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From: "Cufi, Carles" <>
To: Florian Weimer <>
Cc: "" <>,
	"" <>,
	"" <>,
	"Lubos, Robert" <>,
	"Bursztyka, Tomasz" <>,
Subject: RE: Non-packed structures in IP headers
Date: Mon, 4 Oct 2021 10:30:34 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Florian,

Thanks for your response.

> * 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.

That makes sense, but aren't ABI standards different for every architecture? For example, I checked the Arm AAPCS[1] and it states:

"The size of an aggregate shall be the smallest multiple of its alignment that is sufficient to hold all of its

Which, unless I am reading this wrong, means that the compiler would indeed insert padding if the size of the IP headers structs was not a multiple of 4. In this particular case, the struct sizes for the IP headers are 20 and 40 bytes respectively, so there will be no padding inserted. But I only checked a single architecture's ABI (or Procedure Call Standard) documentation, is this true for all archs? 

> 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.

I did see those when browsing the code, thanks for confirming this. It is really padding that I am concerned about, and not alignment.

> Therefore, I do not believe this is an actual problem.

Would the static assert still make sense in order to check this for all architectures?




  parent reply	other threads:[~2021-10-04 10:30 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
2021-10-04 10:41     ` Cufi, Carles
2021-10-04 12:18       ` David Laight
2021-10-04 10:30   ` Cufi, Carles [this message]
2021-10-09  6:56     ` Florian Weimer

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