i915/gvt: Replace one-element array with flexible-array member

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].

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.

Link: KSPP#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>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/Y6Eu2604cqtryP4g@mail.google.com

Author: PauloMigAlmeida

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;