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 Cc: Peter Zijlstra peterz@infradead.org Link: https://lore.kernel.org/175852292275.307379.9040117316112640553.stgit@devnot... Signed-off-by: Masami Hiramatsu (Google) mhiramat@kernel.org Acked-by: Jiri Olsa jolsa@kernel.org Tested-by: Menglong Dong menglong8.dong@gmail.com Acked-by: Menglong Dong menglong8.dong@gmail.com Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.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..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