[binutils-gdb] gdb/unwinders: better support for $pc not saved

[binutils-gdb] gdb/unwinders: better support for $pc not saved

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=5eb2254a1d1d69980601e6530ee63e0e0e6a6c33

commit 5eb2254a1d1d69980601e6530ee63e0e0e6a6c33
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Wed Jan 24 13:52:59 2024 +0000

    gdb/unwinders: better support for $pc not saved
    
    This started with a Red Hat bug report which can be seen here:
    
      https://bugzilla.redhat.com/show_bug.cgi?id=1850710
    
    The problem reported here was using GDB on GNU/Linux for S390, the
    user stepped into JIT generated code.  As they enter the JIT code GDB
    would report 'PC not saved', and this same message would be reported
    after each step/stepi.
    
    Additionally, the user had 'set disassemble-next-line on', and once
    they entered the JIT code this output was not displayed, nor were any
    'display' directives displayed.
    
    The user is not making use of the JIT plugin API to provide debug
    information.  But that's OK, they aren't expecting any source level
    debug here, they are happy to use 'stepi', but the missing 'display'
    directives are a problem, as is the constant 'PC not saved' (error)
    message.
    
    What is happening here is that as GDB is failing to find any debug
    information for the JIT generated code, it is falling back on to the
    S390 prologue unwinder to try and unwind frame #0.  Unfortunately,
    without being able to identify the function boundaries, the S390
    prologue scanner can't help much, in fact, it doesn't even suggest an
    arbitrary previous $pc value (some targets that use a link-register
    will, by default, assume the link-register contains the previous $pc),
    instead the S390 will just say, "sorry, I have no previous $pc value".
    
    The result of this is that when GDB tries to find frame #1 we end
    throwing an error from frame_unwind_pc (the 'PC not saved' error).
    This error is not caught anywhere except at the top-level interpreter
    loop, and so we end up skipping all the 'display' directive handling.
    
    While thinking about this, I wondered, could I trigger the same error
    using the Python Unwinder API?  What happens if a Python unwinder
    claims a frame, but then fails to provide a previous $pc value?
    
    Turns out that exactly the same thing happens, which is great, as that
    means we now have a way to reproduce this bug on any target.  And so
    the test included with this patch does just this.  I have a Python
    unwinder that claims a frame, but doesn't provide any previous
    register values.
    
    I then do two tests, first I stop in the claimed frame (i.e. frame #0
    is the frame that can't be unwound), I perform a few steps, and check
    the backtrace.  And second, I stop in a child of the problem
    frame (i.e. frame #1 is the frame that can't be unwound), and from
    here I check the backtrace.
    
    While all this is going on I have a 'display' directive in place, and
    each time GDB stops I check that the display directive triggers.
    
    Additionally, when checking the backtrace, I am checking that the
    backtrace finishes with the message 'Backtrace stopped: frame did not
    save the PC'.
    
    As for the fix I chose to add a call to frame_unwind_pc directly to
    get_prev_frame_always_1.  Calling frame_unwind_pc will cache the
    unwound $pc value, so this doesn't add much additional work as
    immediately after the new frame_unwind_pc call, we call
    get_prev_frame_maybe_check_cycle, which actually generates the
    previous frame, which will always (I think) require a call to
    frame_unwind_pc anyway.
    
    The reason for adding the frame_unwind_pc call into
    get_prev_frame_always_1, is that if the frame_unwind_pc call fails we
    want to set the frames 'stop_reason', and get_prev_frame_always_1
    seems to be the place where this is done, so I wanted to keep the new
    stop_reason setting code next to all the existing stop_reason setting
    code.
    
    Additionally, once we enter get_prev_frame_maybe_check_cycle we
    actually create the previous frame, then, if it turns out that the
    previous frame can't be created we need to remove the frame .. this
    seemed more complex than just making the check in
    get_prev_frame_always_1.
    
    With this fix in place the original S390 bug is fixed, and also the
    test added in this commit, that uses the Python API, is also fixed.
    
    Reviewed-By: Kevin Buettner <kevinb@redhat.com>

Diff:
---
 gdb/frame.c                             |  32 +++++++++
 gdb/testsuite/gdb.base/pc-not-saved.c   |  48 ++++++++++++++
 gdb/testsuite/gdb.base/pc-not-saved.exp | 113 ++++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.base/pc-not-saved.py  |  71 ++++++++++++++++++++
 4 files changed, 264 insertions(+)

diff --git a/gdb/frame.c b/gdb/frame.c
index 5c7aae9edf4..d3c4c9640aa 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -2423,6 +2423,38 @@ get_prev_frame_always_1 (const frame_info_ptr &this_frame)
 	}
     }
 
+  /* Ensure we can unwind the program counter of THIS_FRAME.  */
+  try
+    {
+      /* Calling frame_unwind_pc for the sentinel frame relies on the
+	 current_frame being set, which at this point it might not be if we
+	 are in the process of setting the current_frame after a stop (see
+	 get_current_frame).
+
+	 The point of this check is to ensure that the unwinder for
+	 THIS_FRAME can actually unwind the $pc, which we assume the
+	 sentinel frame unwinder can always do (it's just a read from the
+	 machine state), so we only call frame_unwind_pc for frames other
+	 than the sentinel (level -1) frame.
+
+	 Additionally, we don't actually care about the value of the
+	 unwound $pc, just that the call completed successfully.  */
+      if (this_frame->level >= 0)
+	frame_unwind_pc (this_frame);
+    }
+  catch (const gdb_exception_error &ex)
+    {
+      if (ex.error == NOT_AVAILABLE_ERROR || ex.error == OPTIMIZED_OUT_ERROR)
+	{
+	  frame_debug_printf ("  -> nullptr // no saved PC");
+	  this_frame->stop_reason = UNWIND_NO_SAVED_PC;
+	  this_frame->prev = nullptr;
+	  return nullptr;
+	}
+
+      throw;
+    }
+
   return get_prev_frame_maybe_check_cycle (this_frame);
 }
 
diff --git a/gdb/testsuite/gdb.base/pc-not-saved.c b/gdb/testsuite/gdb.base/pc-not-saved.c
new file mode 100644
index 00000000000..bc6632a97d7
--- /dev/null
+++ b/gdb/testsuite/gdb.base/pc-not-saved.c
@@ -0,0 +1,48 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2024 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+volatile int global_var = 0;
+
+void
+other_func (void)
+{
+  /* Nothing.  */
+}
+
+void
+break_bt_here (void)
+{
+  /* This is all nonsense; just filler so this function has a body.  */
+  if (global_var != 99)
+    global_var++;
+  if (global_var != 98)
+    global_var++;
+  if (global_var != 97)
+    global_var++;
+  if (global_var != 96)
+    global_var++;
+  other_func ();
+  if (global_var != 95)
+    global_var++;
+}
+
+int
+main (void)
+{
+  break_bt_here ();
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/pc-not-saved.exp b/gdb/testsuite/gdb.base/pc-not-saved.exp
new file mode 100644
index 00000000000..f267a269f1a
--- /dev/null
+++ b/gdb/testsuite/gdb.base/pc-not-saved.exp
@@ -0,0 +1,113 @@
+# Copyright 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test how GDB handles a frame in which the previous-pc value is not
+# available.  Specifically, check that the backtrace correctly reports
+# why the backtrace is truncated, and ensure that 'display' directives
+# still work when 'stepi'-ing through the frame.
+#
+# We do this by registering a Python unwinder which doesn't provide
+# any previous register values.
+
+require allow_python_tests
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
+    return
+}
+
+set remote_python_file \
+    [gdb_remote_download host "${srcdir}/${subdir}/${testfile}.py"]
+
+if { ![runto "break_bt_here"] } {
+    return
+}
+
+# Figuring out the correct frame-id from a Python unwinder is hard.
+# We need to know the function's start address (not too hard), and the
+# stack address on entry to the function, which is much harder to
+# figure out in a cross-target way.
+#
+# So instead we run without any Python unwinder in place and use
+# 'maint print frame-id' to record the frame-id.  We then restart GDB,
+# load the Python unwinder, and tell it to use the frame-id we
+# recorded here.
+set pc unknown
+set cfa unknown
+gdb_test_multiple "maintenance print frame-id" "store break_bt_here frame-id" {
+    -re -wrap "frame-id for frame #0: \\{stack=($hex),code=($hex),\[^\}\]+\\}" {
+	set cfa $expect_out(1,string)
+	set pc $expect_out(1,string)
+    }
+}
+gdb_assert { ![string equal $cfa unknown] } \
+    "check we read the frame's CFA"
+
+gdb_assert { ![string equal $pc unknown] } \
+    "check we read the frame's PC"
+
+# Restart and load the Python unwinder script.
+clean_restart $binfile
+gdb_test_no_output "source ${remote_python_file}" "load python file"
+
+# Tell the Python unwinder to use the frame-id we cached above.
+gdb_test_no_output "python set_break_bt_here_frame_id($pc, $cfa)"
+
+# Run up to the function which the unwinder will claim.
+if { ![runto "break_bt_here"] } {
+    return
+}
+
+# Print the backtrace.  Check that the reason for stopping the
+# backtrace is that the previous $pc is not available.
+gdb_test "bt" \
+    [multi_line \
+	 "^#0  break_bt_here \\(\\) at \[^\r\n\]+" \
+	 "Backtrace stopped: frame did not save the PC"] \
+    "backtrace from break_bt_here function"
+
+# Ensure we can stepi.
+gdb_test "stepi" \
+    "(:?$hex\\s+)?$decimal\\s+\[^\r\n\]+" \
+    "stepi without a display in place"
+
+# Setup a 'display' directive.
+gdb_test "display/i \$pc" \
+    [multi_line \
+	 "^1: x/i \\\$pc" \
+	 "=> $hex <break_bt_here(:?\\+$decimal)?>:\\s+\[^\r\n\]+"]
+
+# Step again, check the 'display' directive is shown.
+gdb_test "stepi" \
+    [multi_line \
+	 "(:?$hex\\s+)?$decimal\\s+\[^\r\n\]+" \
+	 "1: x/i \\\$pc" \
+	 "=> $hex <break_bt_here(:?\\+$decimal)?>:\\s+\[^\r\n\]+"] \
+    "stepi with a display in place"
+
+# Continue to a function that is called from within break_bt_here.
+# The Python unwinder will then be claiming frame #1.
+gdb_breakpoint other_func
+gdb_continue_to_breakpoint "continue to other_func"
+
+# Print the backtrace and check that the reason for stopping the
+# backtrace is that the previous $pc is not available.
+gdb_test "bt" \
+    [multi_line \
+	 "#0  other_func \\(\\) at \[^\r\n\]+" \
+	 "#1  (:?$hex in )?break_bt_here \\(\\) at \[^\r\n\]+" \
+	 "Backtrace stopped: frame did not save the PC"] \
+    "backtrace from other_func function"
diff --git a/gdb/testsuite/gdb.base/pc-not-saved.py b/gdb/testsuite/gdb.base/pc-not-saved.py
new file mode 100644
index 00000000000..65a8e764885
--- /dev/null
+++ b/gdb/testsuite/gdb.base/pc-not-saved.py
@@ -0,0 +1,71 @@
+# Copyright (C) 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import gdb
+from gdb.unwinder import Unwinder, FrameId
+
+# Cached FrameId.  See set_break_bt_here_frame_id for details.
+break_bt_here_frame_id = None
+
+
+def set_break_bt_here_frame_id(pc, cfa):
+    """Call this to pre-calculate the FrameId for the frame our unwinder
+    is going to claim, this avoids us having to actually figure out a
+    frame-id within the unwinder, something which is going to be hard
+    to do in a cross-target way.
+
+    Instead we first run the test without the Python unwinder in
+    place, use 'maint print frame-id' to record the frame-id, then,
+    after loading this Python script, we all this function to record
+    the frame-id that the unwinder should use."""
+    global break_bt_here_frame_id
+    break_bt_here_frame_id = FrameId(cfa, pc)
+
+
+class break_unwinding(Unwinder):
+
+    """An unwinder for the function 'break_bt_here'.  This unwinder will
+    claim any frame for the function in question, but doesn't provide
+    any unwound register values.  Importantly, we don't provide a
+    previous $pc value, this means that if we are stopped in
+    'break_bt_here' then we should fail to unwind beyond frame #0."""
+
+    def __init__(self):
+        Unwinder.__init__(self, "break unwinding")
+
+    def __call__(self, pending_frame):
+        pc_desc = pending_frame.architecture().registers().find("pc")
+        pc = pending_frame.read_register(pc_desc)
+
+        if pc.is_optimized_out:
+            return None
+
+        block = gdb.block_for_pc(pc)
+        if block == None:
+            return None
+        func = block.function
+        if func == None:
+            return None
+        if str(func) != "break_bt_here":
+            return None
+
+        global break_bt_here_frame_id
+        if break_bt_here_frame_id is None:
+            return None
+
+        return pending_frame.create_unwind_info(break_bt_here_frame_id)
+
+
+gdb.unwinder.register_unwinder(None, break_unwinding(), True)