[PATCH] SYSEMU: fix sysaudit / singlestep interaction

      Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

This is simply an adjustment for "Ptrace - i386: fix Syscall Audit interaction
with singlestep" to work on top of SYSEMU patches, too.  On this patch, I have
some doubts: I wonder why we need to alter that way ptrace_disable().

I left the patch this way because it has been extensively tested, but I don't
understand the reason.

The current PTRACE_DETACH handling simply clears child->ptrace; actually this
is not enough because entry.S just looks at the thread_flags; actually,
do_syscall_trace checks current->ptrace but I don't think depending on that is
good, at least for performance, so I think the clearing is done elsewhere.
For instance, on PTRACE_CONT it's done, but doing PTRACE_DETACH without
PTRACE_CONT is possible (and happens when gdb crashes and one kills it
manually).
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
CC: Roland McGrath <roland@redhat.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/ptrace.c

@@ -271,6 +271,8 @@ static void clear_singlestep(struct task_struct *child)
 void ptrace_disable(struct task_struct *child)
 { 
 	clear_singlestep(child);
+	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+	clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
 }
 
 /*
@@ -693,14 +695,29 @@ __attribute__((regparm(3)))
 int do_syscall_trace(struct pt_regs *regs, int entryexit)
 {
 	int is_sysemu = test_thread_flag(TIF_SYSCALL_EMU), ret = 0;
-	/* With TIF_SYSCALL_EMU set we want to ignore TIF_SINGLESTEP */
+	/* With TIF_SYSCALL_EMU set we want to ignore TIF_SINGLESTEP for syscall
+	 * interception. */
 	int is_singlestep = !is_sysemu && test_thread_flag(TIF_SINGLESTEP);
 
 	/* do the secure computing check first */
 	secure_computing(regs->orig_eax);
 
-	if (unlikely(current->audit_context) && entryexit)
+	if (unlikely(current->audit_context)) {
+		if (entryexit)
 			audit_syscall_exit(current, AUDITSC_RESULT(regs->eax), regs->eax);
+		/* Debug traps, when using PTRACE_SINGLESTEP, must be sent only
+		 * on the syscall exit path. Normally, when TIF_SYSCALL_AUDIT is
+		 * not used, entry.S will call us only on syscall exit, not
+		 * entry; so when TIF_SYSCALL_AUDIT is used we must avoid
+		 * calling send_sigtrap() on syscall entry.
+		 *
+		 * Note that when PTRACE_SYSEMU_SINGLESTEP is used,
+		 * is_singlestep is false, despite his name, so we will still do
+		 * the correct thing.
+		 */
+		else if (is_singlestep)
+			goto out;
+	}
 
 	if (!(current->ptrace & PT_PTRACED))
 		goto out;