[Xen-devel] [PATCH 1/3] libxl: events: Pass correct nfds to poll

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

libxl_event.c:eventloop_iteration would pass the allocated pollfds
array size, rather than the used size, to poll (and to
afterpoll_internal).

The effect is that if the number of fds to poll on reduces, libxl will
poll on stale entries.  Because of the way the return value from poll
is processed these stale entries are often harmless because any events
coming back from poll ignored by libxl.  However, it could cause
malfunctions:

It could result in unwanted SIGTTIN/SIGTTOU/SIGPIPE, for example, if
the fd has been reused to refer to an object which can generate those
signals.  Alternatively, it could result in libxl spinning if the
stale entry refers to an fd which happens now to be ready for the
previously-requested operation.

I have tested this with a localhost migration and inspected the strace
output.

Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Cc: Jim Fehlig <jfehlig@xxxxxxxx>
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
---
 tools/libxl/libxl_event.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index bdef7ac..1c48fee 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -1386,7 +1386,7 @@ static int eventloop_iteration(libxl__egc *egc, 
libxl__poller *poller) {
      * can unlock it when it polls.
      */
     EGC_GC;
-    int rc;
+    int rc, nfds;
     struct timeval now;
     
     rc = libxl__gettimeofday(gc, &now);
@@ -1395,7 +1395,7 @@ static int eventloop_iteration(libxl__egc *egc, 
libxl__poller *poller) {
     int timeout;
 
     for (;;) {
-        int nfds = poller->fd_polls_allocd;
+        nfds = poller->fd_polls_allocd;
         timeout = -1;
         rc = beforepoll_internal(gc, poller, &nfds, poller->fd_polls,
                                  &timeout, now);
@@ -1413,7 +1413,7 @@ static int eventloop_iteration(libxl__egc *egc, 
libxl__poller *poller) {
     }
 
     CTX_UNLOCK;
-    rc = poll(poller->fd_polls, poller->fd_polls_allocd, timeout);
+    rc = poll(poller->fd_polls, nfds, timeout);
     CTX_LOCK;
 
     if (rc < 0) {
@@ -1428,8 +1428,7 @@ static int eventloop_iteration(libxl__egc *egc, 
libxl__poller *poller) {
     rc = libxl__gettimeofday(gc, &now);
     if (rc) goto out;
 
-    afterpoll_internal(egc, poller,
-                       poller->fd_polls_allocd, poller->fd_polls, now);
+    afterpoll_internal(egc, poller, nfds, poller->fd_polls, now);
 
     rc = 0;
  out:
-- 
1.7.10.4


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