Subject: Re: [PATCH v6] arm64: implement KPROBES_ON_FTRACE

From: Jisheng Zhang <[email-masked]>
Date: Wed, 25 Dec 2019 09:01:06 +0000

Hi,

On Tue, 24 Dec 2019 19:09:16 +0900 Masami Hiramatsu wrote:

> 
> Hi Jisheng,
> 
> On Mon, 23 Dec 2019 07:47:24 +0000
> Jisheng Zhang <[email-masked]> wrote:
> 
> > Hi Masami,
> >
> > On Wed, 18 Dec 2019 22:25:50 +0900 Masami Hiramatsu wrote:
> >
> >  
> > >
> > >
> > > On Wed, 18 Dec 2019 06:21:35 +0000
> > > Jisheng Zhang <[email-masked]> wrote:
> > >  
> > > > KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
> > > > eliminates the need for a trap, as well as the need to emulate or
> > > > single-step instructions.
> > > >
> > > > Tested on berlin arm64 platform.
> > > >
> > > > ~ # mount -t debugfs debugfs /sys/kernel/debug/
> > > > ~ # cd /sys/kernel/debug/
> > > > /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events
> > > >
> > > > before the patch:
> > > >
> > > > /sys/kernel/debug # cat kprobes/list
> > > > ffffff801009fe28  k  _do_fork+0x0    [DISABLED]
> > > >
> > > > after the patch:
> > > >
> > > > /sys/kernel/debug # cat kprobes/list
> > > > ffffff801009ff54  k  _do_fork+0x4    [DISABLED][FTRACE]  
> > >
> > > BTW, it seems this automatically changes the offset without
> > > user's intention or any warnings. How would you manage if the user
> > > pass a new probe on _do_fork+0x4?  
> >
> > In current implementation, two probes at the same address _do_fork+0x4  
> 
> OK, that is my point.
> 
> > > IOW, it is still the question who really wants to probe on
> > > the _do_fork+"0", if kprobes modifies it automatically,
> > > no one can do that anymore.
> > > This can be happen if the user want to record LR or SP value
> > > at the function call for debug. If kprobe always modifies it,
> > > we will lose the way to do it.  
> >
> > arm64's DYNAMIC_FTRACE_WITH_REGS implementation makes use of GCC
> > -fpatchable-function-entry=2 option to insert two nops. When the function
> > is traced, the first nop will be modified to the LR saver, then the
> > second nop to "bl <ftrace-entry>", commit 3b23e4991fb6("
> > arm64: implement ftrace with regs") explains the mechanism.  
> 
> So both of the instruction at func+0 and func+4 are replaced.
> 
> > So on arm64(in fact any arch makes use of -fpatchable-function-entry will
> > behave similarly), when DYNAMIC_FTRACE_WITH_REGS is enabled, the ftrace location
> > is always on the first 4 bytes offset.
> >
> > I think when users want to register a kprobe on _do_fork+0, what he really want
> > is to kprobe on the patched(by -fpatchable-function-entry)_do_fork+4  
> 
> OK, in this case, kprobe should treat the first 2 instructions as a
> single virtual instruction. This means,
> 
>  - kprobes can probe func+0, but not func+4 if the function is ftraced.
>     (-EILSEQ must be returned)
>  - both debugfs/kprobes/list and tracefs/kprobe_events should show the
>    probed address as func+0. Not func+4.
> 
> Then, user can use kprobes as if there is one big (8-byte) instruction
> at the entry of the function. Since probing on func+4 is rejected and
> the call-site LR/SP is restored in ftrace, there should be no
> contradiction. It should work as if we put a breakpoint on the func + 0.

>From https://lkml.org/lkml/2019/6/18/648, Naveen tried to allow probing on
any ftrace address, is it acceptable on arm64 as well? I will post patches
for this purpose.

Thanks for your review

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