mirror of
https://gitee.com/openharmony/kernel_linux
synced 2025-04-17 23:02:52 +00:00
8387ff2577
We always mixed in the parent pointer into the dentry name hash, but we did it late at lookup time. It turns out that we can simplify that lookup-time action by salting the hash with the parent pointer early instead of late. A few other users of our string hashes also wanted to mix in their own pointers into the hash, and those are updated to use the same mechanism. Hash users that don't have any particular initial salt can just use the NULL pointer as a no-salt. Cc: Vegard Nossum <vegard.nossum@oracle.com> Cc: George Spelvin <linux@sciencehorizons.net> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
77 lines
2.5 KiB
C
77 lines
2.5 KiB
C
#ifndef __LINUX_STRINGHASH_H
|
|
#define __LINUX_STRINGHASH_H
|
|
|
|
#include <linux/compiler.h> /* For __pure */
|
|
#include <linux/types.h> /* For u32, u64 */
|
|
|
|
/*
|
|
* 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)
|
|
*/
|
|
static inline unsigned long end_name_hash(unsigned long hash)
|
|
{
|
|
return (unsigned int)hash;
|
|
}
|
|
|
|
/*
|
|
* 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 */
|