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From: David Laight <David.Laight@ACULAB.COM>
To: 'Florian Weimer' <>,
	"Cufi, Carles" <>
Cc: "" <>,
	"" <>,
	"" <>,
	"Lubos, Robert" <>,
	"Bursztyka, Tomasz" <>,
Subject: RE: Non-packed structures in IP headers
Date: Sat, 2 Oct 2021 15:54:54 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Florian Weimer
> Sent: 01 October 2021 21:10
> * Carles Cufi:
> > I was looking through the structures for IPv{4,6} packet headers and
> > noticed that several of those that seem to be used to parse a packet
> > directly from the wire are not declared as packed. This surprised me
> > because, although I did find that provisions are made so that the
> > alignment of the structure, it is still technically possible for the
> > compiler to inject padding bytes inside those structures, since AFAIK
> > the C standard makes no guarantees about padding unless it's
> > instructed to pack the structure.
> The C standards do not make such guarantees, but the platform ABI
> standards describe struct layout and ensure that there is no padding.
> Linux relies on that not just for networking, but also for the userspace
> ABI, support for separately compiled kernel modules, and in other
> places.

In particular structures are used to map hardware register blocks.

> Sometimes there are alignment concerns in the way these structs are
> used, but I believe the kernel generally controls placement of the data
> that is being worked on, so that does not matter, either.
> Therefore, I do not believe this is an actual problem.

And adding __packed forces the compiler to do byte accesses
(with shifts) on cpu that don't support misaligned memory accesses.

So it really is wrong to specify __packed unless the structure
can be unaligned in memory, or has a 'broken' definition
that has fields that aren't 'naturally aligned'.
In the latter case it is enough to mark the field that requires
the padding before it removed as (IIRC) __aligned(1).
The compiler will then remove the padding but still assume the
field is partially aligned - so my do two 32bit access instead
of 8 8bit ones).


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  reply	other threads:[~2021-10-02 15:55 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2021-10-01 20:10 ` Non-packed structures in IP headers Florian Weimer
2021-10-02 15:54   ` David Laight [this message]
2021-10-04 10:41     ` Cufi, Carles
2021-10-04 12:18       ` David Laight
2021-10-04 10:30   ` Cufi, Carles
2021-10-09  6:56     ` Florian Weimer

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