RE: Non-packed structures in IP headers

From: Florian Weimer

Sent: 01 October 2021 21:10

* Carles Cufi:

quoted

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).

	David

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