linux-kernel - Re: [PATCH] perf: fix symbols__fixup

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Message-ID: <20170316010724.GQ12825@kernel.org>
Date:   Wed, 15 Mar 2017 22:07:24 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: fix symbols__fixup_end heuristic for corner cases

Em Wed, Mar 15, 2017 at 10:53:37PM +0100, Daniel Borkmann escreveu:
> The current symbols__fixup_end() heuristic for the last entry in the
> rb tree is suboptimal as it leads to not being able to recognize the
> symbol in the call graph in a couple of corner cases, for example:

Thanks, will apply, test and push to Ingo via perf/urgent, tomorrow.

- Arnaldo
 
>  i) If the symbol has a start address (f.e. exposed via kallsyms)
>     that is at a page boundary, then the roundup(curr->start, 4096)
>     for the last entry will result in curr->start == curr->end with
>     a symbol length of zero.
> 
> ii) If the symbol has a start address that is shortly before a page
>     boundary, then also here, curr->end - curr->start will just be
>     very few bytes, where it's unrealistic that we could perform a
>     match against.
> 
> Instead, change the heuristic to roundup(curr->start, 4096) + 4096,
> so that we can catch such corner cases and have a better chance to
> find that specific symbol. It's still just best effort as the real
> end of the symbol is unknown to us (and could even be at a larger
> offset than the current range), but better than the current situation.
> 
> Alexei reported that he recently run into case i) with a JITed eBPF
> program (these are all page aligned) as the last symbol which wasn't
> properly shown in the call graph (while other eBPF program symbols
> in the rb tree were displayed correctly). Since this is a generic
> issue, lets try to improve the heuristic a bit.
> 
> Fixes: 2e538c4a1847 ("perf tools: Improve kernel/modules symbol lookup")
> Reported-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  tools/perf/util/symbol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index 70e389b..9b4d8ba 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -202,7 +202,7 @@ void symbols__fixup_end(struct rb_root *symbols)
>  
>  	/* Last entry */
>  	if (curr->end == curr->start)
> -		curr->end = roundup(curr->start, 4096);
> +		curr->end = roundup(curr->start, 4096) + 4096;
>  }
>  
>  void __map_groups__fixup_end(struct map_groups *mg, enum map_type type)
> -- 
> 1.9.3