Re: [Xen-devel] [Xen-users] "xl restore" leaks a file descriptor?

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Aug 11, 2015 at 06:21:18PM +0100, Andrew Cooper wrote:
> On 11/08/15 18:07, Wei Liu wrote:
> > On Tue, Aug 11, 2015 at 04:48:13PM +0100, Ian Campbell wrote:
> >> On Tue, 2015-08-11 at 11:13 -0400, Andrew Armenia wrote:
> >>> It's the checkpoint file - i.e. the command line argument to xl
> >>> restore - that is being leaked.
> >> Thanks.
> >>
> >> [...]
> >>> So the checkpoint file is clearly being leaked.
> >> Indeed. I confirmed this even with the current development version using ls
> >> -l /proc/<pid>/fd which shows an fd open on a deleted file:
> >>
> >> # ps aux| grep xl
> >> root     20465  0.0  0.2 106036   984 ?        SLsl 15:42   0:00 xl 
> >> restore save
> >> # ls -l /proc/20465/fd
> >> [...]
> >> lr-x------. 1 root root 64 Aug 11 15:42 7 -> /root/save
> >> [...]
> >> # rm /root/save
> >> # ls -l /proc/20465/fd
> >> [...]
> >> lr-x------. 1 root root 64 Aug 11 15:42 7 -> /root/save (deleted)
> >> [...]
> >>
> >>>  Its space is not freed
> >>> until the 'xl restore' process is ended by shutting down the domain:
> >> [...]
> >>> It seems like xl restore should close the checkpoint file as soon as
> >>> it's done restoring the domain, allowing the space to be freed, but
> >>> that's clearly not happening.
> >> Right. In fact xl sets the file to be close-on-exec right after opening it,
> >> which is before the daemonisation step, so it ought to be closed
> >> automatically, but isn't for some reason.
> >>
> >> My working theory is that something in the machinery which spawns the save
> >> helper is defeating the use of CLOEXEC, perhaps by dup2() or perhaps by
> >> unsetting CLOEXEC.
> >>
> >> Any way, thanks for reporting. I've copied the devel list and 4.6 RM. Wei
> >> this probably ought to be a blocker for 4.6 (and the fix ought ultimately
> >> to be backported to 4.4 onwards at least).
> >>
> >> NB: This leak seems to be independent of the switch to migration v2.
> >>
> >> Ian.
> > Maybe this is just because we leak a fd.
> >
> > I don't see how CLOEXEC would be of any use if xl doesn't actually exec
> > anything.
> >
> > Below is a PoC patch which seems to fix the problem for me.
> >
> > ---8<---
> > commit 7b5f466d5977dc9f41991ca0c2227023ac07709d
> > Author: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Date:   Tue Aug 11 18:02:25 2015 +0100
> >
> >     xl: close restore_fd when we finish with it
> >     
> >     Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> >
> > diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
> > index 499a05c..525cd24 100644
> > --- a/tools/libxl/xl_cmdimpl.c
> > +++ b/tools/libxl/xl_cmdimpl.c
> > @@ -2846,6 +2846,10 @@ start:
> >          ret = libxl_domain_create_new(ctx, &d_config, &domid,
> >                                        0, autoconnect_console_how);
> >      }
> > +
> > +    if (migrate_fd < 0)
> > +        close(restore_fd);
> > +
> 
> You surely need check for restore_fd >= 0, to avoid a potential EBADF ?
> 

Indeed. When we create a new domain, restore_fd is -1.

Wei.

> ~Andrew

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