Report module for PC before querying for isactivation flag

The call to dwfl_frame_pc potentially advances the internal
unwinder state when we query for the isactivation flag. This can
then lead to breakage when the about-to-be-returned PC lies in
a not-yet-reported module.

To fix this, we now ask first for the PC, which is cheap as it
just forwards a data member of the Dwfl_Frame state. Then we
ensure the module that contains this PC is properly reported to
dwfl. Only then do we ask for the isactivation flag by calling
dwfl_frame_pc a second time.

This fixes many broken backtraces. In perf upstream I have
submitted a similar fix. There, we can more easily compare the
libdw unwinder with libunwind. Here is the corresponding
output of the two:

Working libunwind and libdw with this patch applied:

~~~~~
    heaptrack_gui  2228 135073.400474:     613969 cycles:
                      108c8e [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1093bc [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      109e7b QLocale::QLocale (/usr/lib/libQt5Core.so.5.8.0)
                      1470ff [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      147f67 QSystemLocale::query (/usr/lib/libQt5Core.so.5.8.0)
                      109fbf QLocalePrivate::updateSystemPrivate (/usr/lib/libQt5Core.so.5.8.0)
                      10aa27 QLocale::QLocale (/usr/lib/libQt5Core.so.5.8.0)
                      1e02c3 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      2113bb [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      211505 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1b5df0 QFileInfo::exists (/usr/lib/libQt5Core.so.5.8.0)
                       92eb2 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                       93423 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                       93d2a QLibraryInfo::location (/usr/lib/libQt5Core.so.5.8.0)
                      2170af [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      297c53 QCoreApplicationPrivate::init (/usr/lib/libQt5Core.so.5.8.0)
                       f7cde QGuiApplicationPrivate::init (/usr/lib/libQt5Gui.so.5.8.0)
                      1589e8 QApplicationPrivate::init (/usr/lib/libQt5Widgets.so.5.8.0)
                       78622 main (/home/milian/projects/compiled/other/bin/heaptrack_gui)
                       20439 __libc_start_main (/usr/lib/libc-2.25.so)
                       78299 _start (/home/milian/projects/compiled/other/bin/heaptrack_gui)

    heaptrack_gui  2228 135073.401156:     569521 cycles:
                      131633 QString::endsWith (/usr/lib/libQt5Core.so.5.8.0)
                      1a0701 QDir::cleanPath (/usr/lib/libQt5Core.so.5.8.0)
                      21b82d [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1b3727 QFileInfo::canonicalFilePath (/usr/lib/libQt5Core.so.5.8.0)
                      2780c7 QFactoryLoader::update (/usr/lib/libQt5Core.so.5.8.0)
                      279525 QFactoryLoader::QFactoryLoader (/usr/lib/libQt5Core.so.5.8.0)
                       e5bd0 QPlatformIntegrationFactory::create (/usr/lib/libQt5Gui.so.5.8.0)
                       f5a1c QGuiApplicationPrivate::createPlatformIntegration (/usr/lib/libQt5Gui.so.5.8.0)
                       f650c QGuiApplicationPrivate::createEventDispatcher (/usr/lib/libQt5Gui.so.5.8.0)
                      298524 QCoreApplicationPrivate::init (/usr/lib/libQt5Core.so.5.8.0)
                       f7cde QGuiApplicationPrivate::init (/usr/lib/libQt5Gui.so.5.8.0)
                      1589e8 QApplicationPrivate::init (/usr/lib/libQt5Widgets.so.5.8.0)
                       78622 main (/home/milian/projects/compiled/other/bin/heaptrack_gui)
                       20439 __libc_start_main (/usr/lib/libc-2.25.so)
                       78299 _start (/home/milian/projects/compiled/other/bin/heaptrack_gui)
~~~~~

Broken libdw, i.e. status quo without this patch. Note how the first sample
is randomly missing two frames close to the end. The second stack breaks
completely.

~~~~~
    heaptrack_gui  2228 135073.400474:     613969 cycles:
                      108c8e [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1093bc [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      109e7b QLocale::QLocale (/usr/lib/libQt5Core.so.5.8.0)
                      1470ff [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      147f67 QSystemLocale::query (/usr/lib/libQt5Core.so.5.8.0)
                      109fbf QLocalePrivate::updateSystemPrivate (/usr/lib/libQt5Core.so.5.8.0)
                      10aa27 QLocale::QLocale (/usr/lib/libQt5Core.so.5.8.0)
                      1e02c3 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      2113bb [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      211505 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1b5df0 QFileInfo::exists (/usr/lib/libQt5Core.so.5.8.0)
                       92eb2 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                       93423 [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                       93d2a QLibraryInfo::location (/usr/lib/libQt5Core.so.5.8.0)
                      2170af [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      297c53 QCoreApplicationPrivate::init (/usr/lib/libQt5Core.so.5.8.0)
                       f7cde QGuiApplicationPrivate::init (/usr/lib/libQt5Gui.so.5.8.0)
                       20439 __libc_start_main (/usr/lib/libc-2.25.so)
                       78299 _start (/home/milian/projects/compiled/other/bin/heaptrack_gui)

    heaptrack_gui  2228 135073.401156:     569521 cycles:
                      131633 QString::endsWith (/usr/lib/libQt5Core.so.5.8.0)
                      1a0701 QDir::cleanPath (/usr/lib/libQt5Core.so.5.8.0)
                      21b82d [unknown] (/usr/lib/libQt5Core.so.5.8.0)
                      1b3727 QFileInfo::canonicalFilePath (/usr/lib/libQt5Core.so.5.8.0)
                      2780c7 QFactoryLoader::update (/usr/lib/libQt5Core.so.5.8.0)
                      279525 QFactoryLoader::QFactoryLoader (/usr/lib/libQt5Core.so.5.8.0)
                       e5bd0 QPlatformIntegrationFactory::create (/usr/lib/libQt5Gui.so.5.8.0)
                      723dbf [unknown] ([unknown])
~~~~~

Both issues are fixed with this patch applied.

Change-Id: Ib2a9394ab2a01a5e65860540a675c277ff15dc8c
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 13 insertions, 3 deletions

diff --git a/app/perfunwind.cpp b/app/perfunwind.cpp
index 09d17ce..5839322 100644
--- a/app/perfunwind.cpp
+++ b/app/perfunwind.cpp
@@ -300,8 +300,10 @@ static int frameCallback(Dwfl_Frame *state, void *arg)
     Dwarf_Addr pc = 0;
     PerfUnwind::UnwindInfo *ui = static_cast<PerfUnwind::UnwindInfo *>(arg);
 
-    bool isactivation;
-    if (!dwfl_frame_pc(state, &pc, &isactivation)
+    // do not query for activation directly, as this could potentially advance
+    // the unwinder internally - we must first ensure the module for the pc
+    // is reported
+    if (!dwfl_frame_pc(state, &pc, NULL)
             || (ui->maxFrames != -1 && ui->frames.length() > ui->maxFrames)
             || pc == 0) {
         ui->firstGuessedFrame = ui->frames.length();
@@ -309,9 +311,17 @@ static int frameCallback(Dwfl_Frame *state, void *arg)
         return DWARF_CB_ABORT;
     }
 
+    auto* symbolTable = ui->unwind->symbolTable(ui->sample->pid());
+
+    // ensure the module is reported
+    // if that fails, we will still try to unwind based on frame pointer
+    symbolTable->module(pc);
+
+    // now we can query for the activation flag
+    bool isactivation = false;
+    dwfl_frame_pc(state, &pc, &isactivation);
     Dwarf_Addr pc_adjusted = pc - (isactivation ? 0 : 1);
 
-    auto* symbolTable = ui->unwind->symbolTable(ui->sample->pid());
     // isKernel = false as unwinding generally only works on user code
     bool isInterworking = false;
     const auto frame = symbolTable->lookupFrame(pc_adjusted, false, &isInterworking);