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>

3 files changed, 12 insertions, 12 deletions

diff --git a/fs/ubifs/debug.c b/fs/ubifs/debug.c
index fc718f6178f2..f4d3b568aa64 100644
--- a/fs/ubifs/debug.c
+++ b/fs/ubifs/debug.c
@@ -2467,7 +2467,7 @@ error_dump:
 
 static inline int chance(unsigned int n, unsigned int out_of)
 {
-	return !!((prandom_u32() % out_of) + 1 <= n);
+	return !!(prandom_u32_max(out_of) + 1 <= n);
 
 }
 
@@ -2485,13 +2485,13 @@ static int power_cut_emulated(struct ubifs_info *c, int lnum, int write)
 			if (chance(1, 2)) {
 				d->pc_delay = 1;
 				/* Fail within 1 minute */
-				delay = prandom_u32() % 60000;
+				delay = prandom_u32_max(60000);
 				d->pc_timeout = jiffies;
 				d->pc_timeout += msecs_to_jiffies(delay);
 				ubifs_warn(c, "failing after %lums", delay);
 			} else {
 				d->pc_delay = 2;
-				delay = prandom_u32() % 10000;
+				delay = prandom_u32_max(10000);
 				/* Fail within 10000 operations */
 				d->pc_cnt_max = delay;
 				ubifs_warn(c, "failing after %lu calls", delay);
@@ -2571,7 +2571,7 @@ static int corrupt_data(const struct ubifs_info *c, const void *buf,
 	unsigned int from, to, ffs = chance(1, 2);
 	unsigned char *p = (void *)buf;
 
-	from = prandom_u32() % len;
+	from = prandom_u32_max(len);
 	/* Corruption span max to end of write unit */
 	to = min(len, ALIGN(from + 1, c->max_write_size));
 
diff --git a/fs/ubifs/lpt_commit.c b/fs/ubifs/lpt_commit.c
index d76a19e460cd..cfbc31f709f4 100644
--- a/fs/ubifs/lpt_commit.c
+++ b/fs/ubifs/lpt_commit.c
@@ -1970,28 +1970,28 @@ static int dbg_populate_lsave(struct ubifs_info *c)
 
 	if (!dbg_is_chk_gen(c))
 		return 0;
-	if (prandom_u32() & 3)
+	if (prandom_u32_max(4))
 		return 0;
 
 	for (i = 0; i < c->lsave_cnt; i++)
 		c->lsave[i] = c->main_first;
 
 	list_for_each_entry(lprops, &c->empty_list, list)
-		c->lsave[prandom_u32() % c->lsave_cnt] = lprops->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = lprops->lnum;
 	list_for_each_entry(lprops, &c->freeable_list, list)
-		c->lsave[prandom_u32() % c->lsave_cnt] = lprops->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = lprops->lnum;
 	list_for_each_entry(lprops, &c->frdi_idx_list, list)
-		c->lsave[prandom_u32() % c->lsave_cnt] = lprops->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = lprops->lnum;
 
 	heap = &c->lpt_heap[LPROPS_DIRTY_IDX - 1];
 	for (i = 0; i < heap->cnt; i++)
-		c->lsave[prandom_u32() % c->lsave_cnt] = heap->arr[i]->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = heap->arr[i]->lnum;
 	heap = &c->lpt_heap[LPROPS_DIRTY - 1];
 	for (i = 0; i < heap->cnt; i++)
-		c->lsave[prandom_u32() % c->lsave_cnt] = heap->arr[i]->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = heap->arr[i]->lnum;
 	heap = &c->lpt_heap[LPROPS_FREE - 1];
 	for (i = 0; i < heap->cnt; i++)
-		c->lsave[prandom_u32() % c->lsave_cnt] = heap->arr[i]->lnum;
+		c->lsave[prandom_u32_max(c->lsave_cnt)] = heap->arr[i]->lnum;
 
 	return 1;
 }
diff --git a/fs/ubifs/tnc_commit.c b/fs/ubifs/tnc_commit.c
index 58c92c96ecef..01362ad5f804 100644
--- a/fs/ubifs/tnc_commit.c
+++ b/fs/ubifs/tnc_commit.c
@@ -700,7 +700,7 @@ static int alloc_idx_lebs(struct ubifs_info *c, int cnt)
 		c->ilebs[c->ileb_cnt++] = lnum;
 		dbg_cmt("LEB %d", lnum);
 	}
-	if (dbg_is_chk_index(c) && !(prandom_u32() & 7))
+	if (dbg_is_chk_index(c) && !prandom_u32_max(8))
 		return -ENOSPC;
 	return 0;
 }