csky: Fixup raw_copy_from_user()

If raw_copy_from_user(to, from, N) returns K, callers expect
the first N - K bytes starting at to to have been replaced with
the contents of corresponding area starting at from and the last
K bytes of destination *left* *unmodified*.

What arch/sky/lib/usercopy.c is doing is broken - it can lead to e.g.
data corruption on write(2).

raw_copy_to_user() is inaccurate about return value, which is a bug,
but consequences are less drastic than for raw_copy_from_user().
And just what are those access_ok() doing in there?  I mean, look into
linux/uaccess.h; that's where we do that check (as well as zero tail
on failure in the callers that need zeroing).

AFAICS, all of that shouldn't be hard to fix; something like a patch
below might make a useful starting point.

I would suggest moving these macros into usercopy.c (they are never
used anywhere else) and possibly expanding them there; if you leave
them alive, please at least rename __copy_user_zeroing(). Again,
it must not zero anything on failed read.

Said that, I'm not sure we won't be better off simply turning
usercopy.c into usercopy.S - all that is left there is a couple of
functions, each consisting only of inline asm.

Guo Ren reply:

Yes, raw_copy_from_user is wrong, it's no need zeroing code.

unsigned long _copy_from_user(void *to, const void __user *from,
unsigned long n)
{
        unsigned long res = n;
        might_fault();
        if (likely(access_ok(from, n))) {
                kasan_check_write(to, n);
                res = raw_copy_from_user(to, from, n);
        }
        if (unlikely(res))
                memset(to + (n - res), 0, res);
        return res;
}
EXPORT_SYMBOL(_copy_from_user);

You are right and access_ok() should be removed.

but, how about:
do {
...
        "2:     stw     %3, (%1, 0)     \n"             \
+       "       subi    %0, 4          \n"               \
        "9:     stw     %4, (%1, 4)     \n"             \
+       "       subi    %0, 4          \n"               \
        "10:    stw     %5, (%1, 8)     \n"             \
+       "       subi    %0, 4          \n"               \
        "11:    stw     %6, (%1, 12)    \n"             \
+       "       subi    %0, 4          \n"               \
        "       addi    %2, 16          \n"             \
        "       addi    %1, 16          \n"             \

Don't expand __ex_table

AI Viro reply:

Hey, I've no idea about the instruction scheduling on csky -
if that doesn't slow the things down, all the better.  It's just
that copy_to_user() and friends are on fairly hot codepaths,
and in quite a few situations they will dominate the speed of
e.g. read(2).  So I tried to keep the fast path unchanged.
Up to the architecture maintainers, obviously.  Which would be
you...

As for the fixups size increase (__ex_table size is unchanged)...
You have each of those macros expanded exactly once.
So the size is not a serious argument, IMO - useless complexity
would be, if it is, in fact, useless; the size... not really,
especially since those extra subi will at least offset it.

Again, up to you - asm optimizations of (essentially)
memcpy()-style loops are tricky and can depend upon the
fairly subtle details of architecture.  So even on something
I know reasonably well I would resort to direct experiments
if I can't pass the buck to architecture maintainers.

It *is* worth optimizing - this is where read() from a file
that is already in page cache spends most of the time, etc.

Guo Ren reply:

Thx, after fixup some typo “sub %0, 4”, apply the patch.

TODO:
 - user copy/from codes are still need optimizing.

Signed-off-by: Al Viro <[email protected]>
Signed-off-by: Guo Ren <[email protected]>

Author: Al Viro

arch/csky/include/asm/uaccess.h

@@ -253,7 +253,7 @@ do {								\
 
 extern int __get_user_bad(void);
 
-#define __copy_user(to, from, n)			\
+#define ___copy_to_user(to, from, n)			\
 do {							\
 	int w0, w1, w2, w3;				\
 	asm volatile(					\
@@ -288,31 +288,34 @@ do {							\
 	"       subi    %0, 4           \n"		\
 	"       br      3b              \n"		\
 	"5:     cmpnei  %0, 0           \n"  /* 1B */   \
-	"       bf      8f              \n"		\
+	"       bf      13f             \n"		\
 	"       ldb     %3, (%2, 0)     \n"		\
 	"6:     stb     %3, (%1, 0)     \n"		\
 	"       addi    %2,  1          \n"		\
 	"       addi    %1,  1          \n"		\
 	"       subi    %0,  1          \n"		\
 	"       br      5b              \n"		\
-	"7:     br      8f              \n"		\
+	"7:     subi	%0,  4          \n"		\
+	"8:     subi	%0,  4          \n"		\
+	"12:    subi	%0,  4          \n"		\
+	"       br      13f             \n"		\
 	".section __ex_table, \"a\"     \n"		\
 	".align   2                     \n"		\
-	".long    2b, 7b                \n"		\
-	".long    9b, 7b                \n"		\
-	".long   10b, 7b                \n"		\
+	".long    2b, 13f               \n"		\
+	".long    4b, 13f               \n"		\
+	".long    6b, 13f               \n"		\
+	".long    9b, 12b               \n"		\
+	".long   10b, 8b                \n"		\
 	".long   11b, 7b                \n"		\
-	".long    4b, 7b                \n"		\
-	".long    6b, 7b                \n"		\
 	".previous                      \n"		\
-	"8:                             \n"		\
+	"13:                            \n"		\
 	: "=r"(n), "=r"(to), "=r"(from), "=r"(w0),	\
 	  "=r"(w1), "=r"(w2), "=r"(w3)			\
 	: "0"(n), "1"(to), "2"(from)			\
 	: "memory");					\
 } while (0)
 
-#define __copy_user_zeroing(to, from, n)		\
+#define ___copy_from_user(to, from, n)			\
 do {							\
 	int tmp;					\
 	int nsave;					\
@@ -355,22 +358,22 @@ do {							\
 	"       addi    %1,  1          \n"		\
 	"       subi    %0,  1          \n"		\
 	"       br      5b              \n"		\
-	"8:     mov     %3, %0          \n"		\
-	"       movi    %4, 0           \n"		\
-	"9:     stb     %4, (%1, 0)     \n"		\
-	"       addi    %1, 1           \n"		\
-	"       subi    %3, 1           \n"		\
-	"       cmpnei  %3, 0           \n"		\
-	"       bt      9b              \n"		\
-	"       br      7f              \n"		\
+	"8:     stw     %3, (%1, 0)     \n"		\
+	"       subi    %0, 4           \n"		\
+	"       bf      7f              \n"		\
+	"9:     subi    %0, 8           \n"		\
+	"       bf      7f              \n"		\
+	"13:    stw     %3, (%1, 8)     \n"		\
+	"       subi    %0, 12          \n"		\
+	"       bf      7f              \n"		\
 	".section __ex_table, \"a\"     \n"		\
 	".align   2                     \n"		\
-	".long    2b, 8b                \n"		\
+	".long    2b, 7f                \n"		\
+	".long    4b, 7f                \n"		\
+	".long    6b, 7f                \n"		\
 	".long   10b, 8b                \n"		\
-	".long   11b, 8b                \n"		\
-	".long   12b, 8b                \n"		\
-	".long    4b, 8b                \n"		\
-	".long    6b, 8b                \n"		\
+	".long   11b, 9b                \n"		\
+	".long   12b,13b                \n"		\
 	".previous                      \n"		\
 	"7:                             \n"		\
 	: "=r"(n), "=r"(to), "=r"(from), "=r"(nsave),	\

arch/csky/lib/usercopy.c

@@ -7,19 +7,15 @@
 unsigned long raw_copy_from_user(void *to, const void *from,
 			unsigned long n)
 {
-	if (access_ok(from, n))
-		__copy_user_zeroing(to, from, n);
-	else
-		memset(to, 0, n);
+	___copy_from_user(to, from, n);
 	return n;
 }
 EXPORT_SYMBOL(raw_copy_from_user);
 
 unsigned long raw_copy_to_user(void *to, const void *from,
 			unsigned long n)
 {
-	if (access_ok(to, n))
-		__copy_user(to, from, n);
+	___copy_to_user(to, from, n);
 	return n;
 }
 EXPORT_SYMBOL(raw_copy_to_user);