linux-kernel - Re: [PATCH 2/2] selftests: timens: exec: use 'labs()' over 'abs()'

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Date:   Fri, 5 Nov 2021 13:35:39 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...nel.org>
Cc:     shuah@...nel.org, nathan@...nel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] selftests: timens: exec: use 'labs()' over 'abs()'

On Fri, Nov 5, 2021 at 9:31 AM Anders Roxell <anders.roxell@...aro.org> wrote:
>
> When building selftests/timens with clang, the compiler warn about the
> function abs() see below:
>
> exec.c:33:8: error: absolute value function 'abs' given an argument of type 'long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]
>                         if (abs(tst.tv_sec - now.tv_sec) > 5)
>                             ^
> exec.c:33:8: note: use function 'labs' instead
>                         if (abs(tst.tv_sec - now.tv_sec) > 5)
>                             ^~~
>                             labs

Careful.

Isn't the tv_sec member of `struct timespec` a `time_t` which is 32b
on 32b hosts and 64b on 64b hosts? If I'm recalling that correctly,
then this patch results in a harmless (though unnecessary) sign
extension for 32b targets. That should be fine, but someone like Arnd
should triple check if my concern is valid or not.

So I'm in favor of this patch (dispatching to abs or labs based on 64b
host) would hurt readability.

>
> The note indicates what to do, Rework to use the function 'labs()'.
>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>  tools/testing/selftests/timens/exec.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/timens/exec.c b/tools/testing/selftests/timens/exec.c
> index e40dc5be2f66..d12ff955de0d 100644
> --- a/tools/testing/selftests/timens/exec.c
> +++ b/tools/testing/selftests/timens/exec.c
> @@ -30,7 +30,7 @@ int main(int argc, char *argv[])
>
>                 for (i = 0; i < 2; i++) {
>                         _gettime(CLOCK_MONOTONIC, &tst, i);
> -                       if (abs(tst.tv_sec - now.tv_sec) > 5)
> +                       if (labs(tst.tv_sec - now.tv_sec) > 5)
>                                 return pr_fail("%ld %ld\n", now.tv_sec, tst.tv_sec);
>                 }
>                 return 0;
> @@ -50,7 +50,7 @@ int main(int argc, char *argv[])
>
>         for (i = 0; i < 2; i++) {
>                 _gettime(CLOCK_MONOTONIC, &tst, i);
> -               if (abs(tst.tv_sec - now.tv_sec) > 5)
> +               if (labs(tst.tv_sec - now.tv_sec) > 5)
>                         return pr_fail("%ld %ld\n",
>                                         now.tv_sec, tst.tv_sec);
>         }
> @@ -70,7 +70,7 @@ int main(int argc, char *argv[])
>                 /* Check that a child process is in the new timens. */
>                 for (i = 0; i < 2; i++) {
>                         _gettime(CLOCK_MONOTONIC, &tst, i);
> -                       if (abs(tst.tv_sec - now.tv_sec - OFFSET) > 5)
> +                       if (labs(tst.tv_sec - now.tv_sec - OFFSET) > 5)
>                                 return pr_fail("%ld %ld\n",
>                                                 now.tv_sec + OFFSET, tst.tv_sec);
>                 }
> --
> 2.33.0
>


-- 
Thanks,
~Nick Desaulniers