Re: [PATCH v2] tcp.c: leverage MSG_PEEK with offset kernel capability when available

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jan 19, 2024 at 11:45:05AM +0100, Stefano Brivio wrote:
> On Fri, 19 Jan 2024 11:05:02 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, Jan 18, 2024 at 05:23:26PM +0100, Stefano Brivio wrote:
> > > Not a full review, but a couple of comments, mostly about stuff I also
> > > had in pkt_selfie.c (review of v1):
> > > 
> > > On Thu, 18 Jan 2024 14:05:38 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Sun, Jan 14, 2024 at 01:07:55PM -0500, Jon Maloy wrote:  
> > > > >
> > > > > [...]
> > > > >
> > > > > +
> > > > > +	s[0] = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
> > > > > +	s[1] = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, IPPROTO_TCP);
> > > > > +	if (s[0] < 0 || s[1] < 0) {
> > > > > +		perror("Temporary probe socket creation failed\n");
> > > > > +		goto out;
> > > > > +	}
> > > > > +	if (0 > bind(s[0], &a, sizeof(a))) {    
> > > > 
> > > > Since the socket address is unspecified, why do you need to bind at
> > > > all?  It might be clearer to explicitly set a to localhost + a
> > > > specific port - because you're in a temporary namespace, you can rely
> > > > on every port being available.  
> > > 
> > > There are two advantages of bind() without port, and then getsockname():
> > > first, ip_unprivileged_port_start might have whatever value in our new
> > > namespace (we don't touch it), and I wouldn't take for granted we'll
> > > have CAP_SYS_ADMIN in it for all the possible start-up combinations.
> > > 
> > > Second, there's no need for a magic value.  
> > 
> > Good point.  Note that at present we're not bind()ing to an address
> > either.
> > 
> > > > > +		perror("Temporary probe socket bind() failed\n");
> > > > > +		goto out;
> > > > > +	}
> > > > > +	if (0 > getsockname(s[0], &a, &((socklen_t) { sizeof(a) }))) {
> > > > > +		perror("Temporary probe socket getsockname() failed\n");
> > > > > +		goto out;
> > > > > +	}
> > > > > +	if (0 > listen(s[0], 0)) {
> > > > > +		perror("Temporary probe socket listen() failed\n");
> > > > > +		goto out;
> > > > > +	}
> > > > > +	if (0 <= connect(s[1], &a, sizeof(a)) || errno != EINPROGRESS) {
> > > > > +		perror("Temporary probe socket connect() failed\n");
> > > > > +		goto out;
> > > > > +	}    
> > > > 
> > > > This is assuming that a will now contain the correct address to
> > > > connect to.  Although it will have the right port, I think the address
> > > > may still be unspecified for the listening socket.  
> > > 
> > > Hmm, why? From getsockname(2):
> > > 
> > >        getsockname()  returns  the  current address to which the socket
> > >        sockfd is bound [...]  
> > 
> > But we've only bound ourselves to 0.0.0.0, which while perfectly
> > cromulent for a listening socket, is no good for connect().
> 
> Hah, "cromulent" just embiggened my dictionary! Why not, though? From
> RFC 6890, 2.2.2:
> 
>               +----------------------+----------------------------+
>               | Attribute            | Value                      |
>               +----------------------+----------------------------+
>               | Address Block        | 0.0.0.0/8                  |
>               | Name                 | "This host on this network"|

Huh.  I never realised you could use 0.0.0.0 like that.  I guess that
makes it work, though I still feel like explicitly using localhost
would be clearer.

> and:
> 
>   $ strace -e connect ./pkt_selfie 
>   --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=891325, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
>   connect(5, {sa_family=AF_INET, sin_port=htons(51155), sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EINPROGRESS (Operation now in progress)
>   MSG_PEEK with offset not supported
>   +++ exited with 0 +++
> 
> with pkt_selfie.c from review of v1:
> 
>   https://archives.passt.top/passt-dev/20231206160808.3d312733@elisabeth/
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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