linux-kernel - Re: [PATCH 2/2] tracing: Ignore -Wstring-compare with diagnostic macros

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Date: Tue, 19 Mar 2024 20:37:16 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, ndesaulniers@...gle.com, morbo@...gle.com,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, patches@...ts.linux.dev, Linux Kernel Functional
 Testing <lkft@...aro.org>
Subject: Re: [PATCH 2/2] tracing: Ignore -Wstring-compare with diagnostic
 macros

On Tue, 19 Mar 2024 17:30:41 -0700
Justin Stitt <justinstitt@...gle.com> wrote:


> > diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
> > index 83da83a0c14f..56a4eea5a48e 100644
> > --- a/include/trace/stages/stage6_event_callback.h
> > +++ b/include/trace/stages/stage6_event_callback.h
> > @@ -35,9 +35,14 @@
> >         do {                                                            \
> >                 char *__str__ = __get_str(dst);                         \
> >                 int __len__ = __get_dynamic_array_len(dst) - 1;         \
> > +               __diag_push();                                          \
> > +               __diag_ignore(clang, 11, "-Wstring-compare",            \
> > +                             "__builtin_constant_p() ensures strcmp()" \
> > +                             "will be used for string literals");      \
> >                 WARN_ON_ONCE(__builtin_constant_p(src) ?                \
> >                              strcmp((src), __data_offsets.dst##_ptr_) : \
> >                              (src) != __data_offsets.dst##_ptr_);       \  
> 
> What exactly is the point of the literal string comparison? Why
> doesn't strcmp do the trick?

This is done in the hotpath, and is only for debugging. The string passed
into __string() should be the same as the string passed into __assign_str().

But this is moot as I ended up always using strcmp() even if it will slow
down the recording of the event.

Next merge window the src parameter (along with the strcmp() checks) are
going away.

-- Steve


> 
> > +               __diag_pop();                                           \
> >                 memcpy(__str__, __data_offsets.dst##_ptr_ ? :           \
> >                        EVENT_NULL_STR, __len__);                        \
> >                 __str__[__len__] = '\0';                                \
> >
> > --
> > 2.44.0
> >