linux-kernel - Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number

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Date:	Tue, 15 Dec 2009 11:31:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Michael Neuling <mikey@...ling.org>
Subject: Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number

On Tue, 2009-12-15 at 19:40 +1100, Paul Mackerras wrote:
> It is quite legitimate for CPUs to be numbered sparsely, meaning that
> it possible for an online CPU to have a number which is greater than
> the total count of possible CPUs.
> 
> Currently find_get_context() has a sanity check on the cpu number
> where it checks it against num_possible_cpus().  This test can fail
> for a legitimate cpu number if the cpu_possible_mask is sparsely
> populated.
> 
> This fixes the problem by checking the CPU number against
> nr_cpumask_bits instead, since that is the appropriate check to ensure
> that the cpu number is same to pass to cpu_isset() subsequently.

Cute, do you actually have hardware that does this?

> Reported-by: Michael Neuling <mikey@...ling.org>
> Tested-by: Michael Neuling <mikey@...ling.org>
> Cc: stable@...nel.org
> Signed-off-by: Paul Mackerras <paulus@...ba.org>
> ---
>  kernel/perf_event.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 6b7ddba..78551b3 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -1604,7 +1604,7 @@ static struct perf_event_context *find_get_context(pid_t pid, int cpu)
>  		if (perf_paranoid_cpu() && !capable(CAP_SYS_ADMIN))
>  			return ERR_PTR(-EACCES);
>  
> -		if (cpu < 0 || cpu > num_possible_cpus())
> +		if (cpu < 0 || cpu >= nr_cpumask_bits)
>  			return ERR_PTR(-EINVAL);
>  
>  		/*

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