Re: [PATCH] security: Use || instead of | for boolean expressions

[Rui Teng]

On 6/2/16 10:13 PM, Serge E. Hallyn wrote:
> On Wed, Jun 01, 2016 at 02:03:02PM +0800, Rui Teng wrote:
> > Sparse spits out the following warning:
> > 	security/commoncap.c:989:41: warning: dubious: !x | y
> >
> > Bitwise and logical are equivalent here, but logical was intended.
> > Replacing the bit-wise '|' with the boolean '||' silences the sparse warning.
>
> Hi,
>
> this looks ok, but I'm worried by
>
> > The generated code for both cases is the same.
>
> That cannot be.  The logical result should be the same, but the
> generated code cannot be.

Thanks for cc:ing the author.

I tried to write a sample code to verify it before. Both || and | will
generate the same assembly code.

For example, compiling following code with "gcc -O2 -S main.c", and
replacing || with | can generate the same assembly code.

- main.c ------------

int parse(int a, int b, int c)
{
        if (a || b || c)
                return 1;
        else
                return 0;
}

Of cause, it is only a sample on x86, but even if the generated code is
not the same, the logical will be better than bitwise.
Because (a || b || c) means (a != 0 || b != 0 || c != 0), once a != 0,
the whole expression will be true(short-circuit evaluation).
and (a | b | c) means calculate the bitwise first and check the result
in the end. And since the args are all integer, there is no need to
avoid any short-circuit.


> I'm cc:ing Andy as this code came in with his patch.  Is there an
> actual reason for having used bitwise here?
>
> thanks,
> -serge
>
> > Signed-off-by: Rui Teng <rui.teng@xxxxxxxxxxxxxxxxxx>
> > ---
> >  security/commoncap.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index e7fadde..8f6fb24 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -976,7 +976,7 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> >
> >  	case PR_CAP_AMBIENT:
> >  		if (arg2 == PR_CAP_AMBIENT_CLEAR_ALL) {
> > -			if (arg3 | arg4 | arg5)
> > +			if (arg3 || arg4 || arg5)
> >  				return -EINVAL;
> >
> >  			new = prepare_creds();
> > @@ -986,7 +986,7 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> >  			return commit_creds(new);
> >  		}
> >
> > -		if (((!cap_valid(arg3)) | arg4 | arg5))
> > +		if (((!cap_valid(arg3)) || arg4 || arg5))
> >  			return -EINVAL;
> >
> >  		if (arg2 == PR_CAP_AMBIENT_IS_SET) {
> > --
> > 2.7.4