x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

[Oleg Nesterov]

Hi.

do_debug() is obviously wrong wrt PTRACE_SINGLESTEP/TIF_SINGLESTEP, no?

Afaics this was broken by

        hw-breakpoints: modifying generic debug exception to use 
thread-specific debug registers
        commit 08d68323d1f0c34452e614263b212ca556dae47f

To verify, the "patch" below fixes the stepping for me, not sure what
is the proper fix...

Oleg.

--- arch/x86/kernel/traps.c~    2009-12-18 00:20:49.000000000 +0100
+++ arch/x86/kernel/traps.c     2009-12-18 01:44:05.000000000 +0100
@@ -575,7 +575,7 @@ dotraplinkage void __kprobes do_debug(st
                regs->flags &= ~X86_EFLAGS_TF;
        }
        si_code = get_si_code(tsk->thread.debugreg6);
-       if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS))
+//     if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS))
                send_sigtrap(tsk, regs, error_code, si_code);
        preempt_conditional_cli(regs);