linux-kernel - Re: powerpc: fork && stepping (Was: [RFC,PATCH 0/14] utrace/ptrace)

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Date:	Thu, 26 Nov 2009 19:22:26 +0100
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, utrace-devel@...hat.com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: powerpc: fork && stepping (Was: [RFC,PATCH 0/14] utrace/ptrace)

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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