1 #ifndef __LINUX_STRINGHASH_H 2 #define __LINUX_STRINGHASH_H 3 4 #include <linux/compiler.h> /* For __pure */ 5 #include <linux/types.h> /* For u32, u64 */ 6 #include <linux/hash.h> 7 8 /* 9 * Routines for hashing strings of bytes to a 32-bit hash value. 10 * 11 * These hash functions are NOT GUARANTEED STABLE between kernel 12 * versions, architectures, or even repeated boots of the same kernel. 13 * (E.g. they may depend on boot-time hardware detection or be 14 * deliberately randomized.) 15 * 16 * They are also not intended to be secure against collisions caused by 17 * malicious inputs; much slower hash functions are required for that. 18 * 19 * They are optimized for pathname components, meaning short strings. 20 * Even if a majority of files have longer names, the dynamic profile of 21 * pathname components skews short due to short directory names. 22 * (E.g. /usr/lib/libsesquipedalianism.so.3.141.) 23 */ 24 25 /* 26 * Version 1: one byte at a time. Example of use: 27 * 28 * unsigned long hash = init_name_hash; 29 * while (*p) 30 * hash = partial_name_hash(tolower(*p++), hash); 31 * hash = end_name_hash(hash); 32 * 33 * Although this is designed for bytes, fs/hfsplus/unicode.c 34 * abuses it to hash 16-bit values. 35 */ 36 37 /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */ 38 #define init_name_hash(salt) (unsigned long)(salt) 39 40 /* partial hash update function. Assume roughly 4 bits per character */ 41 static inline unsigned long 42 partial_name_hash(unsigned long c, unsigned long prevhash) 43 { 44 return (prevhash + (c << 4) + (c >> 4)) * 11; 45 } 46 47 /* 48 * Finally: cut down the number of bits to a int value (and try to avoid 49 * losing bits). This also has the property (wanted by the dcache) 50 * that the msbits make a good hash table index. 51 */ 52 static inline unsigned long end_name_hash(unsigned long hash) 53 { 54 return __hash_32((unsigned int)hash); 55 } 56 57 /* 58 * Version 2: One word (32 or 64 bits) at a time. 59 * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h> 60 * exists, which describes major Linux platforms like x86 and ARM), then 61 * this computes a different hash function much faster. 62 * 63 * If not set, this falls back to a wrapper around the preceding. 64 */ 65 extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int); 66 67 /* 68 * A hash_len is a u64 with the hash of a string in the low 69 * half and the length in the high half. 70 */ 71 #define hashlen_hash(hashlen) ((u32)(hashlen)) 72 #define hashlen_len(hashlen) ((u32)((hashlen) >> 32)) 73 #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash)) 74 75 /* Return the "hash_len" (hash and length) of a null-terminated string */ 76 extern u64 __pure hashlen_string(const void *salt, const char *name); 77 78 #endif /* __LINUX_STRINGHASH_H */ 79