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

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Date:   Mon, 12 Jun 2023 17:09:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ingo Molnar <mingo@...nel.org>, Song Liu <song@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2] ftrace: Allow inline functions not inlined to be traced

On Fri, Jun 09 2023 at 17:44, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.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.

I assume this passes the objtool noinstr validation. If so, if would be
helpful to document that.
>  /*
>   * 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

I'm not convinced that this is correct

>  # define __no_sanitize_or_inline __no_kasan_or_inline
>  #else

given that the !__SANITIZE_ADDRESS__ variant is:

>  # define __no_kasan_or_inline __always_inline

which cannot be traced.

> @@ -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

Ditto. 

>  #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

Spaces instead of tab.

> +	 as inline (not __always_inline) that the compiler decides to

and this one has a tab.

> +	 not inline.
> +
>  config BUILDTIME_MCOUNT_SORT
>         bool
>         default y