media: staging/intel-ipu3: css: Fix wrong size comparison imgu_css_fw_init

There is a wrong comparison of the total size of the loaded firmware css->fw->size with the size of a pointer to struct imgu_fw_header.

Turn binary_header into a flexible-array member[1][2], use the struct_size() helper and fix the wrong size comparison. Notice that the loaded firmware needs to contain at least one 'struct imgu_fw_info' item in the binary_header[] array.

It's also worth mentioning that

	"css->fw->size < struct_size(css->fwp, binary_header, 1)"

with binary_header declared as a flexible-array member is equivalent to

	"css->fw->size < sizeof(struct imgu_fw_header)"

with binary_header declared as a one-element array (as in the original code).

The replacement of the one-element array with a flexible-array member also helps with the ongoing efforts to globally enable -Warray-bounds and get us closer to being able to tighten the FORTIFY_SOURCE routines on memcpy().

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

---
Changes in v3:
- Integrate the series into a single patch.
- Adjust relational operator and use struct_size().
- Update Subject line and changelog text.
* Thanks to Dan and Sakari for their feedback. :)

Changes in v2:
- Use flexible array and adjust relational operator in patch 1.

v1:

 drivers/staging/media/ipu3/ipu3-css-fw.c | 7 +++----
 drivers/staging/media/ipu3/ipu3-css-fw.h | 2 +-
 2 files changed, 4 insertions(+), 5 deletions(-)

index 45aff76198e2..981693eed815 100644
@@ -124,12 +124,11 @@ int imgu_css_fw_init(struct imgu_css *css)
 	/* Check and display fw header info */
 
 	css->fwp = (struct imgu_fw_header *)css->fw->data;
-	if (css->fw->size < sizeof(struct imgu_fw_header *) ||
+	if (css->fw->size < struct_size(css->fwp, binary_header, 1) ||
 	    css->fwp->file_header.h_size != sizeof(struct imgu_fw_bi_file_h))
 		goto bad_fw;
-	if (sizeof(struct imgu_fw_bi_file_h) +
-	    css->fwp->file_header.binary_nr * sizeof(struct imgu_fw_info) >
-	    css->fw->size)
+	if (struct_size(css->fwp, binary_header,
+			css->fwp->file_header.binary_nr) > css->fw->size)
 		goto bad_fw;
 
 	dev_info(dev, "loaded firmware version %.64s, %u binaries, %zu bytes\n",

index 3c078f15a295..c0bc57fd678a 100644
@@ -171,7 +171,7 @@ struct imgu_fw_bi_file_h {
 
 struct imgu_fw_header {
 	struct imgu_fw_bi_file_h file_header;
-	struct imgu_fw_info binary_header[1];	/* binary_nr items */
+	struct imgu_fw_info binary_header[];	/* binary_nr items */
 };
 
 /******************* Firmware functions *******************/

2.27.0