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 */ 80