erofs: fix offset truncation when shifting pgoff on 32-bit platforms

On 32-bit platforms, pgoff_t is 32 bits wide, so left-shifting
large arbitrary pgoff_t values by PAGE_SHIFT performs 32-bit arithmetic
and silently truncates the result for pages beyond the 4 GiB boundary.

Cast the page index to loff_t before shifting to produce a correct
64-bit byte offset.

Fixes: 3862929 ("erofs: introduce readmore decompression strategy")
Fixes: 307210c ("erofs: verify metadata accesses for file-backed mounts")
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>

Author: hsiangkao

fs/erofs/data.c

@@ -39,7 +39,7 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 	 * However, the data access range must be verified here in advance.
 	 */
 	if (buf->file) {
-		fpos = index << PAGE_SHIFT;
+		fpos = (loff_t)index << PAGE_SHIFT;
 		err = rw_verify_area(READ, buf->file, &fpos, PAGE_SIZE);
 		if (err < 0)
 			return ERR_PTR(err);

fs/erofs/zdata.c

@@ -1872,7 +1872,7 @@ static void z_erofs_pcluster_readmore(struct z_erofs_frontend *f,
 
 		if (cur < PAGE_SIZE)
 			break;
-		cur = (index << PAGE_SHIFT) - 1;
+		cur = ((loff_t)index << PAGE_SHIFT) - 1;
 	}
 }