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Date: Wed, 8 Apr 2026 09:56:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Venkat Rao Bagalkote <venkat88@linux.ibm.com>,
	bpf <bpf@vger.kernel.org>, Hari Bathini <hbathini@linux.ibm.com>,
	Abhishek Dubey <adubey@linux.ibm.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Ritesh Harjani <riteshh@linux.ibm.com>
Subject: Re: bpf/selftests: test_access_variable_array breaks due to
 sched_domain::span removal
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References: <3e129be5-d61e-4bc4-b691-8d69e2f58de6@linux.ibm.com>
 <CAADnVQ+wZgucXKf5ZFNeSMcgARQVsQ7knVctf5R2GOGDRSEagA@mail.gmail.com>
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In-Reply-To: <adXe4gNBihTOyIjQ@linux.ibm.com>

On Wed, Apr 08, 2026 at 10:21:46AM +0530, Saket Kumar Bhaskar wrote:

> We do have cpumask as [] array in sched_group:
> 
> struct sched_group {
> 	struct sched_group	*next;			/* Must be a circular list */
> 	atomic_t		ref;
> 
> 	unsigned int		group_weight;
> 	unsigned int		cores;
> 	struct sched_group_capacity *sgc;
> 	int			asym_prefer_cpu;	/* CPU of highest priority in group */
> 	int			flags;
> 
> 	/*
> 	 * The CPUs this group covers.
> 	 *
> 	 * NOTE: this field is variable length. (Allocated dynamically
> 	 * by attaching extra space to the end of the structure,
> 	 * depending on how many CPUs the kernel has booted up with)
> 	 */
> 	unsigned long		cpumask[];
> };

For now. But given how utterly broken flex arrays are, this thing will
be gone the second we run into that same thing again.

> Wouldn't be good if it is used as:
> 
> cpumask0 = sg->cpumask[0];

I'm thinking Alexei's point is that this test should not rely on random
kernel code.