linux-kernel - Re: [PATCH bpf-next v3 4/6] mm: introduce BPF kfuncs to access memcg statistics and events

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Message-ID: <CAADnVQJ_WLMRXYV5p4Lk2+nxdC01iAaKQhYecMjx4rXdBeXjNw@mail.gmail.com>
Date: Mon, 22 Dec 2025 16:11:19 -1000
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: bpf <bpf@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>, JP Kobryn <inwardvessel@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Michal Hocko <mhocko@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH bpf-next v3 4/6] mm: introduce BPF kfuncs to access memcg
 statistics and events

On Mon, Dec 22, 2025 at 12:18 PM Roman Gushchin
<roman.gushchin@...ux.dev> wrote:
>
> +       if (idx < 0 || idx >= MEMCG_NR_STAT || !memcg_stat_item_valid(idx))
> +               return (unsigned long)-1;

memcg_stat_item_valid() and memcg_stat_item_valid()
helpers introduced specifically to be used in these kfuncs,
so I feel it's cleaner to do all idx checking within them
instead of splitting the checks like this.
Then it will be easier to see that
memcg_stat_item_valid(idx) access is in bounds when idx < MEMCG_NR_STAT

Also I'd do one check like (u32)idx >= MEMCG_NR_STAT
and drop idx < 0 part. Compiler is probably smart enough to
optimize this way itself, but I'd still do one check.