linux-kernel - Re: [PATCH net] rds: Fix endian annotations across various assignments

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Message-ID: <20250810182752.GM222315@ZenIV>
Date: Sun, 10 Aug 2025 19:27:52 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Ujwal Kundur <ujwal.kundur@...il.com>, g@...iv.linux.org.uk
Cc: allison.henderson@...cle.com, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org,
	netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
	rds-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] rds: Fix endian annotations across various
 assignments

On Sun, Aug 10, 2025 at 07:25:06PM +0100, Al Viro wrote:
> On Sun, Aug 10, 2025 at 06:47:05PM +0100, Al Viro wrote:
> 
> > >  		switch (type) {
> > >  		case RDS_EXTHDR_NPATHS:
> > >  			conn->c_npaths = min_t(int, RDS_MPATH_WORKERS,
> > > -					       be16_to_cpu(buffer.rds_npaths));
> > > +					      (__force __u16)buffer.rds_npaths);
> > 
> > No.  It will break on little-endian.  That's over-the-wire data you are
> > dealing with; it's *NOT* going to be host-endian.  Fix the buggered
> > annotations instead.
> 
> PS: be16_to_cpu() is not the same thing as a cast.  On a big-endian box,
> a 16bit number 1000 (0x3e8) is stored as {3, 0xe8}; on a little-endian it's
> {3, 0xe8} instead; {0xe8, 3} means 59395 there (0xe8 * 256 + 3).
  ^^^^^^^^           ^^^^^^^^^
  {0xe8, 3}          {3, 0xe8}

Sorry, buggered editing.