linux-kernel - Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

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Date:	Fri, 18 Dec 2009 04:06:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, utrace-devel@...hat.com
Subject: Re: x86: do_debug && PTRACE_SINGLESTEP broken by
	08d68323d1f0c34452e614263b212ca556dae47f

On Fri, Dec 18, 2009 at 01:56:50AM +0100, Oleg Nesterov wrote:
> 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);



But I don't understand why it is broken with the check.
If we are in a singlestep exception, dr6 should have its
DR_STEP bit set...

Single stepping works well for me, after a quick check on
gdb. How did you trigger the bug?

Thanks.

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