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Message-ID: <5012D25B.3040302@linutronix.de>
Date: Fri, 27 Jul 2012 19:39:39 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>
CC: linux-kernel@...r.kernel.org,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...hat.com>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] uprobes: don't enable/disable signle step if the user
did it
On 07/26/2012 07:31 PM, Oleg Nesterov wrote:
> Well. I agree, this needs changes. To begin with, uprobe should avoid
> user_enable_single_step() which does access_process_vm(). And I suspect
> uprobes have the problems with TIF_FORCED_TF logic.
Why? Shouldn't wee keep the trap flag if the instruction on which we
placed the uprobe activates it?
>
> But I am not sure about this patch...
>
> On 07/26, Sebastian Andrzej Siewior wrote:
>>
>> @@ -1528,7 +1528,10 @@ static void handle_swbp(struct pt_regs *regs)
>>
>> utask->state = UTASK_SSTEP;
>> if (!pre_ssout(uprobe, regs, bp_vaddr)) {
>> - user_enable_single_step(current);
>> + if (test_tsk_thread_flag(current, TIF_SINGLESTEP))
>> + uprobe->flags |= UPROBE_USER_SSTEP;
>> + else
>> + user_enable_single_step(current);
>
> This is x86 specific, TIF_SINGLESTEP is not defined on every arch.
It is not defined on every arch but I wouldn't say it is 86 specific.
From the architectures which have user_enable_single_step() defined I
see
avr32 TIF_SINGLE_STEP
m68k TIF_DELAYED_TRACE
s390 TIF_SINGLE_STEP
which means those three could rename their flag so things are
consistent. The remaining architectures are
alpha
cris
h8300
score
and they don't set a flag and it seems they change the register
directly.
>
>> @@ -1569,7 +1572,10 @@ static void handle_singlestep(struct uprobe_task *utask, struct pt_regs *regs)
>> put_uprobe(uprobe);
>> utask->active_uprobe = NULL;
>> utask->state = UTASK_RUNNING;
>> - user_disable_single_step(current);
>> + if (uprobe->flags& UPROBE_USER_SSTEP)
>> + uprobe->flags&= ~UPROBE_USER_SSTEP;
>> + else
>> + user_disable_single_step(current);
>
> This is not enough (and I am not sure this is portable).
>
> If SINGLESTEP was set, we should send SIGTRAP here. With this patch
> we return with X86_EFLAGS_TF set, gdb will be notified only after the
> next insn. And if we notify gdb, there is no need to keep X86_EFLAGS_TF.
Sending SIGTRAP is, yes.
> I'm afraid this needs more thinking and new arch-dependant helpers.
>
> Oleg.
>
Sebastian
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