libxl: Reset toolstack_save file position in libxl

toolstack_save data is written to a temporary file in libxl and read
back in libxl-save-helper.  The file position must be reset prior to
reading the file, which is done in libxl-save-helper with lseek.

lseek is unsupported for pipes and sockets, so a wrapper passing such an
fd to libxl-save-helper fails the lseek.  Moving the lseek to libxl
avoids the error, allowing the save to continue.

Signed-off-by: Jason Andryuk <andryuk@aero.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_save_callout.c

@@ -105,6 +105,10 @@ void libxl__xc_domain_save(libxl__egc *egc, libxl__domain_suspend_state *dss,
                                 toolstack_data_buf, toolstack_data_len,
                                 "toolstack data tmpfile", 0);
         if (r) { rc = ERROR_FAIL; goto out; }
+
+        /* file position must be reset before passing to libxl-save-helper. */
+        r = lseek(toolstack_data_fd, 0, SEEK_SET);
+        if (r) { rc = ERROR_FAIL; goto out; }
     }
 
     const unsigned long argnums[] = {

tools/libxl/libxl_save_helper.c

@@ -163,10 +163,9 @@ static uint32_t toolstack_save_len;
 static int toolstack_save_cb(uint32_t domid, uint8_t **buf,
                              uint32_t *len, void *data)
 {
-    assert(toolstack_save_fd > 0);
+    int r;
 
-    int r = lseek(toolstack_save_fd, 0, SEEK_SET);
-    if (r) fail(errno,"rewind toolstack data tmpfile");
+    assert(toolstack_save_fd > 0);
 
     *buf = xmalloc(toolstack_save_len);
     r = read_exactly(toolstack_save_fd, *buf, toolstack_save_len);