Re: [PATCH v1 1/5] treewide: use prandom_u32_max() when possible
From: Kees Cook <hidden>
Date: 2022-10-06 06:28:37
Also in:
dri-devel, linux-f2fs-devel, linux-hams, linux-nvme, lvs-devel
On Wed, Oct 05, 2022 at 11:48:40PM +0200, Jason A. Donenfeld wrote:
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.
Yes please! Since this is a treewide patch, it's helpful for (me at least) doing reviews to detail the mechanism of the transformation. e.g. I imagine this could be done with something like Coccinelle and @no_modulo@ expression E; @@ - (prandom_u32() % (E)) + prandom_u32_max(E)
quoted hunk ↗ jump to hunk
diff --git a/drivers/mtd/ubi/debug.h b/drivers/mtd/ubi/debug.h index 118248a5d7d4..4236c799a47c 100644 --- a/drivers/mtd/ubi/debug.h +++ b/drivers/mtd/ubi/debug.h@@ -73,7 +73,7 @@ static inline int ubi_dbg_is_bgt_disabled(const struct ubi_device *ubi) static inline int ubi_dbg_is_bitflip(const struct ubi_device *ubi) { if (ubi->dbg.emulate_bitflips) - return !(prandom_u32() % 200); + return !(prandom_u32_max(200)); return 0; }
Because some looks automated (why the parens?)
quoted hunk ↗ jump to hunk
@@ -393,14 +387,11 @@ static struct test_driver { static void shuffle_array(int *arr, int n) { - unsigned int rnd; int i, j; for (i = n - 1; i > 0; i--) { - rnd = prandom_u32(); - /* Cut the range. */ - j = rnd % i; + j = prandom_u32_max(i); /* Swap indexes. */ swap(arr[i], arr[j]);
And some by hand. :) Reviewed-by: Kees Cook <redacted> -- Kees Cook