linux-kernel - Re: [PATCH] time: POSIX CPU timers: Ensure that variable is initialized

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Date:   Sun, 14 Jan 2018 20:37:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Max R. P. Grossmann" <m@....pm>
cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org
Subject: Re: [PATCH] time: POSIX CPU timers: Ensure that variable is
 initialized

On Mon, 8 Jan 2018, Max R. P. Grossmann wrote:

> If cpu_timer_sample_group returns -EINVAL, it will not have written into
> *sample. Checking for cpu_timer_sample_group's return value precludes the
> potential use of an uninitialized value of now in the following block.
> Given an invalid clock_idx, the previous code could otherwise overwrite
> *oldval in an undefined manner. This is now prevented. We also exploit
> short-circuiting of && to sample the timer only if the result will
> actually be used to update *oldval.

Sure the explanation makes sense, though you should have mentioned that
_ALL_ current users call this function with a valid clockid.

I'll amend the changelog.


> Signed-off-by: Max R. P. Grossmann <m@....pm>
> ---
>  kernel/time/posix-cpu-timers.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> index 1f27887aa194..e54638be6e19 100644
> --- a/kernel/time/posix-cpu-timers.c
> +++ b/kernel/time/posix-cpu-timers.c
> @@ -1189,9 +1189,8 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
>  	u64 now;
>  
>  	WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
> -	cpu_timer_sample_group(clock_idx, tsk, &now);
>  
> -	if (oldval) {
> +	if (oldval && cpu_timer_sample_group(clock_idx, tsk, &now) != -EINVAL) {
>  		/*
>  		 * We are setting itimer. The *oldval is absolute and we update
>  		 * it to be relative, *newval argument is relative and we update
> -- 
> 2.15.1
> 
>