linux-kernel - Re: [PATCH 5/5] kernel: tracepoints: add support for relative references

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Date:   Mon, 14 Aug 2017 11:23:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Garnier <thgarnie@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Pitre <nico@...aro.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH 5/5] kernel: tracepoints: add support for relative
 references

On Mon, 14 Aug 2017 11:52:31 +0100
Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> To avoid the need for relocating absolute references to tracepoint
> structures at boot time when running relocatable kernels (which may
> take a disproportionate amount of space), add the option to emit
> these tables as relative references instead.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  include/linux/tracepoint.h | 19 +++++++++++++++----
>  kernel/tracepoint.c        | 19 ++++++++++++++-----
>  2 files changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index a26ffbe09e71..d02bf1a695e8 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -228,6 +228,19 @@ extern void syscall_unregfunc(void);
>  		return static_key_false(&__tracepoint_##name.key);	\
>  	}
>  
> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> +#define __TRACEPOINT_ENTRY(name)					 \
> +	asm("	.section \"__tracepoints_ptrs\", \"a\"		     \n" \
> +	    "	.balign 4					     \n" \
> +	    "	.long " VMLINUX_SYMBOL_STR(__tracepoint_##name) " - .\n" \
> +	    "	.previous					     \n")
> +#else
> +#define __TRACEPOINT_ENTRY(name)					 \
> +	static struct tracepoint * const __tracepoint_ptr_##name __used	 \
> +	__attribute__((section("__tracepoints_ptrs"))) =		 \
> +		&__tracepoint_##name
> +#endif
> +
>  /*
>   * We have no guarantee that gcc and the linker won't up-align the tracepoint
>   * structures, so we create an array of pointers that will be used for iteration
> @@ -237,11 +250,9 @@ extern void syscall_unregfunc(void);
>  	static const char __tpstrtab_##name[]				 \
>  	__attribute__((section("__tracepoints_strings"))) = #name;	 \
>  	struct tracepoint __tracepoint_##name				 \
> -	__attribute__((section("__tracepoints"))) =			 \
> +	__attribute__((section("__tracepoints"), used)) =		 \
>  		{ __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
> -	static struct tracepoint * const __tracepoint_ptr_##name __used	 \
> -	__attribute__((section("__tracepoints_ptrs"))) =		 \
> -		&__tracepoint_##name;
> +	__TRACEPOINT_ENTRY(name);
>  
>  #define DEFINE_TRACE(name)						\
>  	DEFINE_TRACE_FN(name, NULL, NULL);
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 685c50ae6300..1c6689603764 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -28,8 +28,8 @@
>  #include <linux/sched/task.h>
>  #include <linux/static_key.h>
>  
> -extern struct tracepoint * const __start___tracepoints_ptrs[];
> -extern struct tracepoint * const __stop___tracepoints_ptrs[];
> +extern const unsigned char __start___tracepoints_ptrs[];
> +extern const unsigned char __stop___tracepoints_ptrs[];

Does this have to be converted to a char array?

>  
>  /* Set to 1 to enable tracepoint debug output */
>  static const int tracepoint_debug;
> @@ -503,17 +503,26 @@ static __init int init_tracepoints(void)
>  __initcall(init_tracepoints);
>  #endif /* CONFIG_MODULES */
>  
> -static void for_each_tracepoint_range(struct tracepoint * const *begin,
> -		struct tracepoint * const *end,
> +static void for_each_tracepoint_range(const void *begin, const void *end,
>  		void (*fct)(struct tracepoint *tp, void *priv),
>  		void *priv)
>  {
> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> +	const signed int *iter;
> +
> +	if (!begin)
> +		return;
> +	for (iter = begin; iter < (signed int *)end; iter++) {

Isn't the typecasts here sufficient, even if the above end is defined
as a tracepoint * const *?

-- Steve

> +		fct((struct tracepoint *)((unsigned long)iter + *iter), priv);
> +	}
> +#else
>  	struct tracepoint * const *iter;
>  
>  	if (!begin)
>  		return;
> -	for (iter = begin; iter < end; iter++)
> +	for (iter = begin; iter < (struct tracepoint * const *)end; iter++)
>  		fct(*iter, priv);
> +#endif
>  }
>  
>  /**