linux-kernel - Re: [PATCH] security: Use || instead of

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Date:	Fri, 3 Jun 2016 00:53:07 +0800
From:	Rui Teng <rui.teng@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	serge.hallyn@...onical.com, james.l.morris@...cle.com,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] security: Use || instead of | for boolean expressions

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@...ux.vnet.ibm.com>
>> ---
>>  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
>