linux-kernel - Re: [PATCH] ftrace: return first found result in lookup

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Message-ID: <20200306121246.5dac2573@gandalf.local.home>
Date:   Fri, 6 Mar 2020 12:12:46 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Artem Savkov <asavkov@...hat.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH] ftrace: return first found result in lookup_rec()

On Fri,  6 Mar 2020 09:10:35 +0100
Artem Savkov <asavkov@...hat.com> wrote:

> It appears that ip ranges can overlap so. In that case lookup_rec()
> returns whatever results it got last even if it found nothing in last
> searched page.
> 
> This breaks an obscure livepatch late module patching usecase:
>   - load livepatch
>   - load the patched module
>   - unload livepatch
>   - try to load livepatch again
> 
> To fix this return from lookup_rec() as soon as it found the record
> containing searched-for ip. This used to be this way prior lookup_rec()
> introduction.
> 
> Fixes: 7e16f581a817 ("ftrace: Separate out functionality from ftrace_location_range()")
> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  kernel/trace/ftrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 3f7ee102868a..b0f5ee1fd6e4 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1547,8 +1547,10 @@ static struct dyn_ftrace *lookup_rec(unsigned long start, unsigned long end)
>  		rec = bsearch(&key, pg->records, pg->index,
>  			      sizeof(struct dyn_ftrace),
>  			      ftrace_cmp_recs);

how about just adding:

		if (rec)
			break;

as that will do the same thing without adding two returns.

-- Steve

> +		if (rec)
> +			return rec;
>  	}
> -	return rec;
> +	return NULL;
>  }
>  
>  /**