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Date: Thu, 3 Aug 2023 07:40:40 +0200
From: Stefano Brivio <sbrivio@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH 01/17] netlink: Split up functionality if nl_link()
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On Thu, 3 Aug 2023 14:29:28 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:
> On Thu, Aug 03, 2023 at 12:09:16PM +1000, David Gibson wrote:
> > On Thu, Aug 03, 2023 at 12:47:29AM +0200, Stefano Brivio wrote:
> [snip]
> > > > -void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu)
> > > > +void nl_link_get_mac(int ns, unsigned int ifi, void *mac)
> > > > {
> > > > - int change = !MAC_IS_ZERO(mac) || up || mtu;
> > > > struct req_t {
> > > > struct nlmsghdr nlh;
> > > > struct ifinfomsg ifm;
> > > > - struct rtattr rta;
> > > > - union {
> > > > - unsigned char mac[ETH_ALEN];
> > > > - struct {
> > > > - unsigned int mtu;
> > > > - } mtu;
> > > > - } set;
> > > > } req = {
> > > > - .nlh.nlmsg_type = change ? RTM_NEWLINK : RTM_GETLINK,
> > > > - .nlh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
> > > > - .nlh.nlmsg_flags = NLM_F_REQUEST | (change ? NLM_F_ACK : 0),
> > > > + .nlh.nlmsg_type = RTM_GETLINK,
> > > > + .nlh.nlmsg_len = sizeof(req),
> > >
> > > I don't think there's a practical issue with this, but there were two
> > > reasons why I used NLMSG_LENGTH(sizeof(struct ifinfomsg)) instead:
> > >
> > > - NLMSG_LENGTH() aligns to 4 bytes, not to whatever
> > > architecture-dependent alignment we might have: the message might
> > > actually be smaller
> >
> > Oof... so. On the one hand, I see the issue; if these are different,
> > I'm not sure what the effect will be. On the other hand, if we use
> > NLMSG_LENGTH and it *is* longer than the structure size, we'll be
> > saying that this message is longer than the datagram containing it.
> > I'm not sure what the effect of that will be either.
>
> Duh, sorry, I realized I had this backwards. NLSMSG_LENGTH() is the
> non-aligned length, sizeof() may include alignment. I'll rework based
> on that understanding.
Right, I was about to write you that... or rather, NLMSG_LENGTH() is
the (presumably) lesser-aligned length. Also, if you check pretty much
any example in iproute2, nlmsg_len is always set like that, using
NLMSG_LENGTH() on the payload.
> > Not really sure what to do about this.
> >
> > > - I see that this works with gcc and clang, but, strictly
> > > speaking, is the size of the struct known "before"
> > > (sequence-point-wise) we're done initialising it? I have a very vague
> > > memory of this not working with gcc 2.9 or suchlike -- which is not a
> > > problem, as long as our new friend C11 actually supports this (but
> > > I'm not entirely sure).
> >
> > I'm pretty sure it's ok, regardless of C11 state. It's not really a
> > question of sequence points: those are about the ordering of run time
> > operations. Even though the structure is being defined inline,
> > determining it's size and layout will still happen at compile time,
> > whereas the initialization is obviously a runtime event.
Ah, sorry, yes, of course. Still I remember that failing spectacularly
in a distant past. But you just checked with gcc 4-ish I guess, so I
guess it would have been fine anyway.
--
Stefano