Re: [PATCH 3/7] tcp_conn: Avoid 7-bit hole in struct tcp_splice_conn

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

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On Wed, Jan 29, 2025 at 08:33:40AM +0100, Stefano Brivio wrote:
> On Wed, 29 Jan 2025 12:02:09 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Jan 28, 2025 at 07:48:33AM +0100, Stefano Brivio wrote:
> > > On Tue, 28 Jan 2025 11:53:09 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Tue, Jan 28, 2025 at 12:15:28AM +0100, Stefano Brivio wrote:  
> > > > > Moving in_epoll out of the common flow data created a 7-bit hole in
> > > > > struct tcp_splice_conn: repack by shrinking @flags by one (otherwise
> > > > > unused) bit.    
> > > > 
> > > > Is this actually necessary for the migration stuff?  Or just a cleanup
> > > > you spotted along the way?  
> > > 
> > > I thought it was helpful to keep the same size on 32-bit, but it looks
> > > like it's not actually needed.
> > > 
> > > Let me drop it from this series as it's just noise and I'm trying to
> > > keep this slim. If we are all happy with it I can apply it. If not I'll
> > > forget about it.  
> > 
> > Eh, I don't care that much either way.
> > 
> > Note, btw, that bit-field packing is another way source and
> > destination could potentially have mismatching data structures.  IIUC
> > bit field packing is described by the ABI and doesn't necessarily
> > match the byte endianness.
> 
> Right, that's actually the reason that brought me to this change: I was
> comparing stuff between x86_64 and armv6l. On the other hand, this part
> of the specific ABI is generally considered stable so I can rely on it.

Uhh.. a specific ABI is stable, yes, but IIUC the whole point of these
endian, word size etc. checks is that you're not counting on it being
an identical ABI at each end.  I'm saying the bit field packing is
another way the ABIs at each end could differ, which is not currently
accounted for.

-- 
David Gibson (he or they)	| 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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