Line data    Source code

       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      128769 : partial_name_hash(unsigned long c, unsigned long prevhash)
      44             : {
      45      128769 :         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       12345 : static inline unsigned int end_name_hash(unsigned long hash)
      54             : {
      55       12345 :         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 */