linux-kernel - Re: [PATCH] test_bpf: Fix a new clang warning about xor-ing two numbers

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Message-ID: <00b21133-196c-f304-14a1-facb24c09103@fb.com>
Date:   Mon, 19 Aug 2019 05:51:09 +0000
From:   Yonghong Song <yhs@...com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] test_bpf: Fix a new clang warning about xor-ing two
 numbers



On 8/18/19 9:34 PM, Nathan Chancellor wrote:
> r369217 in clang added a new warning about potential misuse of the xor
> operator as an exponentiation operator:
> 
> ../lib/test_bpf.c:870:13: warning: result of '10 ^ 300' is 294; did you
> mean '1e300'? [-Wxor-used-as-pow]
>                  { { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
>                         ~~~^~~~~
>                         1e300
> ../lib/test_bpf.c:870:13: note: replace expression with '0xA ^ 300' to
> silence this warning
> ../lib/test_bpf.c:870:31: warning: result of '10 ^ 300' is 294; did you
> mean '1e300'? [-Wxor-used-as-pow]
>                  { { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
>                                           ~~~^~~~~
>                                           1e300
> ../lib/test_bpf.c:870:31: note: replace expression with '0xA ^ 300' to
> silence this warning
> 
> The commit link for this new warning has some good logic behind wanting
> to add it but this instance appears to be a false positive. Adopt its
> suggestion to silence the warning but not change the code. According to
> the differential review link in the clang commit, GCC may eventually
> adopt this warning as well.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/643
> Link: https://github.com/llvm/llvm-project/commit/920890e26812f808a74c60ebc14cc636dac661c1
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Verified that latest trunk clang indeed has this warning, and below 
change indeed fixed the warning in the correct way.

Acked-by: Yonghong Song <yhs@...com>

> ---
> 
> I highly doubt that 1e300 was intented but if it was (or something else
> was), please let me know. Commit history wasn't entirely clear on why
> this expression was used over just a raw number.
> 
>   lib/test_bpf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> index c41705835cba..5ef3eccee27c 100644
> --- a/lib/test_bpf.c
> +++ b/lib/test_bpf.c
> @@ -867,7 +867,7 @@ static struct bpf_test tests[] = {
>   		},
>   		CLASSIC,
>   		{ },
> -		{ { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
> +		{ { 4, 0xA ^ 300 }, { 20, 0xA ^ 300 } },
>   	},
>   	{
>   		"SPILL_FILL",
>