modify syscall nr on-the-fly

Yuriy Tsibizov yuriy.tsibizov at
Mon Aug 20 22:17:29 PDT 2007

2007/8/20, Kostik Belousov <kostikbel at>:
> 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
> > (
> > It forks a child thread that stops
> > (using SIGSTOP), calls getpid() (that will be intercepted by parent thread
> > 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
> > 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).

I know that there is no real use for this feature for native code.


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