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.
pgpM2zjaiw706.pgp
Description: PGP signature