[Qemu-devel] System call from a Guest Linux

[Qemu-devel] System call from a Guest Linux

[Jong Chun Park]

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Hi, all?

First of all, I apologize you for this spam. I'm working with QEMU v0.12.1
with i386 Linux guest on x86-64 Linux host. I'm trying to find a point in
the source code where a system call from the guest OS is handled. If I'm not
mistaken, QEMU disassembles an instruction of INT # in a switch statement of
target-i386/translate.c and then calls helper_sysenter of
target-i386/op_helper.c. The problem is how to tell difference between a
system call of the guest OS and a system call of QEMU (I'm not sure of this,
though). Assume the following code is executed after compilation in the
guest OS:

int main() {
  if (access("hello.txt", R_OK)!=0) {

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[Qemu-devel] [Please discard the previous mail] System call from a Guest Linux

[Jong Chun Park]

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Hi, all?

First of all, I apologize you for the previous incomplete mail and this
spam.  I'm working with QEMU v0.12.1 with i386 Linux guest on x86-64 Linux
host. I'm trying to find a point in the source code where a system call from
the guest OS is handled. If I'm not mistaken, QEMU disassembles an
instruction of INT # in a switch statement of target-i386/translate.c and
then calls helper_sysenter of target-i386/op_helper.c. The problem is how to
tell difference between a system call of the guest OS and a system call of
QEMU (I'm not sure of this, though). Assume the following code is executed
after compilation in the guest OS:

int main() {
   int fd;
   if (access("hello.txt", R_OK) != 0) {
     exit(0);
   }
   fd=open("hello.txt", O_RDONLY);
   close(fd);
   return 0;
}

For this simple sequence of system calls, acess() -> open() -> close(), QEMU
goes through disas_insn() and helper_sysenter() more than 3 times. This
makes me really confused because I believed those should be called 3 times,
though. It'd be greatly appreciated in advance if someone helps me out this.

Thanks a lot,
Jong

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Re: [Qemu-devel] [Please discard the previous mail] System call from a Guest Linux

[Mulyadi Santosa]

Hi Jong

(Sorry, I am not sure how to politely call a Korean name...so
apologize if it turns out to be rude...)

On Thu, Feb 4, 2010 at 1:08 AM, Jong Chun Park <joumon@gmail.com> wrote:
> after compilation in the guest OS:
> int main() {
>    int fd;
>    if (access("hello.txt", R_OK) != 0) {
>      exit(0);
>    }
>    fd=open("hello.txt", O_RDONLY);
>    close(fd);
>    return 0;
> }
>
> For this simple sequence of system calls, acess() -> open() -> close(), QEMU
> goes through disas_insn() and helper_sysenter() more than 3 times. This
> makes me really confused because I believed those should be called 3 times,
> though. It'd be greatly appreciated in advance if someone helps me out this.
> Thanks a lot,
> Jong

Try to run that compile and run that code under strace (without using
Qemu, I mean), then you'll understand how many system calls are
executed.

clues: opening certain libraries, doing mmap() s and so on

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com