Re: [net-next,v1,1/7] net/rds: new extension header: rdma bytes

[Allison Henderson <allison.henderson@oracle.com>]

On Mon, 2026-01-26 at 17:46 +0000, Simon Horman wrote:
> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
> 
> For local reproduction steps see https://urldefense.com/v3/__https://netdev-ai.bots.linux.dev/ai-local.html__;!!ACWV5N9M2RV99hQ!MPuQd8UZOu7Nec0FmSJuOO_TptptoR__bJuwS83q30PC3MeyF4iUF4CGt46bvEfQomEudN-E9bVMtVO05tU$ 
> 
> 
Ok I've got the reproducer working, I'll post an update when I get it resolved.

Thanks for the reviews!
Allison

> ---
> net/rds: new extension header: rdma bytes
> 
> This patch introduces a new extension header type RDS_EXTHDR_RDMA_BYTES
> to allow RDMA initiators to exchange byte counts with their targets for
> statistics and policy purposes. It also expands rds_message_add_extension()
> to support multiple extensions per message.
> 
> > diff --git a/net/rds/rds.h b/net/rds/rds.h
> > index 8a549fe687ac..cadfd7ec0ba9 100644
> > --- a/net/rds/rds.h
> > +++ b/net/rds/rds.h
> 
> [ ... ]
> 
> > @@ -258,6 +259,19 @@ struct rds_ext_header_rdma_dest {
> >  	__be32			h_rdma_offset;
> >  };
> > 
> > +/*
> > + * This extension header tells the peer about delivered RDMA byte count.
> > + */
> > +#define RDS_EXTHDR_RDMA_BYTES	4
> > +
> > +struct rds_ext_header_rdma_bytes {
> > +	__be32		h_rdma_bytes;	/* byte count */
> > +	u8		h_rflags;	/* direction of RDMA, write or read */
> > +};
> 
> This struct has 5 bytes of actual data but sizeof() will return 8 bytes
> due to alignment padding. The 3 padding bytes after h_rflags are not
> initialized when this struct is used in rds_ib_xmit(). Since
> rds_message_add_extension() copies sizeof(struct) bytes via memcpy(),
> those uninitialized padding bytes get transmitted to the peer.
> 
> Should this struct be marked __packed, or should the declaration in
> rds_ib_xmit() use a zeroing initializer like = {} to avoid leaking
> kernel stack contents over the network?
> 
> > diff --git a/net/rds/ib_send.c b/net/rds/ib_send.c
> > index f9d28ddd168d..8282ff61b0b3 100644
> > --- a/net/rds/ib_send.c
> > +++ b/net/rds/ib_send.c
> > @@ -578,10 +578,27 @@ int rds_ib_xmit(struct rds_connection *conn, struct rds_message *rm,
> >  		 * used by the peer to release use-once RDMA MRs. */
> >  		if (rm->rdma.op_active) {
> >  			struct rds_ext_header_rdma ext_hdr;
> > +			struct rds_ext_header_rdma_bytes rdma_bytes_ext_hdr;
> > 
> >  			ext_hdr.h_rdma_rkey = cpu_to_be32(rm->rdma.op_rkey);
> >  			rds_message_add_extension(&rm->m_inc.i_hdr,
> > -					RDS_EXTHDR_RDMA, &ext_hdr, sizeof(ext_hdr));
> > +						  RDS_EXTHDR_RDMA, &ext_hdr);
> > +
> > +			/* prepare the rdma bytes ext header */
> > +			rdma_bytes_ext_hdr.h_rflags = rm->rdma.op_write ?
> > +				RDS_FLAG_RDMA_WR_BYTES : RDS_FLAG_RDMA_RD_BYTES;
> > +			rdma_bytes_ext_hdr.h_rdma_bytes =
> > +				cpu_to_be32(rm->rdma.op_bytes);
> 
> Here rdma_bytes_ext_hdr is declared on the stack without initialization.
> Only h_rflags and h_rdma_bytes are assigned, leaving the 3 padding bytes
> uninitialized. When rds_message_add_extension() calls memcpy() with
> len = sizeof(struct rds_ext_header_rdma_bytes) = 8, those uninitialized
> bytes are copied into the header and sent to the peer.
> 
> [ ... ]