User-mode Linux (Was: modify syscall nr on-the-fly)

[Yuriy Tsibizov]

> 2007/8/21, Yuriy Tsibizov <yuriy.tsibizov at gmail.com>:
> > 2007/8/20, Kostik Belousov <kostikbel at gmail.com>:
> > > On Sat, Aug 18, 2007 at 02:01:26PM +0400, Yuriy Tsibizov wrote:
> > > > I'm trying to get user-mode Linux to run under FreeBSD Linux emulation (on
> > > > i386).
> > > >
> > > > User-mode Linux in it's start-up tests tries to modify syscall number (to be
> > > > called by kernel) on-the-fly
> > > > (http://fxr.watson.org/fxr/source/arch/um/os-Linux/start_up.c?v=linux-2.6).
> > > > It forks a child thread that stops
> > > > (using SIGSTOP), calls getpid() (that will be intercepted by parent thread
> > > > using PTRACE_SYSCALL)
> > > > and return some value based on getpid() results. Main thread waits for
> > > > SIGSTOP in child process and
> > > > enables PTRACE_SYSCALL (I have some code that implements it. It makes some
> > > > incompatible changes
> > > > to PT_SYSCALL that will break FreeBSD applications, but works for Linux
> > > > apps). When main thread
> > > > catches SIGTRAP (generated by ptrace) it tries to modify EAX of child thread
> > > > (with PTRACE_PEEKUSR
> > > > and PTRACE_POKEUSR) to replace getpid syscall with getppid.
> > > >
> > > > is it possible to get updated EAX (and other registers as well)  in
> > > > syscall(...) after ptracestop(...) in PTRACESTOP_SC(...) returns?
> > > >
> > > > Hope for your help,
> > > >
> > > > Yuriy.
> > >
> > > If I understand right what you want, I doubt that existing code would
> > > allow you to change syscall number in debugger process for debuggee.
> > > You shall look at the sys/i386/i386/trap.c, syscall() function [adjust
> > > as needed for other arches]. It calculates callp before doing PTRACESTOP_SC,
> > > as well as copies the syscall arguments into the kernel address space.
> >
> > Yes, I know this. I'm going to recalculate callp after PTRACESTOP_SC.
> > And, there will be no need to copyin from user space -- all syscalls
> > parameters are passed in registers (it will be used only for processes
> > running under Linux emulation).
> Updated registers are available via *frame.
>
> With some hacks (some return codes needed by user-mode Linux are
> hardcoded into kernel) it loads:
[...]
> I'll need two more flags in p_stops to add two optios:
> - respect PTRACE_(OLD)SETOPTIONS PTRACE_O_TRACESYSGOOD (generate
> SIGTRAP | 0x80 instead of plain SIGTRAP)
> - use Linux PTRACE_SYSCALL conventions (clear S_PT_SCE and S_PT_SCX in
> PTRACESTOP_SC)
> to make it more than just a set of hacks to run single program.
>
> PTRACE_(PEEK|POKE)USR seems to need small rewrite too.

patch (against -CURRENT) is available on
http://chibis.persons.gfk.ru/linux/ptrace.diff

You will need to rebuild both kernel an linux module.

Yuriy.