Add 'f_maxcount' to allow filesystems to set a per-file maximum IO size.

fs/file_table.c

@@ -85,6 +85,7 @@ static int old_max;
 			rwlock_init(&f->f_owner.lock);
 			/* f->f_version: 0 */
 			INIT_LIST_HEAD(&f->f_list);
+			f->f_maxcount = INT_MAX;
 			return f;
 		}
 	}

fs/read_write.c

@@ -185,8 +185,12 @@ asmlinkage long sys_llseek(unsigned int fd, unsigned long offset_high,
 
 int rw_verify_area(int read_write, struct file *file, loff_t *ppos, size_t count)
 {
-	struct inode *inode = file->f_dentry->d_inode;
+	struct inode *inode;
 
+	if (count > file->f_maxcount)
+		return -EINVAL;
+
+	inode = file->f_dentry->d_inode;
 	if (inode->i_flock && MANDATORY_LOCK(inode))
 		return locks_mandatory_area(read_write == READ ? FLOCK_VERIFY_READ : FLOCK_VERIFY_WRITE, inode, file, *ppos, count);
 	return 0;

include/linux/fs.h

@@ -586,6 +586,7 @@ struct file {
 	unsigned int		f_uid, f_gid;
 	struct file_ra_state	f_ra;
 
+	size_t			f_maxcount;
 	unsigned long		f_version;
 	void			*f_security;