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From: Huang Pei <huangpei@loongson.cn>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: ambrosehua@gmail.com, Bibo Mao <maobibo@loongson.cn>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mips@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-mm@kvack.org, Jiaxun Yang <jiaxun.yang@flygoat.com>,
	Paul Burton <paulburton@kernel.org>,
	Li Xuefeng <lixuefeng@loongson.cn>,
	Yang Tiezhu <yangtiezhu@loongson.cn>,
	Gao Juxin <gaojuxin@loongson.cn>,
	Fuxin Zhang <zhangfx@lemote.com>, Huacai Chen <chenhc@lemote.com>
Subject: Re: [PATCH] MIPS: fix kernel_stack_pointer()
Date: Tue, 2 Feb 2021 09:32:31 +0800	[thread overview]
Message-ID: <20210202013231.wzyb7clsu7jsze4v@ambrosehua-HP-xw6600-Workstation> (raw)
In-Reply-To: <20210201122352.GA8095@alpha.franken.de>

On Mon, Feb 01, 2021 at 01:23:52PM +0100, Thomas Bogendoerfer wrote:
> On Fri, Jan 29, 2021 at 12:35:07PM +0800, Huang Pei wrote:
> > MIPS always save kernel stack pointer in regs[29]
> > 
> > Signed-off-by: Huang Pei <huangpei@loongson.cn>
> > ---
> >  arch/mips/include/asm/ptrace.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/mips/include/asm/ptrace.h b/arch/mips/include/asm/ptrace.h
> > index 1e76774b36dd..daf3cf244ea9 100644
> > --- a/arch/mips/include/asm/ptrace.h
> > +++ b/arch/mips/include/asm/ptrace.h
> > @@ -53,7 +53,7 @@ struct pt_regs {
> >  
> >  static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
> >  {
> > -	return regs->regs[31];
> > +	return regs->regs[29];
> 
> hmm, I'm still wondering where the trick is... looks like this is used
> for uprobes, so nobody has ever used uprobes or I'm missing something.
> 
> How did you find that ?
> 
> Thomas.
> 
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]


Long story for short, 

+. I think I had fix this bug in 2018, when I backported Uprobe from my
4.4 branch to CentOS 7 3.10. I just knwo it is *not* following MIPS
ABI, but I do not know how it destroy the cool function of
Kprobe/Uprobe, since the failure in porting eBPF from upstream to 3.10
just leave the fix in 3.10, totally forgotten.

+. In 2020, I was told to validate the effect of GNU XHash, and it came
to me that using Uprobe to count the number of "strcmp" called in ld.so,
so I found this fix again.

+. With more work on Kprobe/Kprobe_event/Uprobe, I found it hit only when
accessing arguments of Kprobe/Uprobe, so simple counting numbers of probe
fired would not trigger it



  reply	other threads:[~2021-02-02  1:32 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-29  4:35 Huang Pei
2021-02-01 12:23 ` Thomas Bogendoerfer
2021-02-02  1:32   ` Huang Pei [this message]
2021-02-03 10:36     ` Thomas Bogendoerfer

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