xen 4.3 -> 4.4.0 API change / spurious file handle closure

xen 4.3 -> 4.4.0 API change / spurious file handle closure

[Alex Bligh]

I have an application which works when compiled against
Xen 4.3 but when compiled against Xen 4.4.0 causes a
spurious file handle closure. Specifically fd 0 is
closed when it has nothing to do with Xen. This
happens with a simple call to libxl_ctx_alloc /
libxl_ctx_free.

This can be seen from the snippet below which
simply lists the open fds. In the 'before' call,
the 'ls' command (run from system) opens fd 3
for the directory. In the 'after' call, the
alloc/free has closed fd 0, so fd 0 is reused
for the 'ls' command.

This worked fine under Xen 4.3.

The addition of
  #define LIBXL_API_VERSION 0x040200
does not appear to affect things.

Any idea what's up here?

-- 
Alex Bligh

#include <stdio.h>
#include <stdlib.h>
#include <libxl.h>

/* compile with:
 * gcc -g -O0 xentest.c -o xentest -lxenlight
 */

void
test ()
{
  libxl_ctx *ctx = NULL;
  if (libxl_ctx_alloc (&ctx, LIBXL_VERSION, XTL_NONE, NULL))
    {
      fprintf (stderr, "libxl_ctx_alloc failed");
      exit (1);
    }
  libxl_ctx_free (ctx);

}

int
main (int argc, char **argv)
{
  printf ("Before:\n");
  system ("ls -la /proc/self/fd");
  test ();
  printf ("\nAfter:\n");
  system ("ls -la /proc/self/fd");
  exit (0);
}



Output:


root@node-10-157-128-19:~# ./xentest
Before:
total 0
dr-x------ 2 root root  0 Aug 18 12:14 .
dr-xr-xr-x 9 root root  0 Aug 18 12:14 ..
lrwx------ 1 root root 64 Aug 18 12:14 0 -> /dev/pts/4
lrwx------ 1 root root 64 Aug 18 12:14 1 -> /dev/pts/4
lrwx------ 1 root root 64 Aug 18 12:14 2 -> /dev/pts/4
lr-x------ 1 root root 64 Aug 18 12:14 3 -> /proc/31986/fd

After:
total 0
dr-x------ 2 root root  0 Aug 18 12:14 .
dr-xr-xr-x 9 root root  0 Aug 18 12:14 ..
lr-x------ 1 root root 64 Aug 18 12:14 0 -> /proc/31988/fd
lrwx------ 1 root root 64 Aug 18 12:14 1 -> /dev/pts/4
lrwx------ 1 root root 64 Aug 18 12:14 2 -> /dev/pts/4
root@node-10-157-128-19:~#

Re: xen 4.3 -> 4.4.0 API change / spurious file handle closure

[Andrew Cooper]

On 18/08/14 13:28, Alex Bligh wrote:
> I have an application which works when compiled against
> Xen 4.3 but when compiled against Xen 4.4.0 causes a
> spurious file handle closure. Specifically fd 0 is
> closed when it has nothing to do with Xen. This
> happens with a simple call to libxl_ctx_alloc /
> libxl_ctx_free.
>
> This can be seen from the snippet below which
> simply lists the open fds. In the 'before' call,
> the 'ls' command (run from system) opens fd 3
> for the directory. In the 'after' call, the
> alloc/free has closed fd 0, so fd 0 is reused
> for the 'ls' command.
>
> This worked fine under Xen 4.3.
>
> The addition of
>   #define LIBXL_API_VERSION 0x040200
> does not appear to affect things.
>
> Any idea what's up here?
>

Thats antisocial.  I have reproduced the issue.

Stracing it (with some extra debugging)...

write(1, "Freeing libxl ctx", 17Freeing libxl ctx)       = 17
write(1, "\n", 1
)                       = 1
close(5)                                = 0
close(6)                                = 0
close(7)                                = 0
close(3)                                = 0
close(4)                                = 0
close(0)                                = 0
write(1, "Finished freeing", 16Finished freeing)        = 16

Shows that libxl_ctx_free() is closing fd 0.

I suspect it has an fd which is 0 rather than -1.  I will take a look.

~Andrew