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Message-ID: <CAFhGd8q_4QfjPwGhtfOA40ZGNF7_wf3V43rAvcb_qXQnz1TFOw@mail.gmail.com>
Date: Tue, 19 Mar 2024 17:30:41 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: rostedt@...dmis.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, Mar 19, 2024 at 9:08 AM Nathan Chancellor <nathan@...nel.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@...aro.org>
> Closes: https://lore.kernel.org/all/CA+G9fYs=OTKAZS6g1P1Ewadfr0qoe6LgOVSohqkXmFXotEODdg@mail.gmail.com/
> Signed-off-by: Nathan Chancellor <nathan@...nel.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
>
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