[uml-devel] Re: [patch 1/1] SYSEMU: avoid intercepting syscall on return when using SYSCALL again.

[Bodo Stroesser <bstroesser@fujitsu-siemens.com>]

blaisorblade_spam@yahoo.it wrote:
> From: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
> 
> A guest process switching from using PTRACE_SYSEMU to PTRACE_SYSCALL crashes.
> 
> The problem is in arch/i386/kernel/entry.S. The current SYSEMU patch inhibits
> the syscall-handler to be called, but does not prevent do_syscall_trace to be
> called after this for syscall completion interception.
> 
> The appended patch fixes this. It reuses the flag TIF_SYSCALL_EMU to remember
> "we come from PTRACE_SYSEMU and now are in PTRACE_SYSCALL", since the flag is
> unused in the depicted situation.
> 
> The patch is tested, AFAICS, it works fine, i.e. sysemu can be switched on and
> off dynamically without crash.
> 
> Bodo
> 
> Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
> ---
> 
Fine idea to move one part of the patch to sys_ptrace()!
But maybe you missed to reset TIF_SYSCALL_EMU while resuming with PTRACE_CONT?
Here is a revised patch.

Bodo

---

--- a/arch/i386/kernel/ptrace.c	2004-10-20 16:57:25.000000000 +0200
+++ b/arch/i386/kernel/ptrace.c	2004-10-22 10:57:15.460287887 +0200
@@ -366,16 +366,21 @@ asmlinkage int sys_ptrace(long request,
  		ret = -EIO;
  		if ((unsigned long) data > _NSIG)
  			break;
+		/* If we came here with PTRACE_SYSEMU and now continue with
+		 * PTRACE_SYSCALL, entry.S used to intercept the syscall return. But it
+		 * shouldn't!
+		 * So we don't clear TIF_SYSCALL_EMU, which is always unused in this
+		 * special case, to remember, we came from SYSEMU. That flag
+		 * will be cleared by do_syscall_trace().
+		 */
  		if (request == PTRACE_SYSEMU) {
  			set_tsk_thread_flag(child, TIF_SYSCALL_EMU);
-		}
-		else {
+		} else if (request == PTRACE_CONT) {
  			clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
  		}
  		if (request == PTRACE_SYSCALL) {
  			set_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
-		}
-		else {
+		} else {
  			clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
  		}
  		child->exit_code = data;
@@ -585,7 +590,7 @@ out:
  __attribute__((regparm(3)))
  int do_syscall_trace(struct pt_regs *regs, int entryexit)
  {
-	int is_sysemu;
+	int is_sysemu, is_systrace;
  	if (unlikely(current->audit_context)) {
  		if (!entryexit)
  			audit_syscall_entry(current, regs->orig_eax,
@@ -595,9 +600,19 @@ int do_syscall_trace(struct pt_regs *reg
  			audit_syscall_exit(current, regs->eax);
  	}
  	is_sysemu = test_thread_flag(TIF_SYSCALL_EMU);
+	is_systrace = test_thread_flag(TIF_SYSCALL_TRACE);

-	if (!test_thread_flag(TIF_SYSCALL_TRACE) && !is_sysemu)
+	if (!is_systrace && !is_sysemu)
  		return 0;
+	/* We can detect the case of coming from PTRACE_SYSEMU and now
+	 * running with PTRACE_SYSCALL, by TIF_SYSCALL_EMU being set
+	 * additionally.
+	 * If so let's reset the flag and return without action.
+	 */
+	if (is_sysemu && is_systrace) {
+		clear_thread_flag(TIF_SYSCALL_EMU);
+		return 0;
+	}
  	if (!(current->ptrace & PT_PTRACED))
  		return 0;
  	/* the 0x80 provides a way for the tracing parent to distinguish


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