Uninitialized resid variable in vdev_file_io_start()/vn_rdwr()

[wizeman]

In the userspace implementation of vdev_file_io_start():

142 vdev_file_io_start(zio_t *zio)
143 {
(...)
146         ssize_t resid;
(...)
167         zio->io_error = vn_rdwr(zio->io_type == ZIO_TYPE_READ ?
168             UIO_READ : UIO_WRITE, vf->vf_vnode, zio->io_data,
169             zio->io_size, zio->io_offset, UIO_SYSSPACE,
170             0, RLIM64_INFINITY, kcred, &resid);
171 
172         if (resid != 0 && zio->io_error == 0)
173                 zio->io_error = ENOSPC;

However, in vn_rdwr():

644 vn_rdwr(int uio, vnode_t *vp, void *addr, ssize_t len, offset_t offset,
645         int x1, int x2, rlim64_t x3, void *x4, ssize_t *residp)
646 {                     
(...)
675         if (rc == -1)
676                 return (errno);
677 
678         done += rc;
679 
680         if (residp)
681                 *residp = len - done;

As you can see, the "resid" variable in the first function may never get set before it is checked in the "if" statement.

[behlendorf]

Initialize resid variable

Properly initialize the variable resid for the error case where
vn_rdwr() returns an error with setting *resid. In practice
this is harmless because the contents of resid don't matter
when vn_rdwr() returns an error. But the code does check the
result which means we may or may not end up having the check
zio->io_error. This makes gcc unhappy and having a completely
deterministic code path here is a good thing.

Closed by 2b89d86