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Message-ID: <90fd7cd7-72dc-4df6-88ec-fbc8b64735ad@intel.com>
Date: Wed, 8 May 2024 11:10:46 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Eric Dumazet <edumazet@...gle.com>
CC: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, Kees Cook <keescook@...omium.org>, "Gustavo
A. R. Silva" <gustavoars@...nel.org>, Simon Horman <horms@...nel.org>,
<nex.sw.ncis.osdt.itp.upstreaming@...el.com>,
<linux-hardening@...r.kernel.org>, <netdev@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] netdevice: define and allocate &net_device
_properly_
From: Eric Dumazet <edumazet@...gle.com>
Date: Tue, 7 May 2024 20:21:58 +0200
> On Tue, May 7, 2024 at 2:40 PM Alexander Lobakin
> <aleksander.lobakin@...el.com> wrote:
>>
>> In fact, this structure contains a flexible array at the end, but
>> historically its size, alignment etc., is calculated manually.
>> There are several instances of the structure embedded into other
>> structures, but also there's ongoing effort to remove them and we
>> could in the meantime declare &net_device properly.
>> Declare the array explicitly, use struct_size() and store the array
>> size inside the structure, so that __counted_by() can be applied.
>> Don't use PTR_ALIGN(), as SLUB itself tries its best to ensure the
>> allocated buffer is aligned to what the user expects.
>> Also, change its alignment from %NETDEV_ALIGN to the cacheline size
>> as per several suggestions on the netdev ML.
>>
>> bloat-o-meter for vmlinux:
>>
>> free_netdev 445 440 -5
>> netdev_freemem 24 - -24
>> alloc_netdev_mqs 1481 1450 -31
>>
>> On x86_64 with several NICs of different vendors, I was never able to
>> get a &net_device pointer not aligned to the cacheline size after the
>> change.
>>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> ---
>
> ...
>
>> - p = kvzalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
>> - if (!p)
>> + sizeof_priv = ALIGN(sizeof_priv, SMP_CACHE_BYTES);
>
> If we have a __counted_by(priv_len), why do you ALIGN(sizeof_priv,
> SMP_CACHE_BYTES) ?
To have the whole block of &net_device + private part aligned to a
cacheline size.
>
> If a driver pretends its private part is 4 bytes, we should get a
> warning if 20 bytes are used instead.
Ah okay, so this should be
p = kvzalloc(struct_size(ALIGN(sizeof_priv, SMP_CACHE_BYTES)));
p->priv_len = sizeof_priv;
>
> You added two ____cacheline_aligned already in net_device already.
The whole size passed to kvzalloc() must be cacheline-aligned, otherwise
the MM layer can miscalculate the pointer alignment.
Thanks,
Olek
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