Subject: Re: [PATCH 1/4] kprobes: adjust kprobe addr for KPROBES_ON_FTRACE

From: Jisheng Zhang <[email-masked]>
Date: Tue, 20 Aug 2019 02:05:58 +0000

On Tue, 20 Aug 2019 09:01:30 +0900 Masami Hiramatsu wrote:

> Hi Jisheng,

Hi,

> 
> On Mon, 19 Aug 2019 11:36:09 +0000
> Jisheng Zhang <[email-masked]> wrote:
> 
> > For KPROBES_ON_FTRACE case, we need to adjust the kprobe's addr
> > correspondingly.  
> 
> No, I think you have misunderstood what the ftrace_call_adjust() does.
> Ftrace's rec->ip is already adjusted when initializing it. Kprobes
> checks the list after initialized (adjusted). So you don't need to
> adjust it again.

This is not to adjust the ftarce's rec->ip, but to adjust the struct kprobe
addr member. Because check_kprobe_address_safe()=>arch_check_ftrace_location
will check the kprobe's addr with ftrace's rec->ip. Since ftrace's rec->ip
is already adjusted, there will be mismatch if we don't adjust kprobe's addr
correspondingly.

However, this patch is wrong. I should not update the kprobe's addr
for non-ftrace-entry. Will fix this in next version.

Thanks

> 
> BTW, this type of hidden adjustment should be avoided by design.
> If you find user specifies wrong address, return error instead of
> adjust it silently.
> 
> Thank you,
> 
> >
> > Signed-off-by: Jisheng Zhang <[email-masked]>
> > ---
> >  kernel/kprobes.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 9873fc627d61..f8400753a8a9 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -1560,6 +1560,9 @@ int register_kprobe(struct kprobe *p)
> >       addr = kprobe_addr(p);
> >       if (IS_ERR(addr))
> >               return PTR_ERR(addr);
> > +#ifdef CONFIG_KPROBES_ON_FTRACE
> > +     addr = (kprobe_opcode_t *)ftrace_call_adjust((unsigned long)addr);
> > +#endif
> >       p->addr = addr;
> >
> >       ret = check_kprobe_rereg(p);
> > --
> > 2.23.0.rc1
> >  
> 
> 
> --
> Masami Hiramatsu <[email-masked]>

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