Re: [2.6.31 and later] "struct pid" leak.

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Serge E. Hallyn (serue@us.ibm.com):
> Quoting Oleg Nesterov (oleg@redhat.com):
> > On 03/31, Andrew Morton wrote:
> > >
> > > On Tue, 30 Mar 2010 16:31:13 +0100
> > > Catalin Marinas <catalin.marinas@arm.com> wrote:
> > >
> > > > Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> > > > > I got below report with 2.6.33.1 .
> > > > >
> > > > > unreferenced object 0xde144600 (size 64):
> > > > >   comm "init", pid 1, jiffies 4294678101 (age 291.508s)
> > > > >   hex dump (first 32 bytes):
> > > > >     02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > > > >     00 00 00 00 04 76 ae de d1 76 43 c0 d6 08 00 00  .....v...vC.....
> > > > >   backtrace:
> > > > >     [<c0481704>] create_object+0x121/0x1ef
> > > > >     [<c05f546b>] kmemleak_alloc+0x25/0x42
> > > > >     [<c047e326>] kmemleak_alloc_recursive+0x1c/0x22
> > > > >     [<c047e36e>] kmem_cache_alloc+0x42/0x68
> > > > >     [<c0437701>] alloc_pid+0x19/0x288
> > > > >     [<c0428acc>] copy_process+0x95a/0xdac
> > > > >     [<c04290d8>] do_fork+0x129/0x261
> > > > >     [<c0407de5>] sys_clone+0x1f/0x24
> > > > >     [<c040292d>] ptregs_clone+0x15/0x28
> > > > >     [<ffffffff>] 0xffffffff
> > > > > unreferenced object 0xdfa96a40 (size 64):
> > > > >   comm "login", pid 2259, jiffies 4294719437 (age 250.179s)
> > > > >   hex dump (first 32 bytes):
> > > > >     02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > > > >     00 00 00 00 60 39 ae de d1 76 43 c0 bb 09 00 00  ....`9...vC.....
> > > > >   backtrace:
> > > > >     [<c0481704>] create_object+0x121/0x1ef
> > > > >     [<c05f546b>] kmemleak_alloc+0x25/0x42
> > > > >     [<c047e326>] kmemleak_alloc_recursive+0x1c/0x22
> > > > >     [<c047e36e>] kmem_cache_alloc+0x42/0x68
> > > > >     [<c0437701>] alloc_pid+0x19/0x288
> > > > >     [<c0428acc>] copy_process+0x95a/0xdac
> > > > >     [<c04290d8>] do_fork+0x129/0x261
> > > > >     [<c0407de5>] sys_clone+0x1f/0x24
> > > > >     [<c040292d>] ptregs_clone+0x15/0x28
> > > > >     [<ffffffff>] 0xffffffff
> > > >
> > > > I reported similar leaks last year -
> > > > http://lkml.org/lkml/2009/7/8/422. There is some analysis in the thread
> > > > above of the reference counting but I couldn't figure out where it goes
> > > > wrong. It looks to me like there isn't any reference to a struct pid
> > > > block but its reference count is 2.
> > > >
> > > > There is a bugzilla entry as well -
> > > > https://bugzilla.kernel.org/show_bug.cgi?id=13868
> > > >
> > >
> > > Let's bug some people by cc'ing them ;)
> > 
> > Oh. It is hardly possibly to find the unbalanced get_pid() via grep.
> > 
> > IIRC, I sent the debugging patch which tracks get/put pid, but I can't
> > recall if anybody tried this patch. Hmm, and I can't find that patch
> > or the previous discussion in my maildir...
> > 
> > Catalin, Tetsuo, any chance you can remind me if this patch was used?
> > 
> > Oleg.
> 
> [ probably sounding like a moron, but... ]
> 
> Looking through vt_ioctl.c, I get the feeling that the ttys will
> hang onto vc->vt_pid until either getting a SAK or until someone
> new logs in.  I don't see where logging out will cause a reset_vc().
> So when the logged in task logs out, does vt_pid keep a ref to the
> pid which now no longer exists?
> 
> Came to mind bc I notice that every trace you've sent has included
> /bin/login or X...
> 
> -serge

In particular, if vt_ioctl is called with VT_RELDISP, then
complete_change_console() will get called, which will kill vc->vt_pid,
but it will only call reset_vc(vc) if that kill_pid failed.  reset_vc()
is needed to do our last put_pid().

I could be way off base here, but...

-serge