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SubjectRe: [PATCH v3 1/2] kretprobe: produce sane stack traces
On Sun, 4 Nov 2018 22:59:13 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> The same issue is present in __save_stack_trace
> (arch/x86/kernel/stacktrace.c). This is likely the only reason that --
> as Steven said -- stacktraces wouldn't work with ftrace-graph (and thus
> with the refactor both of you are discussing).

By the way, I was playing with the the orc unwinder and stack traces
from the function graph tracer return code, and got it working with the
below patch. Caution, that patch also has a stack trace hardcoded in
the return path of the function graph tracer, so you don't want to run
function graph tracing without filtering.

You can apply the patch and do:

# cd /sys/kernel/debug/tracing
# echo schedule > set_ftrace_filter
# echo function_graph > current_tracer
# cat trace

3) | schedule() {
rcu_preempt-10 [003] .... 91.160297: <stack trace>
=> ftrace_return_to_handler
=> return_to_handler
=> schedule_timeout
=> rcu_gp_kthread
=> kthread
=> ret_from_fork
3) # 4009.085 us | }
3) | schedule() {
kworker/1:0-17 [001] .... 91.163288: <stack trace>
=> ftrace_return_to_handler
=> return_to_handler
=> worker_thread
=> kthread
=> ret_from_fork
1) # 7000.070 us | }
1) | schedule() {
rcu_preempt-10 [003] .... 91.164311: <stack trace>
=> ftrace_return_to_handler
=> return_to_handler
=> schedule_timeout
=> rcu_gp_kthread
=> kthread
=> ret_from_fork
3) # 4006.540 us | }


Where just adding the stack trace without the other code, these traces
ended at "return_to_handler".

This patch is not for inclusion, it was just a test to see how to make
this work.

-- Steve

diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 91b2cff4b79a..4bcd646ae1f4 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -320,13 +320,15 @@ ENTRY(ftrace_graph_caller)
ENDPROC(ftrace_graph_caller)

ENTRY(return_to_handler)
- UNWIND_HINT_EMPTY
- subq $24, %rsp
+ subq $8, %rsp
+ UNWIND_HINT_FUNC
+ subq $16, %rsp

/* Save the return values */
movq %rax, (%rsp)
movq %rdx, 8(%rsp)
movq %rbp, %rdi
+ leaq 16(%rsp), %rsi

call ftrace_return_to_handler

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 169b3c44ee97..aaeca73218cc 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -242,13 +242,16 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
trace->calltime = current->ret_stack[index].calltime;
trace->overrun = atomic_read(&current->trace_overrun);
trace->depth = index;
+
+ trace_dump_stack(0);
}

/*
* Send the trace to the ring-buffer.
* @return the original return address.
*/
-unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
+unsigned long ftrace_return_to_handler(unsigned long frame_pointer,
+ unsigned long *ptr)
{
struct ftrace_graph_ret trace;
unsigned long ret;
@@ -257,6 +260,8 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
trace.rettime = trace_clock_local();
barrier();
current->curr_ret_stack--;
+ *ptr = ret;
+
/*
* The curr_ret_stack can be less than -1 only if it was
* filtered out and it's about to return from the function.
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