linux-kernel - Re: [PATCH 1/2] drm/imagination: Add and use FLEX_ARRAY

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Message-ID: <8be967cf8bdb9317d6f3bd1f1175b49486c4ccc9.camel@imgtec.com>
Date: Tue, 15 Jul 2025 15:48:33 +0000
From: Alessio Belle <Alessio.Belle@...tec.com>
To: Matt Coster <Matt.Coster@...tec.com>
CC: "tzimmermann@...e.de" <tzimmermann@...e.de>,
        "simona@...ll.ch"
	<simona@...ll.ch>,
        "dri-devel@...ts.freedesktop.org"
	<dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        Frank Binns <Frank.Binns@...tec.com>,
        "maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
        "mripard@...nel.org" <mripard@...nel.org>,
        "airlied@...il.com"
	<airlied@...il.com>
Subject: Re: [PATCH 1/2] drm/imagination: Add and use FLEX_ARRAY_CHECK()

On Wed, 2025-07-09 at 11:04 +0100, Matt Coster wrote:
> It makes little to no sense to use SIZE_CHECK() on flexible structures, so
> let's validate something that actually matters instead.
> 
> Signed-off-by: Matt Coster <matt.coster@...tec.com>

Reviewed-by: Alessio Belle <alessio.belle@...tec.com>

Thanks,
Alessio

> ---
>  drivers/gpu/drm/imagination/pvr_rogue_fwif_check.h | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/imagination/pvr_rogue_fwif_check.h b/drivers/gpu/drm/imagination/pvr_rogue_fwif_check.h
> index 51dc37e78f41d7bdf45d1f434dd1aa5b9eca700a..e72f4064af187e2be3e26722e1ee1ac632087d3d 100644
> --- a/drivers/gpu/drm/imagination/pvr_rogue_fwif_check.h
> +++ b/drivers/gpu/drm/imagination/pvr_rogue_fwif_check.h
> @@ -5,6 +5,8 @@
>  #define PVR_ROGUE_FWIF_CHECK_H
>  
>  #include <linux/build_bug.h>
> +#include <linux/overflow.h>
> +#include <linux/stddef.h>
>  
>  #define OFFSET_CHECK(type, member, offset) \
>  	static_assert(offsetof(type, member) == (offset), \
> @@ -13,6 +15,21 @@
>  #define SIZE_CHECK(type, size) \
>  	static_assert(sizeof(type) == (size), #type " is incorrect size")
>  
> +/*
> + * Where the last member of a struct is a flexible array member, using
> + * SIZE_CHECK() is pointless. If the structure is not already padded to
> + * alignment without the flexible array member, sizeof() will not match the
> + * offset of the flexible array member and the "correct" sizeof() value is
> + * completely meaningless.
> + *
> + * In those instances, use FLEX_ARRAY_CHECK() instead to assert that the final
> + * field is a flexible array member and that it behaves as expected.
> + */
> +#define FLEX_ARRAY_CHECK(type, member)                            \
> +	static_assert(flex_array_size((type *)NULL, member, 1) == \
> +			      sizeof_field(type, member[0]),      \
> +		      #type "->" #member " is incorrect size")
> +
>  OFFSET_CHECK(struct rogue_fwif_file_info_buf, path, 0);
>  OFFSET_CHECK(struct rogue_fwif_file_info_buf, info, 200);
>  OFFSET_CHECK(struct rogue_fwif_file_info_buf, line_num, 400);
> @@ -157,7 +174,7 @@ OFFSET_CHECK(struct rogue_fwif_frag_ctx_state, frag_reg_pm_deallocated_mask_stat
>  OFFSET_CHECK(struct rogue_fwif_frag_ctx_state, frag_reg_dm_pds_mtilefree_status, 4);
>  OFFSET_CHECK(struct rogue_fwif_frag_ctx_state, ctx_state_flags, 8);
>  OFFSET_CHECK(struct rogue_fwif_frag_ctx_state, frag_reg_isp_store, 12);
> -SIZE_CHECK(struct rogue_fwif_frag_ctx_state, 16);
> +FLEX_ARRAY_CHECK(struct rogue_fwif_frag_ctx_state, frag_reg_isp_store);
>  
>  OFFSET_CHECK(struct rogue_fwif_compute_ctx_state, ctx_state_flags, 0);
>  SIZE_CHECK(struct rogue_fwif_compute_ctx_state, 4);
>