Re: [PATCH v2] ftrace: Allow inline functions not inlined to be traced

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 9 Jun 2023 17:44:22 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> Over 10 years ago there were many bugs that caused function tracing to
> crash because some inlined function was not inlined and should not have
> been traced. This made it hard to debug because when the developer tried
> to reproduce it, if their compiler still inlined the function, the bug
> would not trigger. The solution back then was simply to add "notrace" to
> "inline" which would make sure all functions that are marked inline are
> never traced even when the compiler decides to not inline them.
> 
> A lot has changed over the last 10 years.
> 
> 1) ftrace_test_recursion_trylock() is now used by all ftrace hooks which
>    will prevent the recursive crashes from happening that was caused by
>    inlined functions being traced.
> 
> 2) noinstr is now used to mark pretty much all functions that would also
>    cause problems if they are traced.
> 
> Today, it is no longer a problem if an inlined function is not inlined and
> is traced, at least on x86. Removing notrace from inline has been requested
> several times over the years. I believe it is now safe to do so.
> 
> Currently only x86 uses this.
> 
> Acked-by: Song Liu <song@kernel.org>
> Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v1: https://lore.kernel.org/linux-trace-kernel/20230502164102.1a51cdb4@gandalf.local.home
> 
>  - have it opted in by architecture. Currently only x86 adds it. (Mark Rutland)
> 
>  arch/x86/Kconfig               |  1 +
>  include/linux/compiler_types.h | 16 +++++++++++++---
>  kernel/trace/Kconfig           |  7 +++++++
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index da5c081d64a5..1ddebf832534 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -61,6 +61,7 @@ config X86
>  	select ACPI_LEGACY_TABLES_LOOKUP	if ACPI
>  	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
>  	select ARCH_32BIT_OFF_T			if X86_32
> +	select ARCH_CAN_TRACE_INLINE
>  	select ARCH_CLOCKSOURCE_INIT
>  	select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if X86_64 && HUGETLB_PAGE && MIGRATION
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 547ea1ff806e..f827e2a98500 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -169,6 +169,16 @@ struct ftrace_likely_data {
>  #define notrace			__attribute__((__no_instrument_function__))
>  #endif
>  
> +/*
> + * If all inline code not marked as __always_inline is safe to trace,
> + * then allow the architecture to do so.
> + */
> +#ifdef CONFIG_ARCH_CAN_TRACE_INLINE
> +#define __notrace_inline
> +#else
> +#define __notrace_inline	notrace
> +#endif

I think I understand what the purpose of this patch. But I'm confusing the
above change, it looks like a literal contradiction :)
I mean, __notrace_inline functions shouldn't be notrace?

What about call it __may_notrace_inline or __maybe_notrace?

Others looks good to me.

Thank you,

> +
>  /*
>   * it doesn't make sense on ARM (currently the only user of __naked)
>   * to trace naked functions because then mcount is called without
> @@ -184,7 +194,7 @@ struct ftrace_likely_data {
>   * of extern inline functions at link time.
>   * A lot of inline functions can cause havoc with function tracing.
>   */
> -#define inline inline __gnu_inline __inline_maybe_unused notrace
> +#define inline inline __gnu_inline __inline_maybe_unused __notrace_inline
>  
>  /*
>   * gcc provides both __inline__ and __inline as alternate spellings of
> @@ -230,7 +240,7 @@ struct ftrace_likely_data {
>   *     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
>   * '__maybe_unused' allows us to avoid defined-but-not-used warnings.
>   */
> -# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
> +# define __no_kasan_or_inline __no_sanitize_address __notrace_inline __maybe_unused
>  # define __no_sanitize_or_inline __no_kasan_or_inline
>  #else
>  # define __no_kasan_or_inline __always_inline
> @@ -247,7 +257,7 @@ struct ftrace_likely_data {
>   * disable all instrumentation. See Kconfig.kcsan where this is mandatory.
>   */
>  # define __no_kcsan __no_sanitize_thread __disable_sanitizer_instrumentation
> -# define __no_sanitize_or_inline __no_kcsan notrace __maybe_unused
> +# define __no_sanitize_or_inline __no_kcsan __notrace_inline __maybe_unused
>  #else
>  # define __no_kcsan
>  #endif
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index abe5c583bd59..b66ab0e6ce19 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -106,6 +106,13 @@ config HAVE_BUILDTIME_MCOUNT_SORT
>           An architecture selects this if it sorts the mcount_loc section
>  	 at build time.
>  
> +config ARCH_CAN_TRACE_INLINE
> +       bool
> +       help
> +         It is safe for an architecture to trace any function marked
> +	 as inline (not __always_inline) that the compiler decides to
> +	 not inline.
> +
>  config BUILDTIME_MCOUNT_SORT
>         bool
>         default y
> -- 
> 2.39.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>