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[PATCH v2 1/2] tracing: fgraph: Protect return handler from recursion loop

Masami Hiramatsu (Google) posted 2 patches 1 week, 2 days ago
[PATCH v2 1/2] tracing: fgraph: Protect return handler from recursion loop
Posted by Masami Hiramatsu (Google) 1 week, 2 days ago 
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.

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.

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>
Acked-by: Jiri Olsa <jolsa@kernel.org>
---
 Changes in v2:
  - Do not warn on failing ftrace_test_recursion_trylock() because it
    allows one-level nest.
---
 kernel/trace/fgraph.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 1e3b32b1e82c..484ad7a18463 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 can fail because ftrace_test_recursion_trylock() allows one nest
+	 * call. If we are already in a nested call, then we don't probe this and
+	 * just return the original return address.
+	 */
+	if (unlikely(bit < 0))
+		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
Re: [PATCH v2 1/2] tracing: fgraph: Protect return handler from recursion loop
Posted by Masami Hiramatsu (Google) 1 week ago 
Hi Steve,

Can you pick this ? Or I will do?

Thanks,

On Mon, 22 Sep 2025 15:35:22 +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.
> 
> 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.
> 
> 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>
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  Changes in v2:
>   - Do not warn on failing ftrace_test_recursion_trylock() because it
>     allows one-level nest.
> ---
>  kernel/trace/fgraph.c |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 1e3b32b1e82c..484ad7a18463 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 can fail because ftrace_test_recursion_trylock() allows one nest
> +	 * call. If we are already in a nested call, then we don't probe this and
> +	 * just return the original return address.
> +	 */
> +	if (unlikely(bit < 0))
> +		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>
Re: [PATCH v2 1/2] tracing: fgraph: Protect return handler from recursion loop
Posted by menglong.dong@linux.dev 1 week, 2 days ago 
On 2025/9/22 14:35 Masami Hiramatsu (Google) <mhiramat@kernel.org> write:
> 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.
> 
> 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.
> 
> 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>
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  Changes in v2:
>   - Do not warn on failing ftrace_test_recursion_trylock() because it
>     allows one-level nest.
> ---
>  kernel/trace/fgraph.c |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 1e3b32b1e82c..484ad7a18463 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;

The bpf bench testings work fine on this version :)

Tested-by: Menglong Dong <menglong8.dong@gmail.com>
Acked-by: Menglong Dong <menglong8.dong@gmail.com>

Thanks!
Menglong Dong

>  
>  	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 can fail because ftrace_test_recursion_trylock() allows one nest
> +	 * call. If we are already in a nested call, then we don't probe this and
> +	 * just return the original return address.
> +	 */
> +	if (unlikely(bit < 0))
> +		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
> 
> 
>

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