Re: [PATCH] tracing: fgraph: Protect return handler from recursion loop

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Sat, 20 Sep 2025 09:45:15 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Fri, Sep 19, 2025 at 11:27:46AM -0400, Steven Rostedt wrote:
> > On Fri, 19 Sep 2025 20:57:36 +0900
> > "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> > 
> > > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > 
> > > function_graph_enter_regs() prevents itself from recursion by
> > > ftrace_test_recursion_trylock(), but __ftrace_return_to_handler(),
> > > which is called at the exit, does not prevent such recursion.
> > > Therefore, while it can prevent recursive calls from
> > > fgraph_ops::entryfunc(), it is not able to prevent recursive calls
> > > to fgraph from fgraph_ops::retfunc(), resulting in a recursive loop.
> > > This can lead an unexpected recursion bug reported by Menglong.
> > > 
> > >  is_endbr() is called in __ftrace_return_to_handler -> fprobe_return
> > >   -> kprobe_multi_link_exit_handler -> is_endbr.  
> > 
> > So basically its if the handler for the return part calls something that it
> > is tracing, it can trigger the recursion?
> > 
> > > 
> > > To fix this issue, acquire ftrace_test_recursion_trylock() in the
> > > __ftrace_return_to_handler() after unwind the shadow stack to mark
> > > this section must prevent recursive call of fgraph inside user-defined
> > > fgraph_ops::retfunc().
> > > 
> > > This is essentially a fix to commit 4346ba160409 ("fprobe: Rewrite
> > > fprobe on function-graph tracer"), because before that fgraph was
> > > only used from the function graph tracer. Fprobe allowed user to run
> > > any callbacks from fgraph after that commit.
> > 
> > I would actually say it's because before this commit, the return handler
> > callers never called anything that the entry handlers didn't already call.
> > If there was recursion, the entry handler would catch it (and the entry
> > tells fgraph if the exit handler should be called).
> > 
> > The difference here is with fprobes, you can have the exit handler calling
> > functions that the entry handler does not, which exposes more cases where
> > recursion could happen.
> 
> so IIUC we have return kprobe multi probe on is_endbr and now we do:
> 	
> 	is_endbr()
> 	{ -> function_graph_enter_regs installs return probe
> 	  ...
> 	} -> __ftrace_return_to_handler
> 	       fprobe_return
> 	         kprobe_multi_link_exit_handler
> 	           is_endbr
> 		   { -> function_graph_enter_regs installs return probe
> 		     ...
> 		   } -> __ftrace_return_to_handler
> 		          fprobe_return
> 		            kprobe_multi_link_exit_handler
> 			      is_endbr
> 			      { -> function_graph_enter_regs installs return probe
> 			        ...
> 			      } -> __ftrace_return_to_handler
> 			           ... recursion
> 
> 
> with the fix:
> 
> 	is_endbr()
> 	{ -> function_graph_enter_regs installs return probe
> 	  ...
> 	} -> __ftrace_return_to_handler
> 	       fprobe_return
> 	         kprobe_multi_link_exit_handler
> 	           ...
> 	           is_endbr
> 		   { ->  function_graph_enter_regs
> 		           ftrace_test_recursion_trylock fails and we do NOT install return probe
>                      ...
> 		   }
> 
> 
> there's is_endbr call also in kprobe_multi_link_handler, but it won't
> trigger recursion, because function_graph_enter_regs already uses
> ftrace_test_recursion_trylock 
> 
> 
> if above is correct then the fix looks good to me
> 
> Acked-by: Jiri Olsa <jolsa@kernel.org>

Hi Jiri,

I found ftrace_test_recursion_trylock() allows one nest level, can you
make sure it is OK?

Thank you,

> 
> thanks,
> jirka
> 
> 
> > 
> > > 
> > > Reported-by: Menglong Dong <menglong8.dong@gmail.com>
> > > Closes: https://lore.kernel.org/all/20250918120939.1706585-1-dongml2@chinatelecom.cn/
> > > Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > ---
> > >  kernel/trace/fgraph.c |   12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> > > index 1e3b32b1e82c..08dde420635b 100644
> > > --- a/kernel/trace/fgraph.c
> > > +++ b/kernel/trace/fgraph.c
> > > @@ -815,6 +815,7 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > >  	unsigned long bitmap;
> > >  	unsigned long ret;
> > >  	int offset;
> > > +	int bit;
> > >  	int i;
> > >  
> > >  	ret_stack = ftrace_pop_return_trace(&trace, &ret, frame_pointer, &offset);
> > > @@ -829,6 +830,15 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > >  	if (fregs)
> > >  		ftrace_regs_set_instruction_pointer(fregs, ret);
> > >  
> > > +	bit = ftrace_test_recursion_trylock(trace.func, ret);
> > > +	/*
> > > +	 * This must be succeeded because the entry handler returns before
> > > +	 * modifying the return address if it is nested. Anyway, we need to
> > > +	 * avoid calling user callbacks if it is nested.
> > > +	 */
> > > +	if (WARN_ON_ONCE(bit < 0))
> > 
> > I'm not so sure we need the warn on here. We should probably hook it to the
> > recursion detection infrastructure that the function tracer has.
> > 
> > The reason I would say not to have the warn on, is because we don't have a
> > warn on for recursion happening at the entry handler. Because this now is
> > exposed by fprobe allowing different routines to be called at exit than
> > what is used in entry, it can easily be triggered.
> > 
> > -- Steve
> > 
> > 
> > 
> > > +		goto out;
> > > +
> > >  #ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> > >  	trace.retval = ftrace_regs_get_return_value(fregs);
> > >  #endif
> > > @@ -852,6 +862,8 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > >  		}
> > >  	}
> > >  
> > > +	ftrace_test_recursion_unlock(bit);
> > > +out:
> > >  	/*
> > >  	 * The ftrace_graph_return() may still access the current
> > >  	 * ret_stack structure, we need to make sure the update of
> > 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>