Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)

[Al Viro <viro@ftp.linux.org.uk>]

On Mon, Dec 17, 2007 at 06:55:57PM +0100, Eric Dumazet wrote:

> long *mid(long *a, long *b)
> {
> 	return ((a - b) / 2 + a);
> }

... is not actually a middle (you'd want b-a, not a-b there), but anyway

> It gave :
> mid:
>         movq    %rdi, %rdx
>         subq    %rsi, %rdx
>         sarq    $3, %rdx
>         movq    %rdx, %rax
>         shrq    $63, %rax
>         addq    %rdx, %rax
>         sarq    %rax
>         leaq    (%rdi,%rax,8), %rax
>         ret
> 
> while 
> 
> long *mid(long *a, long *b)
> {
> 	return ((a - b) / 2u + a);
> }

... undefined behaviour if a < b

> and while :
> 
> long *mid(long *a, long *b)
> {
> 	return (((unsigned long)(a - b)) / 2 + a);
> }

undefined behaviour, again.