On Tue, Mar 19, 2024 at 9:08 AM Nathan Chancellor <nat...@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 <l...@linaro.org>
> Closes:
> https://lore.kernel.org/all/CA+G9fYs=otkazs6g1p1ewadfr0qoe6lgovsohqkxmfxoteo...@mail.gmail.com/
> Signed-off-by: Nathan Chancellor <nat...@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
>
