linux-kernel - Re: [PATCH v2] RISC-V: Fix /proc/cpuinfo cpumask warning

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Date:   Wed, 12 Oct 2022 05:55:29 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH v2] RISC-V: Fix /proc/cpuinfo cpumask warning

On Wed, Oct 12, 2022 at 10:29:49AM +0200, Andrew Jones wrote:
> Commit 78e5a3399421 ("cpumask: fix checking valid cpu range") has
> started issuing warnings[*] when cpu indices equal to nr_cpu_ids - 1
> are passed to cpumask_next* functions. seq_read_iter() and cpuinfo's
> start and next seq operations implement a pattern like
> 
>   n = cpumask_next(n - 1, mask);
>   show(n);
>   while (1) {
>       ++n;
>       n = cpumask_next(n - 1, mask);
>       if (n >= nr_cpu_ids)
>           break;
>       show(n);
>   }

Can you instead of sudo-code print show the real control flow? What
function hosts the infinite loop?

> which will issue the warning when reading /proc/cpuinfo. Ensure no
> warning is generated by validating the cpu index before calling
> cpumask_next().
> 
> [*] Warnings will only appear with DEBUG_PER_CPU_MAPS enabled.
> 
> Signed-off-by: Andrew Jones <ajones@...tanamicro.com>
> Cc: Yury Norov <yury.norov@...il.com>
> ---
> v2:
>   - Got comments on the x86 equivalent patch and made the same
>     changes to this one
>     - Added all the information I should have in the first place
>       to the commit message [Boris]
>     - Changed style of fix [Boris]
> 
> 
>  arch/riscv/kernel/cpu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index 4aa8cd749441..63138b880b92 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -166,6 +166,9 @@ static void print_mmu(struct seq_file *f)
>  
>  static void *c_start(struct seq_file *m, loff_t *pos)
>  {
> +	if (*pos >= nr_cpu_ids)
> +		return NULL;
> +
>  	*pos = cpumask_next(*pos - 1, cpu_online_mask);
>  	if ((*pos) < nr_cpu_ids)
>  		return (void *)(uintptr_t)(1 + *pos);

OK, as far as I understood your explanations, *pos == nr_cpu_ids
is a valid index because it's used as stop-code for traversing.

However, you're completely silencing cpumask_check(), including
those cases where *pos > nr_cpu_ids. I suspect there's no valid
cases for it. If so, the patch should look like:

 +	if (*pos == nr_cpu_ids)
 +		return NULL;
 +

The same for x86 patch. 

If it comes to v3, can you send both as a series?

Thanks,
Yury