Date: Sat, 18 Aug 2007 14:01:26 +0400 From: "Yuriy Tsibizov" <yuriy.tsibizov@gmail.com> To: hackers@freebsd.org Subject: modify syscall nr on-the-fly Message-ID: <c019b3090708180301r1e4153av72c27bcc9aaceb2e@mail.gmail.com>
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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.
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