linux-kernel - Re: [PATCH perf/core 2/4] perf-probe: Fix to handle aliased symbols in glibc

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Date:	Tue, 3 Mar 2015 22:31:24 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Naohiro Aota <naota@...sp.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH perf/core 2/4] perf-probe: Fix to handle aliased symbols
 in glibc

Hi Masami,

On Mon, Mar 02, 2015 at 09:49:53PM +0900, Masami Hiramatsu wrote:
> Fix perf probe to handle aliased symbols correctly in glibc.
> In the glibc, several symbols are defined as an alias of
> __libc_XXX, e.g. malloc is an alias of __libc_malloc.
> In such cases, dwarf has no subroutine instances of the
> alias functions (e.g. no "malloc" instance), but the map
> has that symbol and its address.
> Thus, if we search the alieased symbol in debuginfo, we
> always fail to find it, but it is in the map.
> 
> To solve this problem, this fails back to address-based
> alternative search, which searches the symbol in the map,
> translates its address to alternative (correct) function
> name by using debuginfo, and retry to find the alternative
> function point from debuginfo.
> 
> This adds fail-back process to --vars, --lines and --add
> options. So, now you can use those on malloc@...c :)

So this is only for binaries that have debuginfo, right?

I have a similar issue with no debuginfo.

  $ perf probe -x /usr/lib/libc.so.6 -V calloc
  The /usr/lib/libc-2.21.so file has no debug information.
  Rebuild with -g, or install an appropriate debuginfo package.
    Error: Failed to show vars.


But it also failed to add a probe to calloc:

  $ perf probe -x /usr/lib/libc.so.6 -a calloc
  Failed to find symbol calloc in /usr/lib/libc-2.21.so
    Error: Failed to add events.
    

Of course there's calloc in the libc binary.

  $ nm /usr/lib/libc.so.6 | grep calloc
  000000000007b1f0 t __calloc
  000000000007b1f0 T __libc_calloc
  000000000007b1f0 W calloc


I think the problem is that calloc is a weak symbol so it'll be
discarded during the symbol loading.  It's because to avoid multiple
symbols (or aliases) at a same address so we choose a better symbol
using heuristics.  But for this case I think we can allow aliases
since it's used only for finding probe points.

Thanks,
Namhyung
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