linux-kernel - Re: Current utrace breaks UML

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Message-ID: <20070523121902.GA4365@c2.user-mode-linux.org>
Date:	Wed, 23 May 2007 08:19:02 -0400
From:	Jeff Dike <jdike@...toit.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Current utrace breaks UML

On Tue, May 22, 2007 at 11:46:11PM -0700, Roland McGrath wrote:
> Do you have a test case for PTRACE_SYSEMU that does not work right?

UML, obviously.  Below is a smaller test.  orig_eax is wrong, so you
can't read the system call number from the process.

With kernel-2.6.20-1.2948, it prints out a bunch of -1's.  On FC5
kernels, you get more reasonable numbers.

				Jeff

-- 
Work email - jdike at linux dot intel dot com


#include <stdlib.h>
#include <stdio.h>
#include <asm/unistd.h>
#include <sys/mman.h>
#include <sys/ptrace.h>
#include <asm/ptrace.h>
#include <sys/wait.h>

#define PTRACE_SYSEMU		  31
#define PAGE_SIZE 4096

static inline long stub_syscall0(long syscall)
{
	long ret;

	__asm__ volatile ("int $0x80" : "=a" (ret) : "0" (syscall));

	return ret;
}

static void child(void)
{
	if(ptrace(PTRACE_TRACEME, 0, 0, 0) < 0){
		perror("traceme");
		exit(1);
	}

	kill(getpid(), SIGUSR1);
	stub_syscall0(__NR_getpid);
}

int main(void)
{
	void *stack;
	unsigned long sp, regs[FRAME_SIZE];
	int pid, n, status;

	stack = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if(stack == MAP_FAILED){
		perror("mmap");
		exit(1);
	}

	sp = (unsigned long) stack + PAGE_SIZE - sizeof(void *);
	pid = fork();
	if(pid < 0)
		perror("fork");
	else if(pid == 0){
		child();
	}
	else {
		while(1){
			n = waitpid(pid, &status, WUNTRACED);
			if(n < 0){
				perror("waitpid");
				exit(1);
			}

			n = ptrace(PTRACE_GETREGS, pid, 0, regs);
			if(n < 0){
				perror("PTRACE_GETREGS");
				exit(1);
			}

			printf("Status 0x%x orig_eax 0x%x\n", status,
			       regs[ORIG_EAX]);

			if(ptrace(PTRACE_SYSEMU, pid, 0, 0)){
				perror("PTRACE_SYSEMU");
				exit(1);
			}
		}
	}

	return 0;
}
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