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

[Justin Stitt <justinstitt@google.com>]

On Tue, Mar 19, 2024 at 9:08 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Commit b1afefa62ca9 ("tracing: Use strcmp() in __assign_str() WARN_ON()
> check") addressed a clang warning, -Wstring-compare, with the use of
> __builtin_constant_p() to dispatch to strcmp() if the source string is a
> string literal and a direct comparison if not. Unfortunately, even with
> this change, the warning is still present because __builtin_constant_p()
> is not evaluated at this stage of the pipeline, so clang still thinks
> the else branch could occur for this situation:
>
>   include/trace/events/sunrpc.h:705:4: error: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Werror,-Wstring-compare]
>   ...
>   include/trace/stages/stage6_event_callback.h:40:15: note: expanded from macro '__assign_str'
>      40 |                              (src) != __data_offsets.dst##_ptr_);       \
>         |                                    ^
>   ...
>
> Use the compiler diagnostic macros to disable this warning around the
> WARN_ON_ONCE() expression since a string comparison function, strcmp(),
> will always be used for the comparison of string literals.
>
> Fixes: b1afefa62ca9 ("tracing: Use strcmp() in __assign_str() WARN_ON() check")
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Closes: https://lore.kernel.org/all/CA+G9fYs=OTKAZS6g1P1Ewadfr0qoe6LgOVSohqkXmFXotEODdg@mail.gmail.com/
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  include/trace/stages/stage6_event_callback.h | 5 +++++
>  1 file changed, 5 insertions(+)
>
> 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?

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