linux-kernel - Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

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Message-Id: <20190205202522.2ab2ea40430e817d4f01cc4e@gmail.com>
Date:   Tue, 5 Feb 2019 20:25:22 -0700
From:   Paul Zimmerman <pauldzim@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        David Miller <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linux-netdev@...r.kernel.org
Subject: Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

On Tue, 05 Feb 2019 18:52:18 -0800, Joe Perches <joe@...ches.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