netdev - RE: bpf pointer alignment validation

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Date:   Wed, 10 May 2017 11:12:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexei Starovoitov' <alexei.starovoitov@...il.com>,
        David Miller <davem@...emloft.net>
CC:     "daniel@...earbox.net" <daniel@...earbox.net>,
        "ast@...com" <ast@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: bpf pointer alignment validation

From: Alexei Starovoitov
> Sent: 10 May 2017 06:58
> > +static u32 calc_align(u32 imm)
> > +{
> > +	u32 align = 1;
> > +
> > +	if (!imm)
> > +		return 1U << 31;
> > +
> > +	while (!(imm & 1)) {
> > +		imm >>= 1;
> > +		align <<= 1;
> > +	}
> > +	return align;
> > +}
> 
> same question as in previous reply.
> Why not to use something like:
> static u32 calc_align(u32 n)
> {
>         if (!n)
>                 return 1U << 31;
>         return n - ((n - 1) & n);
> }

That function needs a comment saying what it returns.
Think I'd write it as:
		return n & ~(n & (n - 1));
(even though that might be one more instruction)

	David