Re: [PATCH] tracing: Use strcmp() in __assign_str() WARN_ON() check

[Steven Rostedt]

On Wed, 13 Mar 2024 13:45:50 -0400
Steven Rostedt <rost...@goodmis.org> wrote:

> Let me test to make sure that when src is a string "like this" that it does
> the strcmp(). Otherwise, we may have to always do the strcmp(), which I
> really would like to avoid.

I added the below patch and enabled sched_switch and it triggered the
warning (expected if it went the strcmp() path). I then changed it to be:

#define __assign_str(dst, src)                                          \
        do {                                                            \
                char *__str__ = __get_str(dst);                         \
                int __len__ = __get_dynamic_array_len(dst) - 1;         \
                WARN_ON_ONCE(__builtin_constant_p(src) ?                \
                             strcmp((src), __data_offsets.dst##_ptr_) : \
-                            (src) != __data_offsets.dst##_ptr_);       \
+                            (src) == __data_offsets.dst##_ptr_);       \
                memcpy(__str__, __data_offsets.dst##_ptr_ ? :           \
                       EVENT_NULL_STR, __len__);                        \
                __str__[__len__] = '\0';                                \
        } while (0)

And the sched_switch did not trigger (expected). So it seems that it should
not be a problem.

Note, I only tested this with gcc and not clang.

But I guess there's also the case where we have:

        __assign_str(str, field ? field : "NULL")

But hopefully that's not an issue.

-- Steve

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index dbb01b4b7451..eaacd0c4e899 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -236,6 +236,7 @@ TRACE_EVENT(sched_switch,
                __array(        char,   next_comm,      TASK_COMM_LEN   )
                __field(        pid_t,  next_pid                        )
                __field(        int,    next_prio                       )
+               __string( test, "this")
        ),
 
        TP_fast_assign(
@@ -246,6 +247,7 @@ TRACE_EVENT(sched_switch,
                memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
                __entry->next_pid       = next->pid;
                __entry->next_prio      = next->prio;
+               __assign_str(test, "this");
                /* XXX SCHED_DEADLINE */
        ),
 
diff --git a/include/trace/stages/stage6_event_callback.h 
b/include/trace/stages/stage6_event_callback.h
index 83da83a0c14f..cf301c723fd0 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -36,7 +36,7 @@
                char *__str__ = __get_str(dst);                         \
                int __len__ = __get_dynamic_array_len(dst) - 1;         \
                WARN_ON_ONCE(__builtin_constant_p(src) ?                \
-                            strcmp((src), __data_offsets.dst##_ptr_) : \
+                            !strcmp((src), __data_offsets.dst##_ptr_) :        
\
                             (src) != __data_offsets.dst##_ptr_);       \
                memcpy(__str__, __data_offsets.dst##_ptr_ ? :           \
                       EVENT_NULL_STR, __len__);                        \