Date: Tue, 21 Aug 2007 08:48:29 +0400 From: "Yuriy Tsibizov" <yuriy.tsibizov@gmail.com> To: "Kostik Belousov" <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: modify syscall nr on-the-fly Message-ID: <c019b3090708202148l5390b4fex7e17f6a645253b25@mail.gmail.com> In-Reply-To: <20070820160853.GS2738@deviant.kiev.zoral.com.ua> References: <c019b3090708180301r1e4153av72c27bcc9aaceb2e@mail.gmail.com> <20070820160853.GS2738@deviant.kiev.zoral.com.ua>
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2007/8/20, Kostik Belousov <kostikbel@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). I know that there is no real use for this feature for native code. Yuriy
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