linux-kernel - Re: [PATCH][next] trace: osnoise: Fix u64 less than zero comparison

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Message-ID: <c74e711e-71c9-df9c-8406-b9e92ef12da0@redhat.com>
Date:   Tue, 29 Jun 2021 19:19:25 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Colin King <colin.king@...onical.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH][next] trace: osnoise: Fix u64 less than zero comparison

On 6/29/21 6:52 PM, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> The less than zero comparison of the u64 variable 'noise' is always
> false because the variable is unsigned. Since the time_sub macro
> can potentially return an -ve vale, make the variable a s64 to
> fix the issue.

Ops! concurrent bug fixing.

Dan Carpenter reported the same bug (and another problem), and I was working in
the patches... I saw yours after sending his ones:

https://lore.kernel.org/lkml/acd7cd6e7d56b798a298c3bc8139a390b3c4ab52.1624986368.git.bristot@redhat.com/

The patches do the same fix, but there it also:

 - Made also max_noise s64 (it is snapshot of noise).
 - Arranged the declarations in the inverted christmas tree.

> Addresses-Coverity: ("Unsigned compared against 0")
> Fixes: bce29ac9ce0b ("trace: Add osnoise tracer")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>

Steven, can we merge the flags?

-- Daniel

> ---
>  kernel/trace/trace_osnoise.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> index 38aa5e208ffd..02c984560ceb 100644
> --- a/kernel/trace/trace_osnoise.c
> +++ b/kernel/trace/trace_osnoise.c
> @@ -1040,11 +1040,11 @@ static void osnoise_stop_tracing(void)
>  static int run_osnoise(void)
>  {
>  	struct osnoise_variables *osn_var = this_cpu_osn_var();
> -	u64 noise = 0, sum_noise = 0, max_noise = 0;
> +	u64 sum_noise = 0, max_noise = 0;
>  	struct trace_array *tr = osnoise_trace;
>  	u64 start, sample, last_sample;
>  	u64 last_int_count, int_count;
> -	s64 total, last_total = 0;
> +	s64 noise = 0, total, last_total = 0;
>  	struct osnoise_sample s;
>  	unsigned int threshold;
>  	int hw_count = 0;
>