linux-kernel - Re: [PATCH v3 0/6] Tracing vs CR2

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Message-ID: <009b1e7e-3278-6acb-40f7-cb5b1245a4ba@etsukata.com>
Date:   Sat, 20 Jul 2019 23:23:36 +0900
From:   Eiichi Tsukata <devel@...ukata.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Vegard Nossum <vegard.nossum@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux_lkml_grp@...cle.com, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        He Zhe <zhe.he@...driver.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v3 0/6] Tracing vs CR2


On 2019/07/20 21:49, Andy Lutomirski wrote:
> On Fri, Jul 19, 2019 at 8:59 PM Eiichi Tsukata <devel@...ukata.com> wrote:
>>
...
>>
>> ====
>>
>> debug() // dr6: 0xffff4ff0, user_mode: 1
>>   TRACE_IRQS_OFF
>>     arch_stack_user_walk()
>>       debug()  // dr6: 0xffff4ff1 == 0xffff4ff0 | 0xffff0ff1 ... (*)
>>         do_debug()
>>           WARN_ON_ONCE
>>   do_debug() // dr6: 0xffff0ff0(cleared in the above do_debug())
> 
> The dr6 register will indeed be cleared like this, but the dr6
> variable should still be 0xffff4ff0.

I should have use DR6 to mean it is a register, not variable.
"dr6" was ambiguous.

> 
>>
...
>>
>> Note: printk() in do_debug() can cause infinite loop(printk() ->
>> irq_disable() -> do_debug() -> printk() ...), so printk_deferred()
>> was preferable.
>>
> 
> Shouldn't that be fixed with my patches?  It should only be able to
> recurse two deep: do_debug() from user mode can indeed trip
> breakpoints, but the next do_debug() will clear DR7 in paranoid_entry.
> 

Sorry, I missed that. Now I confirmed your patches fixed the loop.

Thanks

Eiichi