[PATCH v6 03/15] fbdev: efifb: move the BMP headers to <video/bmp.h>

["Màxim Pedraza Padilla" <maximpedraza@gmail.com>]

The DRM splash client parses the same BMP headers as efifb. Rather than
keep a second copy of the structures, move them to include/video/bmp.h
with the magic number.

BMP is little endian whatever the CPU, so the header gives the fields
as __le16 and __le32, and efifb now reads them with
get_unaligned_le16() and get_unaligned_le32(). Width and height are
read signed, as the format has them. No change on the little endian
systems EFI runs on.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Màxim Pedraza Padilla <maximpedraza@gmail.com>
---

Notes:
    TODO for the efifb maintainers, not changed here so that this patch
    only moves the headers: the bound check
    
    	if ((bitmap_offset + bmp_pitch * bmp_height) > bgrt_image_size)
    
    is done in u32 and can wrap with a crafted BGRT image, for instance
    with a bitmap_offset close to 4 GiB, and so can round_up(3 * bmp_width,
    4) and the product with a huge width; image_offset_x + bmp_width against
    lfb_width is a u32 sum too. efifb_copy_bmp() would then read outside the
    mapped image. The DRM splash client checks the same headers with
    check_add_overflow() and check_mul_overflow(). I can send a separate fix
    if wanted.

 drivers/video/fbdev/efifb.c | 49 ++++++++++++++-----------------------
 include/video/bmp.h         | 40 ++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 31 deletions(-)
 create mode 100644 include/video/bmp.h

diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
index 3411be06d3e6..fd348a72f780 100644
--- a/drivers/video/fbdev/efifb.c
+++ b/drivers/video/fbdev/efifb.c
@@ -16,32 +16,13 @@
 #include <linux/platform_device.h>
 #include <linux/printk.h>
 #include <linux/sysfb.h>
+#include <linux/unaligned.h>
+#include <video/bmp.h>
 #include <video/vga.h>
 #include <asm/efi.h>
 #include <drm/drm_connector.h>  /* For DRM_MODE_PANEL_ORIENTATION_* */
 #include <drm/drm_panel_quirks.h> /* For drm_get_panel_orientation_quirk */
 
-struct bmp_file_header {
-	u16 id;
-	u32 file_size;
-	u32 reserved;
-	u32 bitmap_offset;
-} __packed;
-
-struct bmp_dib_header {
-	u32 dib_header_size;
-	s32 width;
-	s32 height;
-	u16 planes;
-	u16 bpp;
-	u32 compression;
-	u32 bitmap_size;
-	u32 horz_resolution;
-	u32 vert_resolution;
-	u32 colors_used;
-	u32 colors_important;
-} __packed;
-
 static bool use_bgrt = true;
 static bool request_mem_succeeded = false;
 static u64 mem_flags = EFI_MEMORY_WC | EFI_MEMORY_UC;
@@ -145,9 +126,10 @@ static bool efifb_bgrt_sanity_check(const struct screen_info *si, u32 bmp_width)
 
 static void efifb_show_boot_graphics(struct fb_info *info, const struct screen_info *si)
 {
-	u32 bmp_width, bmp_height, bmp_pitch, dst_x, y, src_y;
+	u32 bmp_width, bmp_height, bmp_pitch, bitmap_offset, dst_x, y, src_y;
 	struct bmp_file_header *file_header;
 	struct bmp_dib_header *dib_header;
+	s32 width, height;
 	void *bgrt_image = NULL;
 	u8 *dst = info->screen_base;
 
@@ -186,20 +168,25 @@ static void efifb_show_boot_graphics(struct fb_info *info, const struct screen_i
 		goto error;
 
 	file_header = bgrt_image;
-	if (file_header->id != 0x4d42 || file_header->reserved != 0)
+	if (get_unaligned_le16(&file_header->id) != BMP_FILE_MAGIC_ID ||
+	    get_unaligned_le32(&file_header->reserved) != 0)
 		goto error;
+	bitmap_offset = get_unaligned_le32(&file_header->bitmap_offset);
 
 	dib_header = bgrt_image + sizeof(*file_header);
-	if (dib_header->dib_header_size != 40 || dib_header->width < 0 ||
-	    dib_header->planes != 1 || dib_header->bpp != 24 ||
-	    dib_header->compression != 0)
+	width = get_unaligned_le32(&dib_header->width);
+	height = get_unaligned_le32(&dib_header->height);
+	if (get_unaligned_le32(&dib_header->dib_header_size) != 40 ||
+	    width < 0 || get_unaligned_le16(&dib_header->planes) != 1 ||
+	    get_unaligned_le16(&dib_header->bpp) != 24 ||
+	    get_unaligned_le32(&dib_header->compression) != 0)
 		goto error;
 
-	bmp_width = dib_header->width;
-	bmp_height = abs(dib_header->height);
+	bmp_width = width;
+	bmp_height = abs(height);
 	bmp_pitch = round_up(3 * bmp_width, 4);
 
-	if ((file_header->bitmap_offset + bmp_pitch * bmp_height) >
+	if ((bitmap_offset + bmp_pitch * bmp_height) >
 				bgrt_image_size)
 		goto error;
 
@@ -222,12 +209,12 @@ static void efifb_show_boot_graphics(struct fb_info *info, const struct screen_i
 
 		src_y = y - bgrt_tab.image_offset_y;
 		/* Positive header height means upside down row order */
-		if (dib_header->height > 0)
+		if (height > 0)
 			src_y = (bmp_height - 1) - src_y;
 
 		memset(dst, 0, bgrt_tab.image_offset_x * 4);
 		dst_x = bgrt_tab.image_offset_x;
-		efifb_copy_bmp(bgrt_image + file_header->bitmap_offset +
+		efifb_copy_bmp(bgrt_image + bitmap_offset +
 					    src_y * bmp_pitch,
 			       (u32 *)dst + dst_x, bmp_width, si);
 		dst_x += bmp_width;
diff --git a/include/video/bmp.h b/include/video/bmp.h
new file mode 100644
index 000000000000..8aa249f2302f
--- /dev/null
+++ b/include/video/bmp.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _VIDEO_BMP_H
+#define _VIDEO_BMP_H
+
+#include <linux/types.h>
+
+/* "BM", the first two bytes of a BMP file */
+#define BMP_FILE_MAGIC_ID 0x4d42
+
+/*
+ * The headers at the start of a BMP file. Every field is little endian,
+ * whatever the CPU, and the headers need not be aligned in memory: read them
+ * with get_unaligned_le16() and get_unaligned_le32().
+ */
+struct bmp_file_header {
+	__le16 id;
+	__le32 file_size;
+	__le32 reserved;
+	__le32 bitmap_offset;
+} __packed;
+
+/*
+ * BITMAPINFOHEADER. Width and height are signed: a negative width means
+ * nothing, and a negative height that the rows go top to bottom.
+ */
+struct bmp_dib_header {
+	__le32 dib_header_size;
+	__le32 width;
+	__le32 height;
+	__le16 planes;
+	__le16 bpp;
+	__le32 compression;
+	__le32 bitmap_size;
+	__le32 horz_resolution;
+	__le32 vert_resolution;
+	__le32 colors_used;
+	__le32 colors_important;
+} __packed;
+
+#endif /* _VIDEO_BMP_H */
-- 
2.39.5