[BUG] alignment problem in net/core/flow.c:flow_key_compare

[BUG] alignment problem in net/core/flow.c:flow_key_compare

[Måns Rullgård]

There is a problem with alignment in the flow_key_compare function in
net/core/flow.c.  It takes arguments of type struct flowi * and casts
them to flow_compare_t *, which is 64 bits on 64-bit machines.  It
then proceeds to read and compare 64-bit values from these pointers.
The problem is that struct flowi only requires 32-bit alignment, so
these reads cause numerous unaligned exceptions.  On average, I get
nearly 1000 unaligned exceptions per second.

The solutions I see are either to force the alignment of struct flowi
to 64 bits, or to use 32-bit access in flow_key_compare.

-- 
Måns Rullgård
mru@kth.se

Re: [BUG] alignment problem in net/core/flow.c:flow_key_compare

[Måns Rullgård]

mru@kth.se (Måns Rullgård) writes:

> There is a problem with alignment in the flow_key_compare function in
> net/core/flow.c.  It takes arguments of type struct flowi * and casts
> them to flow_compare_t *, which is 64 bits on 64-bit machines.  It
> then proceeds to read and compare 64-bit values from these pointers.
> The problem is that struct flowi only requires 32-bit alignment, so
> these reads cause numerous unaligned exceptions.  On average, I get
> nearly 1000 unaligned exceptions per second.
>
> The solutions I see are either to force the alignment of struct flowi
> to 64 bits, or to use 32-bit access in flow_key_compare.

I forgot to mention that this is kernel 2.6.4.

-- 
Måns Rullgård
mru@kth.se

Re: [BUG] alignment problem in net/core/flow.c:flow_key_compare

[David S. Miller]

On Thu, 18 Mar 2004 12:25:57 +0100
mru@kth.se (Måns Rullgård) wrote:

> mru@kth.se (Måns Rullgård) writes:
> 
> > The solutions I see are either to force the alignment of struct flowi
> > to 64 bits, or to use 32-bit access in flow_key_compare.
> 
> I forgot to mention that this is kernel 2.6.4.

Yes, just add an alignment attribute of some kind to the struct
is probably the best idea.

Re: [BUG] alignment problem in net/core/flow.c:flow_key_compare

[Måns Rullgård]

"David S. Miller" <davem@redhat.com> writes:

> On Thu, 18 Mar 2004 12:25:57 +0100
> mru@kth.se (Måns Rullgård) wrote:
>
>> mru@kth.se (Måns Rullgård) writes:
>> 
>> > The solutions I see are either to force the alignment of struct flowi
>> > to 64 bits, or to use 32-bit access in flow_key_compare.
>> 
>> I forgot to mention that this is kernel 2.6.4.
>
> Yes, just add an alignment attribute of some kind to the struct
> is probably the best idea.

What's the proper way of doing that in kernel code?  Should I use gcc
__attribute__ directly or is there some macro that's preferred?

-- 
Måns Rullgård
mru@kth.se