Re: [PATCH 3/3] ftrace/jprobes/x86: Fix conflict between jprobes and function graph tracing

[Steven Rostedt]

On Thu, 15 Jan 2015 20:57:29 +0900
Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> 
> > If the function tracer traces the jprobe handler, the hook function
> > for that handler will not be called, and its saved return address
> > will be used for the next function. This will result in a kernel
> > crash.
> 
> Actually, both jprobe (user-defined) handler and jprobe_return()
> doesn't execute "ret".

Yep I new that. But a notrace on jprobe_return isn't that big of a
deal. It's not a function we really need to trace, as it really doesn't
do anything but being a 'hack' for jprobes to return properly.

> So, right after run out the jprobe handler with function-graph tracer,
> on the top of its hidden stack, there are at least 2(*) unused return
> addresses, one is for jprobe handler (this should be same as the
> probed function's return address) and other is jprobe_return()'s
> return address. (*: this can be more than 2 if jprobe_return is
> called from a function which is called from jprobe handler)
> 
> So, the hidden stack may be as below;
> 
> [jprobe_return() return address]
> [probed function return address]
> [probed function return address]
> 
> After jumping back to the probed function, the function return is
> trapped by the function-graph tracer, and it uses jprobe_return()'s
> return address. Since usually jprobe_return() is called at the end of
> the function, CPU will execute "ret" soon again(if someone puts a BUG
> right after jprobe_return(), the kernel will show that BUG), and it
> returns to the caller of the probed function.
> However, there still be the probed function return address on the
> hidden stack! This means that the next "ret" will go back to the same
> address but with modified register and stacks.

Yep, I discovered all this with my patch that allows function tracing
with jprobes.

> 
> > To solve this, pause function tracing before the jprobe handler is
> > called and unpause it before it returns back to the function it
> > probed.
> 
> Agreed, but it also could drop some NMI events. That is downside.
> 
> > Some other updates:
> > 
> > Used a variable "saved_sp" to hold kcb->jprobe_saved_sp. This makes
> > the code look a bit cleaner and easier to understand (various tries
> > to fix this bug required this change).
> 
> OK.
> 
> > Note, if fentry is being used, jprobes will change the ip address
> > before the function graph tracer runs and it will not be able to
> > trace the function that the jprobe is probing.
> 
> yes, it should be fixed.
> 
> BTW, my last part of IPMODIFY patches (which is not yet merged)
> can solve this a different way. It sets IPMODIFY flag only to jprobe.
> So, if function-graph tracer sets IPMODIFY flag, user can not enable
> function-graph tracer when jprobe is used.

Nah, I don't want to stop jprobes due to function graph tracing or vice
versa.

function graph tracer doesn't change the regs->ip, so it doesn't need
the flag. But I sent out a patch that fixes this for this case. Let me
know what you think of that one.


> 
> https://lkml.org/lkml/2014/10/9/210
> 
> Anyway, this patch is better to go stable trees.
> 
> Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> 

Thanks!

-- Steve
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