Re: [PATCH] perf: fix symbols__fixup_end heuristic for corner cases

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Mar 15, 2017 at 10:53:37PM +0100, Daniel Borkmann wrote:
> 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:
> 
>  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@kernel.org>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Acked-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Alexei Starovoitov <ast@kernel.org>

thanks!