[05/24] treewide: use prandom_u32_max() when possible, part 1

Commit Message

Darrick J. Wong Nov. 9, 2022, 2:06 a.m. UTC

From: Jason A. Donenfeld <[e-mail]>

Source kernel commit: 81895a65ec63ee1daec3255dc1a06675d2fbe915

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 <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Yury Norov <[e-mail]>
Reviewed-by: KP Singh <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Reviewed-by: Christoph Böhmwalder <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>
Acked-by: Ulf Hansson <[e-mail]>
Acked-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>
---
 libxfs/xfs_alloc.c  |    2 +-
 libxfs/xfs_ialloc.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

Patch

diff --git a/libxfs/xfs_alloc.c b/libxfs/xfs_alloc.c
index 271c48dd83..3e310ce3e5 100644
--- a/libxfs/xfs_alloc.c
+++ b/libxfs/xfs_alloc.c
@@ -1516,7 +1516,7 @@  xfs_alloc_ag_vextent_lastblock(
 
 #ifdef DEBUG
 	/* Randomly don't execute the first algorithm. */
-	if (prandom_u32() & 1)
+	if (prandom_u32_max(2))
 		return 0;
 #endif
 
diff --git a/libxfs/xfs_ialloc.c b/libxfs/xfs_ialloc.c
index f27de1fdd0..18f3dea5f5 100644
--- a/libxfs/xfs_ialloc.c
+++ b/libxfs/xfs_ialloc.c
@@ -631,7 +631,7 @@  xfs_ialloc_ag_alloc(
 	/* randomly do sparse inode allocations */
 	if (xfs_has_sparseinodes(tp->t_mountp) &&
 	    igeo->ialloc_min_blks < igeo->ialloc_blks)
-		do_sparse = prandom_u32() & 1;
+		do_sparse = prandom_u32_max(2);
 #endif
 
 	/*