linux-kernel - Re: [tip:x86/asm] x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP

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Date:	Fri, 27 Feb 2015 10:21:22 +0800
From:	Wang Nan <wangnan0@...wei.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, Li Zefan <lizefan@...wei.com>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/traps: Enable DEBUG_STACK after cpu_init()
 for TRAP_DB/BP

On 2015/2/26 23:12, Andy Lutomirski wrote:
> On Thu, Feb 26, 2015 at 5:12 AM, tip-bot for Wang Nan <tipbot@...or.com> wrote:
>> Commit-ID:  b4d8327024637cb2a1f7910dcb5d0ad7a096f473
>> Gitweb:     http://git.kernel.org/tip/b4d8327024637cb2a1f7910dcb5d0ad7a096f473
>> Author:     Wang Nan <wangnan0@...wei.com>
>> AuthorDate: Thu, 26 Feb 2015 13:49:39 +0800
>> Committer:  Ingo Molnar <mingo@...nel.org>
>> CommitDate: Thu, 26 Feb 2015 12:29:20 +0100
>>
>> x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP
>>
>> Before this patch early_trap_init() installs DEBUG_STACK for
>> X86_TRAP_BP and X86_TRAP_DB. However, DEBUG_STACK doesn't work
>> correctly until cpu_init() <-- trap_init().
>>
>> This patch passes 0 to set_intr_gate_ist() and
>> set_system_intr_gate_ist() instead of DEBUG_STACK to let it use
>> same stack as kernel, and installs DEBUG_STACK for them in
>> trap_init().
>>
> 
> Sorry, I'm late to the party.  This patch, while technically correct
> AFAICT, is really ugly, because the whole point is that it *doesn't*
> use ist.  In other words:
> 
>> +       set_intr_gate_ist(X86_TRAP_DB, &debug, 0);
> 
> That should be set_intr_gate(X86_TRAP_DB, &debug);
> 

I considered this, but found that set_intr_gate() and set_intr_gate_ist(..., 0)
behaviors differently: set_intr_gate() -> _trace_set_gate() requires 'trace_##addr'
to be installed to trace_idt_table, while set_intr_gate_ist(..., 0) puts 'addr'.
into trace_idt_table() through _set_gate().
Therefore, if we want to use set_intr_gate() we need to provide a trace_debug entry
for it, which will be overwritten in trap_init() so it is in fact a useless symbol.

Anyway, I'd like to post a v3 patch follow your advise. Please keep an eye on it.

Thank you!

>>         /* int3 can be called from all */
>> -       set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
>> +       set_system_intr_gate_ist(X86_TRAP_BP, &int3, 0);
> 
> Similarly, this should be set_system_gate.
> 
>>  #ifdef CONFIG_X86_32
>>         set_intr_gate(X86_TRAP_PF, page_fault);
>>  #endif
>> @@ -1005,6 +1013,15 @@ void __init trap_init(void)
>>          */
>>         cpu_init();
>>
>> +       /*
>> +        * X86_TRAP_DB and X86_TRAP_BP have been set
>> +        * in early_trap_init(). However, DEBUG_STACK works only after
>> +        * cpu_init() loads TSS. See comments in early_trap_init().
> 
> It's not that DEBUG_STACK only works after the TSS is loaded; it's
> that the IST mechanism only works after TSS is loaded.
> 
> --Andy
> 


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