linux-kernel - Re: [PATCH 2/2] exec: allow core_pipe recursion check to look for a value of 1 rather than 0 (v2)

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Date:	Mon, 1 Feb 2010 11:29:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	jmoskovc@...hat.com, mingo@...hat.com, drbd-dev@...ts.linbit.com,
	benh@...nel.crashing.org, t.sailer@...mni.ethz.ch, abelay@....edu,
	gregkh@...e.de, spock@...too.org, viro@...iv.linux.org.uk,
	neilb@...e.de, mfasheh@...e.com, menage@...gle.com,
	shemminger@...ux-foundation.org, takedakn@...data.co.jp
Subject: Re: [PATCH 2/2] exec: allow core_pipe recursion check to look for
	a value of 1 rather than 0 (v2)

On 01/31, Neil Horman wrote:
>
> On Sun, Jan 31, 2010 at 04:50:01PM +0100, Oleg Nesterov wrote:
> > On 01/29, Neil Horman wrote:
> > >
> > >  void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> > >  {
> > > ...
> > > -		if (call_usermodehelper_pipe(helper_argv[0], helper_argv, NULL,
> > > -				&cprm.file)) {
> > > +		cprm.file = NULL;
> >
> > it is already NULL,
> >
> Are we sure, it was declared on the stack.

it must be NULL, or compiler is buggy. it was declared as "= { ... }".

> I think its safer to ensure that its
> NULL.

OK, agreed. I mentioned this just in case.

> > > +		if (call_usermodehelper_fns(helper_argv[0], helper_argv, NULL,
> > > +					    UMH_WAIT_EXEC, umh_pipe_setup,
> > > +					    NULL, &cprm)) {
> > > +			if (cprm.file)
> > > +				filp_close(cprm.file, NULL);
> >
> > Hmm. Looks like this change fixes the bug by accident.
> >
> > Before this patch, I think we leak info->stdin if kernel_thread() fails
> > in __call_usermodehelper() pathes.
> >
> I think we did that in call_usermodehelper_pipe.

Afaics, no. Well yes, call_usermodehelper_pipe() closes write_pipe,
but I meant nobody closes read_pipe, info->stdin, if we fail before
____call_usermodehelper() is called.

> > Completely off-topic, but I think __call_usermodehelper(UMH_NO_WAIT) is
> > buggy. if kernel_thread() failes it should do call_usermodehelper_freeinfo().
> > Also, UMH_WAIT_EXEC should set ->retval in this case.
> >
> I went down that path last time I changed this code, Andrew and I decided that
> yes it was buggy, but someone (can't recall how) smacked me around a bit and
> explained how it worked (some odd artifact behavior of the scheduler).  Its in
> the lkml archives if you want to get the whole story.

Hmm. I strongly believe this is buggy, and the scheduler can't help in any
way. Fortunately, kernel_thread() must "never" fail...

Oh. And in theory, it is better to change wait_for_helper(). It should
do allow_signal(SIGCHLD) after kernel_thread(). Otherwise, kernel_thread()
can fail if user-space sends SIGCHLD to the forking thread.

> > Cough. And why call_usermodehelper_exec() has this strange ->path[0] == '\0'
> > check?
> >
> That I can't explain.  I figured I'd let that sleeping dog lie until this got
> striaghtened out and fix it separately if it needed it
> Neil

Yes, yes, agreed. As I said, this has nothing to do with this series,
even if I am right these (minor) bugs should be fixed separately.

Oleg.

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