Re: [PATCH] net/mlx5e: Avoid field-overflowing memcpy()

[Saeed Mahameed <saeed@xxxxxxxxxx>]

On Fri, 2021-08-06 at 15:45 -0700, Kees Cook wrote:
> On Fri, Aug 06, 2021 at 03:17:56PM -0700, Saeed Mahameed wrote:
> > On Fri, 2021-08-06 at 14:50 -0700, Kees Cook wrote:
> > > [...]
> > > So, split the memcpy() so the compiler can reason about the buffer
> > > sizes.
> > > 
> > > "pahole" shows no size nor member offset changes to struct > >
> > > mlx5e_tx_wqe
> > > nor struct mlx5e_umr_wqe. "objdump -d" shows no meaningful object
> > > code changes (i.e. only source line number induced differences and
> > > optimizations).
> > 
> > spiting the memcpy doesn't induce any performance degradation ? extra
> > instruction to copy the 1st 2 bytes ? 
> 
> Not meaningfully, but strictly speaking, yes, it's a different series
> of
> instructions.
> 
> > [...]
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > 
> > why only here ? mlx5 has at least 3 other places where we use this
> > unbound memcpy .. 
> 
> Can you point them out? I've been fixing only the ones I've been able
> to
> find through instrumentation (generally speaking, those with fixed
> sizes).
> 

we will need to examine each change carefully to look for performance
degradation and maybe run some micro-benchmark tests in house before i
can ack this patch. 

$ git grep -n "eseg->inline_hdr.start"
drivers/infiniband/hw/mlx5/wr.c:129:            copysz = min_t(u64,
*cur_edge - (void *)eseg->inline_hdr.start,
drivers/infiniband/hw/mlx5/wr.c:131:            memcpy(eseg-
>inline_hdr.start, pdata, copysz);
drivers/infiniband/hw/mlx5/wr.c:133:                          
sizeof(eseg->inline_hdr.start) + copysz, 16);
drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c:344:          
memcpy(eseg->inline_hdr.start, xdptxd->data, MLX5E_XDP_MIN_INLINE);
drivers/net/ethernet/mellanox/mlx5/core/en_tx.c:510:                  
mlx5e_insert_vlan(eseg->inline_hdr.start, skb, attr->ihs);
drivers/net/ethernet/mellanox/mlx5/core/en_tx.c:514:                  
memcpy(eseg->inline_hdr.start, skb->data, attr->ihs);
drivers/net/ethernet/mellanox/mlx5/core/en_tx.c:1033:          
memcpy(eseg->inline_hdr.start, skb->data, attr.ihs);