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 */