Date: Thu, 26 Nov 2009 19:22:26 +0100
From: Veaceslav Falico <vfalico@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       Christoph Hellwig <hch@infradead.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <peterz@infradead.org>,
       Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org,
       utrace-devel@redhat.com,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: powerpc: fork && stepping (Was: [RFC,PATCH 0/14] utrace/ptrace)
Message-ID: <20091126182226.GF12355@darkmag.usersys.redhat.com>
References: <20091124200127.GA5751@redhat.com> <20091125080342.GD2660@in.ibm.com> <20091125154052.GA6734@redhat.com> <20091126075335.GA18508@in.ibm.com> <20091126145051.GB4382@redhat.com> <20091126172524.GA14768@redhat.com>
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On Thu, Nov 26, 2009 at 06:25:24PM +0100, Oleg Nesterov wrote:
> On 11/26, Oleg Nesterov wrote:
> >
> > On 11/26, Ananth N Mavinakayanahalli wrote:
> > >
> > > step-fork: step-fork.c:56: handler_fail: Assertion `0' failed.
> > > /bin/sh: line 5: 17325 Aborted                 ${dir}$tst
> > > FAIL: step-fork
> >
> > Good to know, thanks again Ananth.
> >
> > I'll take a look. Since I know nothing about powerpc, I can't
> > promise the quick fix ;)
> >
> > The bug was found by code inspection, but the fix is not trivial
> > because it depends on arch/, and it turns out the arch-independent
> > fix in
> >
> > 	ptrace-copy_process-should-disable-stepping.patch
> > 	http://marc.info/?l=linux-mm-commits&m=125789789322573
> >
> > doesn't work.
> 
> Just noticed the test-case fails in handler_fail(). Most probably
> this means it is killed by SIGALRM because either parent or child
> hang in wait(). Perhaps we have another (ppc specific?) bug, but
> currently I do not understand how this is possible, this should
> not be arch-dependent.

I can confirm that we have another bug on ppc arch. The test case below
is spinning forever, 

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <assert.h>

int main(void)
{
        int pid, status;

        if (!(pid = fork())) {
                assert(ptrace(PTRACE_TRACEME) == 0);
                kill(getpid(), SIGSTOP);

                if (!fork())
                        return 0;

                printf("fork passed..\n");

                return 0;
        }

        for (;;) {
                assert(pid == wait(&status));
                if (WIFEXITED(status))
                        break;
                assert(ptrace(PTRACE_SINGLESTEP, pid, 0,0) == 0);
        }

        printf("Parent exit.\n");

        return 0;
}

it doesn't hang, the parent is spinning around for, the test case
isn't printing anything. Seems like fork() can't complete under
PTRACE_SINGLESTEP.

--
Veaceslav 
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