* Re: [uml-devel] Question about system call trap in UML
2007-03-06 23:32 [uml-devel] Question about system call trap in UML Haifeng He
@ 2007-03-06 23:52 ` Blaisorblade
0 siblings, 0 replies; 2+ messages in thread
From: Blaisorblade @ 2007-03-06 23:52 UTC (permalink / raw)
To: user-mode-linux-devel
On Wednesday 07 March 2007 00:32, Haifeng He wrote:
> Hi,
>
> I am beginner of UML. I have a question about how the system call
> trap happens in UML in skas0 mode. Suppose a user application
> within UML initialed a system call. How does UML track the
> system call and is the host OS the one to actually handle the system
> call?
The base tool is ptrace (see man 2 ptrace).
Basically ptrace(PTRACE_SYSCALL) (also used by the strace tool - look at man
strace) is used to intercept the syscalls to the host. Registers are modified
so that getpid() is executed (it is a syscall doing no harm), and the syscall
is then executed by the UML kernel.
arch/um/os-Linux/skas/process.c contains the core code: the main loop in
userspace() runs for the process lifetime, once for each process.
handle_trap() does the above trick with getpid and ptrace and calls
handle_syscall(), which performs the actual syscall.
Assume local_using_sysemu == 0; that does not change the core, it's just a
faster API to tell the host "don't execute this syscall" than changing the
syscall to getpid().
No difference about this exists in SKAS0 and SKAS3, that I remember (I think
it's also valid for TT mode, with a few differences).
> Particularly, I found a file: arch/um/sys-i386/stub.S. Is the file used
> for handling syscalls in user application?
No. That code is injected in all user applications within UML.
When we need that the user process does a certain syscall, we change EIP to
point to that piece of code, write the syscalls to perform and their
arguments in a fixed location, and resume the process with ptrace().
The 'virtual RAM' of a UML instance is represented by a file, which is mapped
with MAP_SHARED.
So, memory mappings for userspace processes are installed by making that stub
call mmap() with certain arguments decided by UML; munmap() and mprotect()
are also used.
Since demand paging is used, when mmap is called no mapping is installed; when
a page is allocated and page tables are modified, we can call mmap() to
install the mapping.
> Can somebody explain the
> process to me?
The above is just a little start, to let you explore more and ask more
specific questions.
Bye
--
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
Chiacchiera con i tuoi amici in tempo reale!
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