| Re: [BRIDGE] Unaligned access on IA64 when comparing ethernet addresses [message #12114 is a reply to message #12112] |
Tue, 17 April 2007 21:24   |
Eric Dumazet
Messages: 36
Registered: July 2006
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Member |
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David Miller a écrit :
> From: Stephen Hemminger <shemminger@linux-foundation.org>
> Date: Tue, 17 Apr 2007 13:37:23 -0700
>
>> The previous patch relied on the bridge id being aligned by
>> the compiler (which happens as a side effect). So please use
>> this instead.
>>
>> compare_ether_addr() implicitly requires that the addresses
>> passed are 2-bytes aligned in memory.
>>
>> This is not true for br_stp_change_bridge_id() and
>> br_stp_recalculate_bridge_id() in which one of the addresses
>> is unsigned char *, and thus may not be 2-bytes aligned.
>>
>> Signed-off-by: Evgeny Kravtsunov <emkravts@openvz.org>
>> Signed-off-by: Kirill Korotaev <dev@openvz.org>
>> Signed-off-by: Pavel Emelianov <xemul@openvz.org>
>> Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
>
> bridge_id would be aligned by luck, because it is composed of char's
> there is no explicit reason it should be aligned on at least an
> unsigned short boundary.
>
> I like the other patch much better, it provided explicit alignment and
> is guarenteed to get rid of the problem.
>
> Indeed, I wrote a test program on 32-bit Sparc to validate this:
>
> struct bridge_id {
> unsigned char a[6];
> unsigned char b[6];
> };
>
> extern void bar(unsigned char *, unsigned char *);
>
> void foo(void)
> {
> unsigned char a;
> struct bridge_id b;
>
> bar(&b.a[0], &a);
> }
>
> foo() gets compiled like this:
>
> foo:
> save %sp, -120, %sp
> add %fp, -21, %o0
> call bar, 0
> add %fp, -9, %o1
> jmp %i7+8
> restore
>
> See? The bridge_id (passed in via %o0) is on an odd byte boundary
> on the stack.
>
> So your patch isn't fixing the bug at all.
>
> I'm going to apply the original patch, because that one will
> actually fix the problem and was actually tested on a system
> that saw the problem.
I suspect you missed part of Stephen patch :
(maybe some mailer problem...)
--- linux-2.6.orig/net/bridge/br_private.h 2007-04-17
13:26:48.000000000 -0700 +++ linux-2.6/net/bridge/br_private.h
2007-04-17 13:30:29.000000000 -0700 @@ -36,7 +36,7 @@
{
unsigned char prio[2];
unsigned char addr[6];
-};
+} __attribute__((aligned(8)));
struct mac_addr
{
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