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

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Message-ID: <CAM0EoM=fZVr4ROKZ+tA9A=yxcx6LnNVFzTb+_brFv9c-CiRfdA@mail.gmail.com>
Date:   Mon, 21 Aug 2023 10:35:29 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        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 Fri, Aug 18, 2023 at 10:38 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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

This is a "bug fix" given that the structure had no zero array
construct as was implied by d61491a51f7e . I didnt want to call it out
as a bug fix (for -net) because existing code was not harmful but
allocated extra memory which this patch gives back.
The other instances have a legit need for "flexible array".

cheers,
jamal
> --
> pw-bot: cr