treewide: use prandom_u32_max() when possible, part 1

Rather than incurring a division or requesting too many random bytes for
the given range, use the prandom_u32_max() function, which only takes
the minimum required bytes from the RNG and avoids divisions. This was
done mechanically with this coccinelle script:

@basic@
expression E;
type T;
identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32";
typedef u64;
@@
(
- ((T)get_random_u32() % (E))
+ prandom_u32_max(E)
|
- ((T)get_random_u32() & ((E) - 1))
+ prandom_u32_max(E * XXX_MAKE_SURE_E_IS_POW2)
|
- ((u64)(E) * get_random_u32() >> 32)
+ prandom_u32_max(E)
|
- ((T)get_random_u32() & ~PAGE_MASK)
+ prandom_u32_max(PAGE_SIZE)
)

@multi_line@
identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32";
identifier RAND;
expression E;
@@

-       RAND = get_random_u32();
        ... when != RAND
-       RAND %= (E);
+       RAND = prandom_u32_max(E);

// Find a potential literal
@literal_mask@
expression LITERAL;
type T;
identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32";
position p;
@@

        ((T)get_random_u32()@p & (LITERAL))

// Add one to the literal.
@script:python add_one@
literal << literal_mask.LITERAL;
RESULT;
@@

value = None
if literal.startswith('0x'):
        value = int(literal, 16)
elif literal[0] in '123456789':
        value = int(literal, 10)
if value is None:
        print("I don't know how to handle %s" % (literal))
        cocci.include_match(False)
elif value == 2**32 - 1 or value == 2**31 - 1 or value == 2**24 - 1 or value == 2**16 - 1 or value == 2**8 - 1:
        print("Skipping 0x%x for cleanup elsewhere" % (value))
        cocci.include_match(False)
elif value & (value + 1) != 0:
        print("Skipping 0x%x because it's not a power of two minus one" % (value))
        cocci.include_match(False)
elif literal.startswith('0x'):
        coccinelle.RESULT = cocci.make_expr("0x%x" % (value + 1))
else:
        coccinelle.RESULT = cocci.make_expr("%d" % (value + 1))

// Replace the literal mask with the calculated result.
@plus_one@
expression literal_mask.LITERAL;
position literal_mask.p;
expression add_one.RESULT;
identifier FUNC;
@@

-       (FUNC()@p & (LITERAL))
+       prandom_u32_max(RESULT)

@collapse_ret@
type T;
identifier VAR;
expression E;
@@

 {
-       T VAR;
-       VAR = (E);
-       return VAR;
+       return E;
 }

@drop_var@
type T;
identifier VAR;
@@

 {
-       T VAR;
        ... when != VAR
 }

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: KP Singh <kpsingh@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz> # for ext4 and sbitmap
Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com> # for drbd
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com> # for s390
Acked-by: Ulf Hansson <ulf.hansson@linaro.org> # for mmc
Acked-by: Darrick J. Wong <djwong@kernel.org> # for xfs
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 4 insertions, 4 deletions

diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
index f25692def781..2503ae2ae65d 100644
--- a/mm/kasan/kasan_test.c
+++ b/mm/kasan/kasan_test.c
@@ -1292,7 +1292,7 @@ static void match_all_not_assigned(struct kunit *test)
 	KASAN_TEST_NEEDS_CONFIG_OFF(test, CONFIG_KASAN_GENERIC);
 
 	for (i = 0; i < 256; i++) {
-		size = (get_random_int() % 1024) + 1;
+		size = prandom_u32_max(1024) + 1;
 		ptr = kmalloc(size, GFP_KERNEL);
 		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 		KUNIT_EXPECT_GE(test, (u8)get_tag(ptr), (u8)KASAN_TAG_MIN);
@@ -1301,7 +1301,7 @@ static void match_all_not_assigned(struct kunit *test)
 	}
 
 	for (i = 0; i < 256; i++) {
-		order = (get_random_int() % 4) + 1;
+		order = prandom_u32_max(4) + 1;
 		pages = alloc_pages(GFP_KERNEL, order);
 		ptr = page_address(pages);
 		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
@@ -1314,7 +1314,7 @@ static void match_all_not_assigned(struct kunit *test)
 		return;
 
 	for (i = 0; i < 256; i++) {
-		size = (get_random_int() % 1024) + 1;
+		size = prandom_u32_max(1024) + 1;
 		ptr = vmalloc(size);
 		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 		KUNIT_EXPECT_GE(test, (u8)get_tag(ptr), (u8)KASAN_TAG_MIN);
diff --git a/mm/slub.c b/mm/slub.c
index 96dd392d7f99..157527d7101b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1881,7 +1881,7 @@ static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
 		return false;
 
 	freelist_count = oo_objects(s->oo);
-	pos = get_random_int() % freelist_count;
+	pos = prandom_u32_max(freelist_count);
 
 	page_limit = slab->objects * s->size;
 	start = fixup_red_left(s, slab_address(slab));