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

[Kees Cook]

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).

-- 
Kees Cook