netdev - Re: [PATCH net] lib: packing: fix shift wrapping in bit

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Message-ID: <20221207122254.otq7biekqz2nzhgl@skbuf>
Date:   Wed, 7 Dec 2022 14:22:54 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Dan Carpenter <error27@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] lib: packing: fix shift wrapping in bit_reverse()

On Wed, Dec 07, 2022 at 03:21:04PM +0300, Dan Carpenter wrote:
> On Wed, Dec 07, 2022 at 02:19:36PM +0200, Vladimir Oltean wrote:
> > On Wed, Dec 07, 2022 at 02:23:28PM +0300, Dan Carpenter 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.
> > > 
> > > Fixes: 554aae35007e ("lib: Add support for generic packing operations")
> > > Signed-off-by: Dan Carpenter <error27@...il.com>
> > > ---
> > >  lib/packing.c | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/lib/packing.c b/lib/packing.c
> > > index 9a72f4bbf0e2..9d7418052f5a 100644
> > > --- a/lib/packing.c
> > > +++ b/lib/packing.c
> > > @@ -32,12 +32,11 @@ static int get_reverse_lsw32_offset(int offset, size_t len)
> > >  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));
> > > +		if (val & BIT_ULL(1))
> > 
> > hmm, why 1 and not i?
> 
> Because I'm a moron.  Let me resend.

Wait a second, I deliberately wrote the code without conditionals.
Let me look at the code disassembly before and after the patch and see
what they look like.