Re: [PATCH] dma: map_benchmark: turn dma_sg_map_param buf into a flexible array

[Rosen Penev]

On Tue, Jun 2, 2026 at 9:52 AM Kees Cook <k...@kernel.org> wrote:
>
> On Mon, May 25, 2026 at 03:06:28PM -0700, Rosen Penev wrote:
> > The buf pointer was kmalloc_array()'d immediately after the parent
> > struct allocation, with the count (granule, validated to 1..1024 by
> > the ioctl) trivially available beforehand.  Move buf to the struct
> > tail as a flexible array member and fold the two allocations into a
> > single kzalloc_flex(), dropping the kfree(params->buf) in both the
> > prepare error path and unprepare.
> >
> > Add __counted_by for extra runtime analysis.
> >
> > Assisted-by: Claude:Opus-4.7
> > Signed-off-by: Rosen Penev <ros...@gmail.com>
> > ---
> >  kernel/dma/map_benchmark.c | 29 +++++++++++++----------------
> >  1 file changed, 13 insertions(+), 16 deletions(-)
> >
> > diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
> > index 29eeb5fdf199..a65da5c7710c 100644
> > --- a/kernel/dma/map_benchmark.c
> > +++ b/kernel/dma/map_benchmark.c
> > @@ -121,35 +121,35 @@ static struct map_benchmark_ops 
> > dma_single_map_benchmark_ops = {
> >  struct dma_sg_map_param {
> >       struct sg_table sgt;
> >       struct device *dev;
> > -     void **buf;
> >       u32 npages;
> >       u32 dma_dir;
> > +     void *buf[] __counted_by(npages);
> >  };
> >
> >  static void *dma_sg_map_benchmark_prepare(struct map_benchmark_data *map)
> >  {
> > +     struct dma_sg_map_param *params;
> >       struct scatterlist *sg;
> > +     u32 npages;
> >       int i;
> >
> > -     struct dma_sg_map_param *params = kzalloc(sizeof(*params), 
> > GFP_KERNEL);
> > -
> > -     if (!params)
> > -             return NULL;
> >       /*
> >        * Set the number of scatterlist entries based on the granule.
> >        * In SG mode, 'granule' represents the number of scatterlist entries.
> >        * Each scatterlist entry corresponds to a single page.
> >        */
> > -     params->npages = map->bparam.granule;
> > +     npages = map->bparam.granule;
> > +
> > +     params = kzalloc_flex(*params, buf, npages);
> > +     if (!params)
> > +             return NULL;
> > +
> > +     params->npages = npages;
> >       params->dma_dir = map->bparam.dma_dir;
> >       params->dev = map->dev;
> > -     params->buf = kmalloc_array(params->npages, sizeof(*params->buf),
> > -                                 GFP_KERNEL);
> > -     if (!params->buf)
> > -             goto out;
> >
> > -     if (sg_alloc_table(&params->sgt, params->npages, GFP_KERNEL))
> > -             goto free_buf;
> > +     if (sg_alloc_table(&params->sgt, npages, GFP_KERNEL))
>
> nit: I think it's better to use the params->npages here just because it
> is obviously tied to params->sgt, and reduces code churn for this patch
> (i.e. that line would be left alone).
Fixed. I only did it to reduce line length.
>
> Otherwise looks good.
>
> -Kees
>
> > +             goto free_params;
> >
> >       for_each_sgtable_sg(&params->sgt, sg, i) {
> >               params->buf[i] = (void *)__get_free_page(GFP_KERNEL);
> > @@ -166,9 +166,7 @@ static void *dma_sg_map_benchmark_prepare(struct 
> > map_benchmark_data *map)
> >               free_page((unsigned long)params->buf[i]);
> >
> >       sg_free_table(&params->sgt);
> > -free_buf:
> > -     kfree(params->buf);
> > -out:
> > +free_params:
> >       kfree(params);
> >       return NULL;
> >  }
> > @@ -183,7 +181,6 @@ static void dma_sg_map_benchmark_unprepare(void *mparam)
> >
> >       sg_free_table(&params->sgt);
> >
> > -     kfree(params->buf);
> >       kfree(params);
> >  }
> >
> > --
> > 2.54.0
> >
>
> --
> Kees Cook