hash = end_name_hash(hash);
  33 *
  34 * Although this is designed for bytes, fs/hfsplus/unicode.c
  35 * abuses it to hash 16-bit values.
  36 */
  37
  38/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
  39#define init_name_hash(salt)            (unsigned long)(salt)
  40
  41/* partial hash update function. Assume roughly 4 bits per character */
  42static inline unsigned long
  43partial_name_hash(unsigned long c, unsigned long prevhash)
  44{
  45        return (prevhash + (c << 4) + (c >> 4)) * 11;
  46}
  47
  48/*
  49 * Finally: cut down the number of bits to a int value (and try to avoid
  50 * losing bits).  This also has the property (wanted by the dcache)
  51 * that the msbits make a good hash table index.
  52 */
  53static inline unsigned int end_name_hash(unsigned long hash)
  54{
  55        return hash_long(hash, 32);
  56}
  57
  58/*
  59 * Version 2: One word (32 or 64 bits) at a time.
  60 * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
  61 * exists, which describes major Linux platforms like x86 and ARM), then
  62 * this computes a different hash function much faster.
  63 *
  64 * If not set, this falls back to a wrapper around the preceding.
  65 */
  66extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int);
  67
  68/*
  69 * A hash_len is a u64 with the hash of a string in the low
  70 * half and the length in the high half.
  71 */
  72#define hashlen_hash(hashlen) ((u32)(hashlen))
  73#define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
  74#define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
  75
  76/* Return the "hash_len" (hash and length) of a null-terminated string */
  77extern u64 __pure hashlen_string(const void *salt, const char *name);
  78
  79#endif  /* __LINUX_STRINGHASH_H */
  80