fs/namei.c: Add hashlen_string() function

We'd like to make more use of the highly-optimized dcache hash functions
throughout the kernel, rather than have every subsystem create its own,
and a function that hashes basic null-terminated strings is required
for that.

(The name is to emphasize that it returns both hash and length.)

It's actually useful in the dcache itself, specifically d_alloc_name().
Other uses in the next patch.

full_name_hash() is also tweaked to make it more generally useful:
1) Take a "char *" rather than "unsigned char *" argument, to
   be consistent with hash_name().
2) Handle zero-length inputs.  If we want more callers, we don't want
   to make them worry about corner cases.

Signed-off-by: George Spelvin <linux@sciencehorizons.net>

fs/namei.c

@@ -1822,19 +1822,20 @@ static inline unsigned long mix_hash(unsigned long hash)
 
 #endif
 
-unsigned int full_name_hash(const unsigned char *name, unsigned int len)
+/* Return the hash of a string of known length */
+unsigned int full_name_hash(const char *name, unsigned int len)
 {
 	unsigned long a, hash = 0;
 
 	for (;;) {
+		if (!len)
+			goto done;
 		a = load_unaligned_zeropad(name);
 		if (len < sizeof(unsigned long))
 			break;
 		hash = mix_hash(hash + a);
 		name += sizeof(unsigned long);
 		len -= sizeof(unsigned long);
-		if (!len)
-			goto done;
 	}
 	hash += a & bytemask_from_count(len);
 done:
@@ -1842,6 +1843,29 @@ done:
 }
 EXPORT_SYMBOL(full_name_hash);
 
+/* Return the "hash_len" (hash and length) of a null-terminated string */
+u64 hashlen_string(const char *name)
+{
+	unsigned long a, adata, mask, hash, len;
+	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
+
+	hash = a = 0;
+	len = -sizeof(unsigned long);
+	do {
+		hash = mix_hash(hash + a);
+		len += sizeof(unsigned long);
+		a = load_unaligned_zeropad(name+len);
+	} while (!has_zero(a, &adata, &constants));
+
+	adata = prep_zero_mask(a, adata, &constants);
+	mask = create_zero_mask(adata);
+	hash += a & zero_bytemask(mask);
+	len += find_zero(mask);
+
+	return hashlen_create(fold_hash(hash), len);
+}
+EXPORT_SYMBOL(hashlen_string);
+
 /*
  * Calculate the length and hash of the path component, and
  * return the "hash_len" as the result.
@@ -1872,15 +1896,32 @@ static inline u64 hash_name(const char *name)
 
 #else
 
-unsigned int full_name_hash(const unsigned char *name, unsigned int len)
+/* Return the hash of a string of known length */
+unsigned int full_name_hash(const char *name, unsigned int len)
 {
 	unsigned long hash = init_name_hash();
 	while (len--)
-		hash = partial_name_hash(*name++, hash);
+		hash = partial_name_hash((unsigned char)*name++, hash);
 	return end_name_hash(hash);
 }
 EXPORT_SYMBOL(full_name_hash);
 
+/* Return the "hash_len" (hash and length) of a null-terminated string */
+u64 hash_string(const char *name)
+{
+	unsigned long hash = init_name_hash();
+	unsigned long len = 0, c;
+
+	c = (unsigned char)*name;
+	do {
+		len++;
+		hash = partial_name_hash(c, hash);
+		c = (unsigned char)name[len];
+	} while (c);
+	return hashlen_create(end_name_hash(hash), len);
+}
+EXPORT_SYMBOL(hash_string);
+
 /*
  * We know there's a real path component here of at least
  * one character.