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

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Date:	Fri, 27 Nov 2009 18:46:27 +0100
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Paul Mackerras <paulus@...ba.org>,
	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 11:37:03PM +0100, Oleg Nesterov wrote:
>
> Could you look at this
>
>   ptrace-copy_process-should-disable-stepping.patch
>   http://marc.info/?l=linux-mm-commits&m=125789789322573
>
> patch? It is not clear to me how we can modify the test-case to
> verify it fixes the original problem for powerpc.

I modified the test-case, it confirms that
ptrace-copy_process-should-disable-stepping.patch fixes the
problem with TIF_SINGLESTEP copied by fork() on powerpc.

Probably we need a similar fix for step-fork.c in ptrace-tests.

Modified the original testcase to call fork via syscall(__NR_fork),
to avoid the looping inside libc's fork() on powerpc.
The parent singlesteps until he sees that the child has forked, after
that the parent PTRACE_CONTs until the child exits.

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

int main(void)
{
	void *addr_after_fork = &&after_fork;
	int pid, status;

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

		if (!syscall(__NR_fork)) {
			/* kernel bug: this child will be killed by SIGTRAP */
			printf("Hello world\n");
			return 43;
		}

after_fork:	wait(&status);
		return WEXITSTATUS(status);
	}

	for (;;) {
		siginfo_t info;

		assert(pid == wait(&status));
		assert(ptrace(PTRACE_GETSIGINFO, pid, 0,&info) == 0);
		if (info.si_addr == addr_after_fork)
			break;
		assert(ptrace(PTRACE_SINGLESTEP, pid, 0,0) == 0);
	}

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

	assert(WEXITSTATUS(status) == 43);
	return 0;
}

--
Veaceslav
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