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 */
  42 static inline unsigned long
  43 partial_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  */
  53 static 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  */
  66 extern 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 */
  77 extern u64 __pure hashlen_string(const void *salt, const char *name);
  78
  79 #endif  /* __LINUX_STRINGHASH_H */