ubi: Fix failure attaching when vid_hdr offset equals to (sub)page size

Following process will make ubi attaching failed since commit
1b42b1a3 ("ubi: ensure that VID header offset ... size"):

ID="0xec,0xa1,0x00,0x15" # 128M 128KB 2KB
modprobe nandsim id_bytes=$ID
flash_eraseall /dev/mtd0
modprobe ubi mtd="0,2048"  # set vid_hdr offset as 2048 (one page)
(dmesg):
  ubi0 error: ubi_attach_mtd_dev [ubi]: VID header offset 2048 too large.
  UBI error: cannot attach mtd0
  UBI error: cannot initialize UBI, error -22

Rework original solution, the key point is making sure
'vid_hdr_shift + UBI_VID_HDR_SIZE < ubi->vid_hdr_alsize',
so we should check vid_hdr_shift rather not vid_hdr_offset.
Then, ubi still support (sub)page aligined VID header offset.

Fixes: 1b42b1a3 ("ubi: ensure that VID header offset ... size")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Tested-by: Nicolas Schichan <nschichan@freebox.fr>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com> # v5.10, v4.19
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/build.c

@@ -666,12 +666,6 @@ static int io_init(struct ubi_device *ubi, int max_beb_per1024)
 	ubi->ec_hdr_alsize = ALIGN(UBI_EC_HDR_SIZE, ubi->hdrs_min_io_size);
 	ubi->vid_hdr_alsize = ALIGN(UBI_VID_HDR_SIZE, ubi->hdrs_min_io_size);
 
-	if (ubi->vid_hdr_offset && ((ubi->vid_hdr_offset + UBI_VID_HDR_SIZE) >
-	    ubi->vid_hdr_alsize)) {
-		ubi_err(ubi, "VID header offset %d too large.", ubi->vid_hdr_offset);
-		return -EINVAL;
-	}
-
 	dbg_gen("min_io_size      %d", ubi->min_io_size);
 	dbg_gen("max_write_size   %d", ubi->max_write_size);
 	dbg_gen("hdrs_min_io_size %d", ubi->hdrs_min_io_size);
@@ -689,6 +683,21 @@ static int io_init(struct ubi_device *ubi, int max_beb_per1024)
 						ubi->vid_hdr_aloffset;
 	}
 
+	/*
+	 * Memory allocation for VID header is ubi->vid_hdr_alsize
+	 * which is described in comments in io.c.
+	 * Make sure VID header shift + UBI_VID_HDR_SIZE not exceeds
+	 * ubi->vid_hdr_alsize, so that all vid header operations
+	 * won't access memory out of bounds.
+	 */
+	if ((ubi->vid_hdr_shift + UBI_VID_HDR_SIZE) > ubi->vid_hdr_alsize) {
+		ubi_err(ubi, "Invalid VID header offset %d, VID header shift(%d)"
+			" + VID header size(%zu) > VID header aligned size(%d).",
+			ubi->vid_hdr_offset, ubi->vid_hdr_shift,
+			UBI_VID_HDR_SIZE, ubi->vid_hdr_alsize);
+		return -EINVAL;
+	}
+
 	/* Similar for the data offset */
 	ubi->leb_start = ubi->vid_hdr_offset + UBI_VID_HDR_SIZE;
 	ubi->leb_start = ALIGN(ubi->leb_start, ubi->min_io_size);