linux-kernel - Re: [RFC 01/16] ftrace: Add function to find fentry of function

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Date:	Wed, 30 Apr 2014 10:48:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	linux-kernel@...r.kernel.org, jirislaby@...il.com,
	Vojtech Pavlik <vojtech@...e.cz>, Michael Matz <matz@...e.de>,
	Jiri Kosina <jkosina@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC 01/16] ftrace: Add function to find fentry of function

On Wed, 30 Apr 2014 16:30:34 +0200
Jiri Slaby <jslaby@...e.cz> wrote:

> This is needed for kgr to find fentry location to be "ftraced". We use
> this to find a place where to jump to a new/old code location.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> ---
>  include/linux/ftrace.h |  1 +
>  kernel/trace/ftrace.c  | 29 +++++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index ae9504b4b67d..8b447493b6a5 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -299,6 +299,7 @@ extern void
>  unregister_ftrace_function_probe_func(char *glob, struct ftrace_probe_ops *ops);
>  extern void unregister_ftrace_function_probe_all(char *glob);
>  
> +extern unsigned long ftrace_function_to_fentry(unsigned long addr);
>  extern int ftrace_text_reserved(const void *start, const void *end);
>  
>  extern int ftrace_nr_registered_ops(void);
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 4a54a25afa2f..9968695cdcf9 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1495,6 +1495,35 @@ ftrace_ops_test(struct ftrace_ops *ops, unsigned long ip, void *regs)
>  		}				\
>  	}
>  
> +/**
> + * ftrace_function_to_fentry -- lookup fentry location for a function
> + * @addr: function address to find a fentry in
> + *
> + * Perform a lookup in a list of fentry callsites to find one that fits a
> + * specified function @addr. It returns the corresponding fentry callsite or
> + * zero on failure.
> + */
> +unsigned long ftrace_function_to_fentry(unsigned long addr)
> +{
> +	const struct dyn_ftrace *rec;
> +	const struct ftrace_page *pg;
> +	unsigned long ret = 0;
> +
> +	mutex_lock(&ftrace_lock);
> +	do_for_each_ftrace_rec(pg, rec) {

The records are sorted within a pg. You can optimize this a lot if you
just test the first and last record and see if it is in the range. If
not, then skip to the next page. If it is, you can use a bsearch as
well, to save on the lookups.

-- Steve

> +		unsigned long off;
> +		if (!kallsyms_lookup_size_offset(rec->ip, NULL, &off))
> +			continue;
> +		if (addr + off == rec->ip) {
> +			ret = rec->ip;
> +			goto end;
> +		}
> +	} while_for_each_ftrace_rec()
> +end:
> +	mutex_unlock(&ftrace_lock);
> +
> +	return ret;
> +}
>  
>  static int ftrace_cmp_recs(const void *a, const void *b)
>  {

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