Subject: Re: [PATCH v2 56/63] RDMA/mlx5: Use struct_group() to zero struct mlx5_ib_mr

From: Kees Cook <[email-masked]>
Date: Thu, 19 Aug 2021 09:19:08 -0700

On Thu, Aug 19, 2021 at 09:27:16AM -0300, Jason Gunthorpe wrote:
> On Tue, Aug 17, 2021 at 11:05:26PM -0700, Kees Cook wrote:
> > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > field bounds checking for memset(), avoid intentionally writing across
> > neighboring fields.
> > 
> > Add struct_group() to mark region of struct mlx5_ib_mr that should be
> > initialized to zero.
> > 
> > Cc: Leon Romanovsky <[email-masked]>
> > Cc: Doug Ledford <[email-masked]>
> > Cc: Jason Gunthorpe <[email-masked]>
> > Cc: [email-masked]
> > Signed-off-by: Kees Cook <[email-masked]>
> > ---
> >  drivers/infiniband/hw/mlx5/mlx5_ib.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/infiniband/hw/mlx5/mlx5_ib.h b/drivers/infiniband/hw/mlx5/mlx5_ib.h
> > index bf20a388eabe..f63bf204a7a1 100644
> > --- a/drivers/infiniband/hw/mlx5/mlx5_ib.h
> > +++ b/drivers/infiniband/hw/mlx5/mlx5_ib.h
> > @@ -644,6 +644,7 @@ struct mlx5_ib_mr {
> >  	struct ib_umem *umem;
> >  
> >  	/* This is zero'd when the MR is allocated */
> > +	struct_group(cleared,
> >  	union {
> >  		/* Used only while the MR is in the cache */
> >  		struct {
> > @@ -691,12 +692,13 @@ struct mlx5_ib_mr {
> >  			bool is_odp_implicit;
> >  		};
> >  	};
> > +	);
> >  };
> >  
> >  /* Zero the fields in the mr that are variant depending on usage */
> >  static inline void mlx5_clear_mr(struct mlx5_ib_mr *mr)
> >  {
> > -	memset(mr->out, 0, sizeof(*mr) - offsetof(struct mlx5_ib_mr, out));
> > +	memset(&mr->cleared, 0, sizeof(mr->cleared));
> >  }
> 
> Why not use the memset_after(mr->umem) here?

I can certainly do that instead. In this series I've tended to opt
for groupings so the position of future struct member additions are
explicitly chosen. (i.e. reducing the chance that a zeroing of the new
member be a surprise.)

-Kees

-- 
Kees Cook