[Qemu-devel] USE_CODE_COPY breaks clone

[Qemu-devel] USE_CODE_COPY breaks clone

[Rafael Ávila de Espíndola]

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The cvs version of qemu (i386->i386) generates a segmentation fault in the 
emulated code of the attached test program. Commenting the definition of 
USE_CODE_COPY make the test execute successfully.

I think that the problem is in the fact that cpu_init is called in the parent 
process and this functions calls modify_ldt.

Where can I found some documentation about modify_ldt? What is USE_CODE_COPY 
supposed to do?

I will try to call cpu_init from within the new thread and see if it works.

One more question: What do you think about breaking the do_fork function in 
do_fork, do_clone and do_vfork? Presently it tries to guess if it should call 
fork or clone, but do_syscall knows the answer :)

Thanks for any comment,
Rafael

[-- Attachment #2: teste.c --]
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#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

int f(void *oie) {
    printf("%s\n",(char *)oie);
    return 0;
}

int main() {
    const int stack_size = 8192;
    int ret;
    void *buf = malloc(stack_size);
    void *buff_end = buf + stack_size;
    unsigned int flags =CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND;
    char msg[] = "tata";
    ret = clone(f, buff_end, flags, (void *) msg);
    assert(ret != -1);
    printf("blit\n");
    return 0;
}

Re: [Qemu-devel] USE_CODE_COPY breaks clone

[Rafael Ávila de Espíndola]

On Wednesday 01 September 2004 14:32, Rafael Ávila de Espíndola wrote:
> I think that the problem is in the fact that cpu_init is called in the
> parent process and this functions calls modify_ldt.
>
> Where can I found some documentation about modify_ldt? What is
> USE_CODE_COPY supposed to do?
Ok. It translate i386 to i386 code by copying it.

> I will try to call cpu_init from within the new thread and see if it works.
No it doesn't. If clone is called with CLONE_VM then the kernel does  
mm = oldmm;
and both processes share the ldt. So the parent ldt is "also" changed when 
cpu_init is called from clone_func.

I could not find out why it is necessary to modify the ltd of the process for 
code copy to work. Is there some documentation? Could someone explain?

Does someone have an Idea of how to solve this?

Thanks,
Rafael