word-at-a-time: make the interfaces truly generic
This changes the interfaces in <asm/word-at-a-time.h> to be a bit more
complicated, but a lot more generic.
In particular, it allows us to really do the operations efficiently on
both little-endian and big-endian machines, pretty much regardless of
machine details. For example, if you can rely on a fast population
count instruction on your architecture, this will allow you to make your
optimized <asm/word-at-a-time.h> file with that.
NOTE! The "generic" version in include/asm-generic/word-at-a-time.h is
not truly generic, it actually only works on big-endian. Why? Because
on little-endian the generic algorithms are wasteful, since you can
inevitably do better. The x86 implementation is an example of that.
(The only truly non-generic part of the asm-generic implementation is
the "find_zero()" function, and you could make a little-endian version
of it. And if the Kbuild infrastructure allowed us to pick a particular
header file, that would be lovely)
The <asm/word-at-a-time.h> functions are as follows:
- WORD_AT_A_TIME_CONSTANTS: specific constants that the algorithm
uses.
- has_zero(): take a word, and determine if it has a zero byte in it.
It gets the word, the pointer to the constant pool, and a pointer to
an intermediate "data" field it can set.
This is the "quick-and-dirty" zero tester: it's what is run inside
the hot loops.
- "prep_zero_mask()": take the word, the data that has_zero() produced,
and the constant pool, and generate an *exact* mask of which byte had
the first zero. This is run directly *outside* the loop, and allows
the "has_zero()" function to answer the "is there a zero byte"
question without necessarily getting exactly *which* byte is the
first one to contain a zero.
If you do multiple byte lookups concurrently (eg "hash_name()", which
looks for both NUL and '/' bytes), after you've done the prep_zero_mask()
phase, the result of those can be or'ed together to get the "either
or" case.
- The result from "prep_zero_mask()" can then be fed into "find_zero()"
(to find the byte offset of the first byte that was zero) or into
"zero_bytemask()" (to find the bytemask of the bytes preceding the
zero byte).
The existence of zero_bytemask() is optional, and is not necessary
for the normal string routines. But dentry name hashing needs it, so
if you enable DENTRY_WORD_AT_A_TIME you need to expose it.
This changes the generic strncpy_from_user() function and the dentry
hashing functions to use these modified word-at-a-time interfaces. This
gets us back to the optimized state of the x86 strncpy that we lost in
the previous commit when moving over to the generic version.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index c4c09b0..bb2b201 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -4,37 +4,7 @@
#include <linux/errno.h>
#include <asm/byteorder.h>
-
-static inline long find_zero(unsigned long mask)
-{
- long byte = 0;
-
-#ifdef __BIG_ENDIAN
-#ifdef CONFIG_64BIT
- if (mask >> 32)
- mask >>= 32;
- else
- byte = 4;
-#endif
- if (mask >> 16)
- mask >>= 16;
- else
- byte += 2;
- return (mask >> 8) ? byte : byte + 1;
-#else
-#ifdef CONFIG_64BIT
- if (!((unsigned int) mask)) {
- mask >>= 32;
- byte = 4;
- }
-#endif
- if (!(mask & 0xffff)) {
- mask >>= 16;
- byte += 2;
- }
- return (mask & 0xff) ? byte : byte + 1;
-#endif
-}
+#include <asm/word-at-a-time.h>
#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
#define IS_UNALIGNED(src, dst) 0
@@ -51,8 +21,7 @@
*/
static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
{
- const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1;
- const unsigned long low_bits = REPEAT_BYTE(0x7f);
+ const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
long res = 0;
/*
@@ -66,18 +35,16 @@
goto byte_at_a_time;
while (max >= sizeof(unsigned long)) {
- unsigned long c, v, rhs;
+ unsigned long c, data;
/* Fall back to byte-at-a-time if we get a page fault */
if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
break;
- rhs = c | low_bits;
- v = (c + high_bits) & ~rhs;
*(unsigned long *)(dst+res) = c;
- if (v) {
- v = (c & low_bits) + low_bits;
- v = ~(v | rhs);
- return res + find_zero(v);
+ if (has_zero(c, &data, &constants)) {
+ data = prep_zero_mask(c, data, &constants);
+ data = create_zero_mask(data);
+ return res + find_zero(data);
}
res += sizeof(unsigned long);
max -= sizeof(unsigned long);