Re: [uml-devel] Explaination of system call function flow in TT mode

[Blaisorblade <blaisorblade@yahoo.it>]

On Monday 16 May 2005 20:52, Young Koh wrote:
> Hi, Thanks for the reply. Let me have one following question.
>
> As my understand, in SKAS mode, tracer and tracee are different host
> processes, and they have UML kernel code and application code
> respectively. (in TT mode, tracee has both kernel and application
> code)
Yes, exactly.
> Suppose the tracee attmpts to invoke a host system call. then, it will
> be intercepted by the tracer as in TT mode. (In TT mode, the tracer
> turns off tracing and sends SIGUSR2 to the tracee, and then, the
> signal handler in the tracee will execute the system call.)
Ok, perfect.
> but, in 
> SKAS mode, because the tracee doesn't have the UML kernel code, only
> the tracer knows about the system call code. The tracer cannot simply
> send a signal to make the tracee execute the system call. Then how
> does it do? Does the tracer execute the system call for the tracee?

Exactly. in arch/um/kernel/skas/process.c, userspace() loops doing 
PTRACE_SYSCALL, and when SIGTRAP | 0x80 is received by waiting on the tracee, 
and it's handled by handle_trap(), which reads the syscall params in the 
registers saved by userspace. You'll probably find the 0x80 strange, since 
ptrace(2) documents SIGTRAP; but in recent UML kernels we set 
PTRACE_O_SYSGOOD so we get the additional 0x80 (which does not happen on 
normal SIGTRAP, due for instance to singlestepping).

See, in arch/i386/kernel/ptrace.c:do_syscall_trace(), the call to 
ptrace_notify().
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade




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