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[Qemu-devel] Signal management in qemu-user

From: Alex Barcelo
Subject: [Qemu-devel] Signal management in qemu-user
Date: Thu, 17 May 2012 11:23:42 +0200

I'm working in a "big" (=complex, strange) project[1] and come across a bug in signal management. I have been able to narrow it down to this program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <malloc.h>
#include <signal.h>

unsigned char *testfun;

int main ( void )
    unsigned int ra;
    // We block the SIGSEGV signal, used by qemu-user
    sigset_t set;
    sigaddset(&set, 11);
    sigprocmask(SIG_BLOCK, &set, NULL);
    mprotect(testfun, 1024, PROT_READ|PROT_EXEC|PROT_WRITE);

    //400687: b8 0d 00 00 00          mov    $0xd,%eax
    //40068d: c3                      retq
    testfun[ 0]=0xb8;
    testfun[ 1]=0x0d;
    testfun[ 2]=0x00;
    testfun[ 3]=0x00;
    testfun[ 4]=0x00;
    testfun[ 5]=0xc3;
    printf ( "0x%02X\n",
             ((unsigned int (*)())testfun)() );
    //400687: b8 20 00 00 00          mov    $0x20,%eax
    //40068d: c3                      retq
    // This self-modifying code will break because of the sigsegv signal block
    testfun[ 1]=0x20;
    printf ( "0x%02X\n",
             ((unsigned int (*)())testfun)() );

Running it in a i386 machine works and gives an output of "0x0d\n0x20". Running it in a qemu-i386 segfaults. Because the self-modifying code raises a SIGSEGV in the qemu (I understand that it is the method used by qemu to handle self-modifying code). But the sigprocmask disables the SIGSEGV and the qemu-user... does nothing to avoid it. So the SIGSEGV is unmanaged and breaks the program.

I will work in this bug (assuming that nobody is working on it). I suppose that I have to make qemu aware of the signals on the guest and the signals on himself. And do NOT allow the guest doing dangerous manipulations (intercepting the syscall).

Is there something I have to take into acoount? Am I right in my assumptions? Some guru to give me some advice ;) ?

[1] The project is puting a qemu-system-i386 crosscompiled in a qemu-ppc. The "first guest" has LOTS of self-modifying code (qemu-system). I have been bughunting to achieve this "qemu-tower". And correct signal management is a must (as qemu uses lots of signal management, and an incomplete implementation on qemu-user means segfault and things breaking).

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