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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Milian Wolff <milian.wolff@kdab.com>, Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.4 126/167] perf report: Ensure the perf DSO
 mapping matches what libdw sees
Thread-Topic: [PATCH AUTOSEL for 4.4 126/167] perf report: Ensure the perf DSO
 mapping matches what libdw sees
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Date:   Mon, 19 Mar 2018 16:08:00 +0000
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From: Milian Wolff <milian.wolff@kdab.com>

[ Upstream commit 2538b9e2450ae255337c04356e9e0f8cb9ec48d9 ]

In some situations the libdw unwinder stopped working properly.  I.e.
with libunwind we see:

~~~~~
heaptrack_gui  2228 135073.400112:     641314 cycles:
	            e8ed _dl_fixup (/usr/lib/ld-2.25.so)
	           15f06 _dl_runtime_resolve_sse_vex (/usr/lib/ld-2.25.so)
	           ed94c KDynamicJobTracker::KDynamicJobTracker (/home/milian/proj=
ects/compiled/kf5/lib64/libKF5KIOWidgets.so.5.35.0)
	           608f3 _GLOBAL__sub_I_kdynamicjobtracker.cpp (/home/milian/proje=
cts/compiled/kf5/lib64/libKF5KIOWidgets.so.5.35.0)
	            f199 call_init.part.0 (/usr/lib/ld-2.25.so)
	            f2a5 _dl_init (/usr/lib/ld-2.25.so)
	             db9 _dl_start_user (/usr/lib/ld-2.25.so)
~~~~~

But with libdw and without this patch this sample is not properly
unwound:

~~~~~
heaptrack_gui  2228 135073.400112:     641314 cycles:
	            e8ed _dl_fixup (/usr/lib/ld-2.25.so)
	           15f06 _dl_runtime_resolve_sse_vex (/usr/lib/ld-2.25.so)
	           ed94c KDynamicJobTracker::KDynamicJobTracker (/home/milian/proj=
ects/compiled/kf5/lib64/libKF5KIOWidgets.so.5.35.0)
~~~~~

Debug output showed me that libdw found a module for the last frame
address, but it thinks it belongs to /usr/lib/ld-2.25.so. This patch
double-checks what libdw sees and what perf knows. If the mappings
mismatch, we now report the elf known to perf. This fixes the situation
above, and the libdw unwinder produces the same stack as libunwind.

Signed-off-by: Milian Wolff <milian.wolff@kdab.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20170602143753.16907-1-milian.wolff@kdab.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 tools/perf/util/unwind-libdw.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.=
c
index 2dcfe9a7c8d0..60edec383281 100644
--- a/tools/perf/util/unwind-libdw.c
+++ b/tools/perf/util/unwind-libdw.c
@@ -37,6 +37,14 @@ static int __report_module(struct addr_location *al, u64=
 ip,
 		return 0;
=20
 	mod =3D dwfl_addrmodule(ui->dwfl, ip);
+	if (mod) {
+		Dwarf_Addr s;
+
+		dwfl_module_info(mod, NULL, &s, NULL, NULL, NULL, NULL, NULL);
+		if (s !=3D al->map->start)
+			mod =3D 0;
+	}
+
 	if (!mod)
 		mod =3D dwfl_report_elf(ui->dwfl, dso->short_name,
 				      dso->long_name, -1, al->map->start,
--=20
2.14.1