Re: [PATCH net] lib: packing: fix shift wrapping in bit_reverse()

[Uladzislau Koshchanka <koshchanka@xxxxxxxxx>]

On Wed, 7 Dec 2022 at 14:30, Dan Carpenter <error27@xxxxxxxxx> wrote:
>
> The bit_reverse() function is clearly supposed to be able to handle
> 64 bit values, but the types for "(1 << i)" and "bit << (width - i - 1)"
> are not enough to handle more than 32 bits.

It seems from the surrounding code that this function is only called
for width of up to a byte (but correct me if I'm wrong). There are
fast implementations of bit-reverse in include/linux/bitrev.h. It's
better to just remove this function entirely and call bitrev8, which
is just a precalc-table lookup. While at it, also sort includes.

Signed-off-by: Uladzislau Koshchanka <koshchanka@xxxxxxxxx>

 lib/packing.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/lib/packing.c b/lib/packing.c
index 9a72f4bbf0e2..47ea47c1198a 100644
--- a/lib/packing.c
+++ b/lib/packing.c
@@ -2,10 +2,11 @@
 /* Copyright 2016-2018 NXP
  * Copyright (c) 2018-2019, Vladimir Oltean <olteanv@xxxxxxxxx>
  */
-#include <linux/packing.h>
-#include <linux/module.h>
 #include <linux/bitops.h>
+#include <linux/bitrev.h>
 #include <linux/errno.h>
+#include <linux/module.h>
+#include <linux/packing.h>
 #include <linux/types.h>

 static int get_le_offset(int offset)
@@ -29,19 +30,6 @@ static int get_reverse_lsw32_offset(int offset, size_t len)
        return word_index * 4 + offset;
 }

-static u64 bit_reverse(u64 val, unsigned int width)
-{
-       u64 new_val = 0;
-       unsigned int bit;
-       unsigned int i;
-
-       for (i = 0; i < width; i++) {
-               bit = (val & (1 << i)) != 0;
-               new_val |= (bit << (width - i - 1));
-       }
-       return new_val;
-}
-
 static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit,
                                       int *box_end_bit, u8 *box_mask)
 {
@@ -49,7 +37,7 @@ static void adjust_for_msb_right_quirk(u64
*to_write, int *box_start_bit,
        int new_box_start_bit, new_box_end_bit;

        *to_write >>= *box_end_bit;
-       *to_write = bit_reverse(*to_write, box_bit_width);
+       *to_write = bitrev8(*to_write) >> (8 - box_bit_width);
        *to_write <<= *box_end_bit;

        new_box_end_bit   = box_bit_width - *box_start_bit - 1;