From: Paul Zimmerman <pauldzim@gmail.com>
To: Joe Perches <joe@perches.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
Heiner Kallweit <hkallweit1@gmail.com>,
David Miller <davem@davemloft.net>,
linux-kernel@vger.kernel.org, linux-netdev@vger.kernel.org
Subject: Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing
Date: Tue, 5 Feb 2019 20:25:22 -0700 [thread overview]
Message-ID: <20190205202522.2ab2ea40430e817d4f01cc4e@gmail.com> (raw)
In-Reply-To: <9e4332bf41d5d58a4fa6cf138fc9ea3b2f9a488f.camel@perches.com>
On Tue, 05 Feb 2019 18:52:18 -0800, Joe Perches <joe@perches.com> wrote:
> On Tue, 2019-02-05 at 19:27 -0700, Paul Zimmerman wrote:
>> On Tue, 2019-02-05, Joe Perches wrote:
>>> On Tue, 2019-02-05 at 12:04 -0800, Eric Dumazet wrote:
>>> > On 02/05/2019 10:42 AM, Joe Perches wrote:
>>> > > It's declared after a pointer so it is already is 2 byte aligned.
>>> > >
>>> > > A lot of drivers wouldn't work otherwise.
>>> >
>>> > Maybe these drivers are only used on arches where this does not matter.
>>>
>>> Possible.
>>>
>>> I had only grepped through the sources looking for
>>> declarations using:
>>>
>>> $ git grep -B1 '\[ETH_ALEN\];' -- '*.c' | grep -A1 '\*'
>>>
>>> It's quite a few files in net/ too btw.
>>>
>>> I still think adding __align(<even#>) is unnecessary here unless
>>> it follows something like a bool or a u8.
>>
>> Um, guys, this is practically C-101.
>>
>> From C99, 6.7.2.1:
>>
>> > 13/ Within a structure object, the non-bit-field members and the units in
>> > which bit-fields reside have addresses that increase in the order in which
>> > they are declared. A pointer to a structure object, suitably converted,
>> > points to its initial member (or if that member is a bit-field, then to the
>> > unit in which it resides), and vice versa. There may be unnamed padding
>> > within a structure object, but not at its beginning.
>>
>> AFAIK there is no such language in the spec regarding variable layout on
>> the stack. So Joe, you are totally off-base here.
>
> We're not talking about the spec, see the void * arithmetic
> bit, we're talking about what gcc and clang actually do.
Sorry, I see I was a bit unclear. In an earlier message, which I neglected
to quote, you said:
> It's declared after a pointer so it is already is 2 byte aligned.
> A lot of drivers wouldn't work otherwise.
But it's declared after a pointer *on the stack* (local variable), not in
a structure. I was trying to say that there is nothing in the C spec that
says that local variables have any kind of ordering guarantee, unlike struct
members. And I have never seen any kernel code that relies on the ordering
of local variables to work correctly.
I used to work a lot with low-level C/assembly code, and I know for a fact
that GCC does not lay out stack variables in the same order that they are
declared.
-- Paul
next prev parent reply other threads:[~2019-02-06 3:25 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-06 2:27 [PATCH v2 2/2] r8169: Avoid pointer aliasing Paul Zimmerman
2019-02-06 2:52 ` Joe Perches
2019-02-06 3:25 ` Paul Zimmerman [this message]
2019-02-06 3:49 ` Joe Perches
-- strict thread matches above, loose matches on Subject: below --
2019-02-04 16:42 [PATCH v2 1/2] r8169: Load MAC address from device tree if present Thierry Reding
2019-02-04 16:42 ` [PATCH v2 2/2] r8169: Avoid pointer aliasing Thierry Reding
2019-02-04 16:59 ` Andrew Lunn
2019-02-04 18:44 ` Heiner Kallweit
2019-02-05 3:20 ` David Miller
2019-02-05 18:42 ` Joe Perches
2019-02-05 19:14 ` David Miller
2019-02-05 19:19 ` Joe Perches
2019-02-06 7:25 ` Michal Kubecek
2019-02-05 20:04 ` Eric Dumazet
2019-02-05 20:18 ` Joe Perches
2019-02-05 20:23 ` Eric Dumazet
2019-02-05 20:23 ` Heiner Kallweit
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