linux-kernel - Re: [PATCH][next] net: sched: cls_u32: Fix allocation in u32

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Message-ID: <20230818193810.102a2581@kernel.org>
Date:   Fri, 18 Aug 2023 19:38:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] net: sched: cls_u32: Fix allocation in u32_init()

On Thu, 17 Aug 2023 09:58:53 -0600 Gustavo A. R. Silva wrote:
> Subject: [PATCH][next] net: sched: cls_u32: Fix allocation in u32_init()
> Date: Thu, 17 Aug 2023 09:58:53 -0600
> 
> Replace struct_size() with sizeof(), and avoid allocating 8 too many
> bytes.

What are you fixing?

> The following difference in binary output is expected and reflects the
> desired change:
> 
> | net/sched/cls_u32.o
> | @@ -6148,7 +6148,7 @@
> | include/linux/slab.h:599
> |     2cf5:      mov    0x0(%rip),%rdi        # 2cfc <u32_init+0xfc>
> |                        2cf8: R_X86_64_PC32     kmalloc_caches+0xc
> |-    2cfc:      mov    $0x98,%edx
> |+    2cfc:      mov    $0x90,%edx

Sure, but why are you doing this? And how do you know the change is
correct?

There are 2 other instances where we allocate 1 entry or +1 entry.
Are they not all wrong?

Also some walking code seems to walk <= divisor, divisor IIUC being
the array bound - 1?

Jamal acked so changes are this is right, but I'd really like to
understand what's going on, and I shouldn't have to ask you all 
these questions :S
-- 
pw-bot: cr