linux-kernel - RE: [PATCH] perf symbols: Cannot disassemble some routines when debuginfo present

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Date:   Fri, 23 Nov 2018 10:24:21 -0800 (PST)
From:   Eric Saint Etienne <eric.saint.etienne@...cle.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Darren Kenny <darren.kenny@...cle.com>,
        Eric Saint-Etienne <eric.saintetienne@...il.com>
Subject: RE: [PATCH] perf symbols: Cannot disassemble some routines when
 debuginfo present

> > +		/*
> > +		 * When using -ffunction-sections, only .text gets loaded by
> > +		 * map_groups__find() into al->map. Consequently al->map address
> > +		 * range encompass the whole code.
> > +		 *
> > +		 * But map__load() has just loaded many function maps by
> > +		 * splitting al->map, which reduced al->map range drastically.
> > +		 * Very likely the target address is now in one of those newly
> > +		 * created function maps, so we need to lookup the map again
> > +		 * to find that new map.
> > +		 */
> 
> hum, so map__load actualy can split the map to create new maps?
> 
> cold you please point me to that code? I haven't touch this area for some
> time and I can't find it

The split happens in dso_process_kernel_symbol() in symbol-elf.c where we
call map_groups__find_by_name() to find an existing map, but with
-ffunction-sections and a symbol belonging to new (function) map, such map
doesn't exist yet so we end up creating one and adjusting existing maps
accordingly because adjust_kernel_syms is set. Makes sense?

As of 4.20-rc3 the call chain is as follows:
event:c:1573	   map__load()
map.c:315	   dso__load()
symobl.c:1528	   dso__load_kernel_sym()
symbol.c:1896	   dso__load_vmlinux_path()
		   (or we directly call dso__load_vmlinux() at line 1892)
symbol.c:1744	   dso__load_vmlinux()
symbol.c:1719	   dso__load_sym()
symbol-elf.c:1090  dso_process_kernel_symbol()

-eric