Re: [PATCH] [next] i915/gvt: Replace one-element array with flexible-array member

[Zhenyu Wang <zhenyuw@linux.intel.com>]

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On 2022.12.20 16:41:15 +1300, Paulo Miguel Almeida wrote:
> One-element arrays are deprecated, and we are replacing them with
> flexible array members instead. So, replace one-element array with
> flexible-array member in struct gvt_firmware_header and refactor the
> rest of the code accordingly.
> 
> Additionally, previous implementation was allocating 8 bytes more than
> required to represent firmware_header + cfg_space data + mmio data.
> 
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> ---
> To make reviewing this patch easier, I'm pasting before/after struct
> sizes.
> 
> pahole -C gvt_firmware_header before/drivers/gpu/drm/i915/gvt/firmware.o 
> struct gvt_firmware_header {
> 	u64                        magic;                /*     0     8 */
> 	u32                        crc32;                /*     8     4 */
> 	u32                        version;              /*    12     4 */
> 	u64                        cfg_space_size;       /*    16     8 */
> 	u64                        cfg_space_offset;     /*    24     8 */
> 	u64                        mmio_size;            /*    32     8 */
> 	u64                        mmio_offset;          /*    40     8 */
> 	unsigned char              data[1];              /*    48     1 */
> 
> 	/* size: 56, cachelines: 1, members: 8 */
> 	/* padding: 7 */
> 	/* last cacheline: 56 bytes */
> };
> 
> pahole -C gvt_firmware_header after/drivers/gpu/drm/i915/gvt/firmware.o 
> struct gvt_firmware_header {
> 	u64                        magic;                /*     0     8 */
> 	u32                        crc32;                /*     8     4 */
> 	u32                        version;              /*    12     4 */
> 	u64                        cfg_space_size;       /*    16     8 */
> 	u64                        cfg_space_offset;     /*    24     8 */
> 	u64                        mmio_size;            /*    32     8 */
> 	u64                        mmio_offset;          /*    40     8 */
> 	unsigned char              data[];               /*    48     0 */
> 
> 	/* size: 48, cachelines: 1, members: 8 */
> 	/* last cacheline: 48 bytes */
> };
> 
> As you can see the additional byte of the fake-flexible array (data[1])
> forced the compiler to pad the struct but those bytes aren't actually used
> as first & last bytes (of both cfg_space and mmio) are controlled by the
> <>_size and <>_offset members present in the gvt_firmware_header struct.
> ---
>  drivers/gpu/drm/i915/gvt/firmware.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gvt/firmware.c b/drivers/gpu/drm/i915/gvt/firmware.c
> index a683c22d5b64..dce93738e98a 100644
> --- a/drivers/gpu/drm/i915/gvt/firmware.c
> +++ b/drivers/gpu/drm/i915/gvt/firmware.c
> @@ -45,7 +45,7 @@ struct gvt_firmware_header {
>  	u64 cfg_space_offset;	/* offset in the file */
>  	u64 mmio_size;
>  	u64 mmio_offset;	/* offset in the file */
> -	unsigned char data[1];
> +	unsigned char data[];
>  };
>  
>  #define dev_to_drm_minor(d) dev_get_drvdata((d))
> @@ -77,7 +77,7 @@ static int expose_firmware_sysfs(struct intel_gvt *gvt)
>  	unsigned long size, crc32_start;
>  	int ret;
>  
> -	size = sizeof(*h) + info->mmio_size + info->cfg_space_size;
> +	size = offsetof(struct gvt_firmware_header, data) + info->mmio_size + info->cfg_space_size;
>  	firmware = vzalloc(size);
>  	if (!firmware)
>  		return -ENOMEM;
> -- 

Looks good to me.
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>

Thanks!

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