linux-kernel - Re: [PATCH linux-next] tools/tracing: fix application of sizeof to pointer

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Date:   Fri, 20 Aug 2021 09:00:09 +0000
From:   <Viktor.Rosendahl@....de>
To:     <jing.yangyang@....com.cn>, <rostedt@...dmis.org>,
        <cgel.zte@...il.com>
CC:     <vulab@...as.ac.cn>, <colin.king@...onical.com>,
        <linux-kernel@...r.kernel.org>, <zealci@....com.cn>
Subject: Re: [PATCH linux-next] tools/tracing: fix application of sizeof to
 pointer

Hi Yangyang,

On Thu, 2021-08-19 at 23:00 -0700, CGEL wrote:
> From: jing yangyang <jing.yangyang@....com.cn>
> 
> sizeof when applied to a pointer typed expression gives the size of
> the pointer.
> 
> ./tools/tracing/latency/latency-collector.c:1541:10-16:ERROR application of
> sizeof to pointer
> 
> This issue was detected with the help of Coccinelle.
> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: jing yangyang <jing.yangyang@....com.cn>
> ---
>  tools/tracing/latency/latency-collector.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/tracing/latency/latency-collector.c
> b/tools/tracing/latency/latency-collector.c
> index 3a2e6bb..64d531d 100644
> --- a/tools/tracing/latency/latency-collector.c
> +++ b/tools/tracing/latency/latency-collector.c
> @@ -1538,7 +1538,7 @@ static void tracing_loop(void)
>  				mutex_lock(&print_mtx);
>  				check_signals();
>  				write_or_die(fd_stdout, queue_full_warning,
> -					     sizeof(queue_full_warning));
> +					     sizeof(*queue_full_warning));

The old code would give a size of 8, i.e. the size of the pointer. Your
suggestion will give a size of 1, i.e. the size of the first character of the
error message. So instead of ouputing "Could no" we would only write out "C".

What we want is the length of the error message. This could be achieved in two
ways:

1. By changing the sizeof(queue_full_warning) to strlen(queue_full_warning).

2. By changing the definition of queue_full_warning to be an array, in that case
we would like to use sizeof(queue_full_warning) - 1, the "- 1" comes from the
fact that we don't want to write out the terminating null character.

I think the first approach with strlen() is the better solution because it's
shorter and modern compilers will do the strlen() calculation of constant
strings at compile time anyway.

best regards,

Viktor 

>  				mutex_unlock(&print_mtx);
>  			}
>  			modified--;