Re: [uml-devel] Question about system call trap in UML

[Blaisorblade <blaisorblade@yahoo.it>]

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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