| /* SPDX-License-Identifier: GPL-2.0 */ |
| #ifndef __LINUX_STRINGHASH_H |
| #define __LINUX_STRINGHASH_H |
| |
| #include <linux/compiler.h> /* For __pure */ |
| #include <linux/types.h> /* For u32, u64 */ |
| #include <linux/hash.h> |
| |
| /* |
| * Routines for hashing strings of bytes to a 32-bit hash value. |
| * |
| * These hash functions are NOT GUARANTEED STABLE between kernel |
| * versions, architectures, or even repeated boots of the same kernel. |
| * (E.g. they may depend on boot-time hardware detection or be |
| * deliberately randomized.) |
| * |
| * They are also not intended to be secure against collisions caused by |
| * malicious inputs; much slower hash functions are required for that. |
| * |
| * They are optimized for pathname components, meaning short strings. |
| * Even if a majority of files have longer names, the dynamic profile of |
| * pathname components skews short due to short directory names. |
| * (E.g. /usr/lib/libsesquipedalianism.so.3.141.) |
| */ |
| |
| /* |
| * Version 1: one byte at a time. Example of use: |
| * |
| * unsigned long hash = init_name_hash; |
| * while (*p) |
| * hash = partial_name_hash(tolower(*p++), hash); |
| * hash = end_name_hash(hash); |
| * |
| * Although this is designed for bytes, fs/hfsplus/unicode.c |
| * abuses it to hash 16-bit values. |
| */ |
| |
| /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */ |
| #define init_name_hash(salt) (unsigned long)(salt) |
| |
| /* partial hash update function. Assume roughly 4 bits per character */ |
| static inline unsigned long |
| partial_name_hash(unsigned long c, unsigned long prevhash) |
| { |
| return (prevhash + (c << 4) + (c >> 4)) * 11; |
| } |
| |
| /* |
| * Finally: cut down the number of bits to a int value (and try to avoid |
| * losing bits). This also has the property (wanted by the dcache) |
| * that the msbits make a good hash table index. |
| */ |
| static inline unsigned int end_name_hash(unsigned long hash) |
| { |
| return hash_long(hash, 32); |
| } |
| |
| /* |
| * Version 2: One word (32 or 64 bits) at a time. |
| * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h> |
| * exists, which describes major Linux platforms like x86 and ARM), then |
| * this computes a different hash function much faster. |
| * |
| * If not set, this falls back to a wrapper around the preceding. |
| */ |
| extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int); |
| |
| /* |
| * A hash_len is a u64 with the hash of a string in the low |
| * half and the length in the high half. |
| */ |
| #define hashlen_hash(hashlen) ((u32)(hashlen)) |
| #define hashlen_len(hashlen) ((u32)((hashlen) >> 32)) |
| #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash)) |
| |
| /* Return the "hash_len" (hash and length) of a null-terminated string */ |
| extern u64 __pure hashlen_string(const void *salt, const char *name); |
| |
| #endif /* __LINUX_STRINGHASH_H */ |