linux-kernel - Re: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save

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Date:	Tue, 17 Jul 2012 11:08:23 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386
 function calls

(2012/07/14 3:47), Steven Rostedt wrote:
> On Thu, 2012-07-12 at 21:39 +0900, Masami Hiramatsu wrote:
> 
>> /*
>>  * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
>>  * when it traps.  The previous stack will be directly underneath the saved
>>  * registers, and 'sp/ss' won't even have been saved. Thus the '&regs->sp'.
>>  *
>>  * This is valid only for kernel mode traps.
>>  */
>> static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
>> {
>> #ifdef CONFIG_X86_32
>>         return (unsigned long)(&regs->sp);
>> #else
>>         return regs->sp;
>> #endif
>> }
> 
> I found that regs_get_register() doesn't honor this either. Thus,
> kprobes in tracing gets this:
> 
>  # echo 'p:ftrace sys_read+4 s=%sp' > /debug/tracing/kprobe_events
>  # echo 1 > /debug/tracing/events/kprobes/enable
>  # cat trace
>             sshd-1345  [000] d...   489.117168: ftrace: (sys_read+0x4/0x70) s=b7e96768
>             sshd-1345  [000] d...   489.117191: ftrace: (sys_read+0x4/0x70) s=b7e96768
>              cat-1447  [000] d...   489.117392: ftrace: (sys_read+0x4/0x70) s=5a7
>              cat-1447  [001] d...   489.118023: ftrace: (sys_read+0x4/0x70) s=b77ad05f
>             less-1448  [000] d...   489.118079: ftrace: (sys_read+0x4/0x70) s=b7762e06
>             less-1448  [000] d...   489.118117: ftrace: (sys_read+0x4/0x70) s=b7764970
> 

Yes, that is by design, since I made it so. :)
Instead of %sp, kprobe tracer provides $stack special argument
for stack address, because "sp" is not always means the stack
address on every arch.

Thanks,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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