Date: Sat, 4 Aug 2001 12:23:04 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Brian Somers <brian@Awfulhak.org> Cc: Julian Elischer <julian@elischer.org>, Thomas Pornin <Thomas.Pornin@ens.fr>, <freebsd-net@FreeBSD.ORG>, <freebsd-alpha@FreeBSD.ORG>, Brian Somers <brian@freebsd-services.com>, <FreeBSD-gnats-submit@FreeBSD.ORG>, Brian Somers <brian@freebsd-services.com> Subject: Re: kern/27767: (was: PPPoE + Alpha + 32/64 bits) Message-ID: <20010804121134.G16179-100000@besplex.bde.org> In-Reply-To: <200108022338.f72Nc6u09427@hak.lan.Awfulhak.org>
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On Fri, 3 Aug 2001, Brian Somers wrote:
> > [bde wrote]
> > Here I don't see how packing the struct helps on alphas. Doesn't it
> > cause traps by misaligning uniqtag.data.pointer? I think you need
> > something like:
>
> It doesn't cause a trap as the compiler is (now) smart enough to
> handle it properly. Note where we have the alignment problem here:
I guess it knows how to handle this in packed structs and not in many
other cases.
> > struct {
> > struct pppoe_tag hdr;
> > char data[sizeof(void *)];
> > } uniqtag;
> > void *pointer;
> > ...
> > pointer = sp;
> > memcpy(uniqtag.data, &pointer, sizeof(uniqtag.data));
>
> That's probably better. I went with the __attribute((packed)) stuff
> because it was already done that way in the rest of the code.
I think the packed attributes in ng_pppoe.h are no-ops on most machines.
> In this specific case however, it may be best to just send the variable
> plus the alignment padding as the tag:
>
> struct {
> struct pppoe_tag hdr;
> union uniq data;
> } uniqtag;
>
> ......
> uniqtag.hdr.tag_type = PTT_HOST_UNIQ;
> uniqtag.hdr.tag_len = htons((u_int16_t)(sizeof(uniqtag) - sizeof(uniqtag.hdr));
> uniqtag.data.pointer = sp;
You might also have to zero everything to ensure that there is no stack
garbage in the message.
Bruce
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