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

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Mar 12, 2024 at 11:30:02AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> The WARN_ON() check in __assign_str() to catch where the source variable
> to the macro doesn't match the source variable to __string() gives an
> error in clang:
> 
> >> include/trace/events/sunrpc.h:703:4: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]
>      670 |                         __assign_str(progname, "unknown");
> 
> That's because the __assign_str() macro has:
> 
>    WARN_ON_ONCE((src) != __data_offsets.dst##_ptr_);
> 
> Where "src" is a string literal. Clang warns when comparing a string
> literal directly as it is undefined to what the value of the literal is.
> 
> Since this is still to make sure the same string that goes to __string()
> is the same as __assign_str(), for string literals do a test for that and
> then use strcmp() in those cases
> 
> Note that this depends on commit 51270d573a8d ("tracing/net_sched: Fix
> tracepoints that save qdisc_dev() as a string") being applied, as this was
> what found that bug.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202402292111.KIdExylU-lkp@intel.com/
> Fixes: 433e1d88a3be ("tracing: Add warning if string in __assign_str() does not match __string()")

Is this change destined for 6.9 or 6.10? I applied it to current
trace/core (eb1533d156d3) along with 51270d573a8d but the warning is
still present. I even tried

    __builtin_choose_expr(__is_constexpr((src)),
                         strcmp((src), __data_offsets.dst##_ptr_),
                         (src) != __data_offsets.dst##_ptr_));

but not even that silenced the warning. I think we will still need a
diag directive to fully silence this warning.

> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  include/trace/stages/stage6_event_callback.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
> index a0c15f67eabe..83da83a0c14f 100644
> --- a/include/trace/stages/stage6_event_callback.h
> +++ b/include/trace/stages/stage6_event_callback.h
> @@ -35,7 +35,9 @@
>  	do {								\
>  		char *__str__ = __get_str(dst);				\
>  		int __len__ = __get_dynamic_array_len(dst) - 1;		\
> -		WARN_ON_ONCE((src) != __data_offsets.dst##_ptr_);	\
> +		WARN_ON_ONCE(__builtin_constant_p(src) ?		\
> +			     strcmp((src), __data_offsets.dst##_ptr_) :	\
> +			     (src) != __data_offsets.dst##_ptr_);	\
>  		memcpy(__str__, __data_offsets.dst##_ptr_ ? :		\
>  		       EVENT_NULL_STR, __len__);			\
>  		__str__[__len__] = '\0';				\
> -- 
> 2.43.0
>