[PATCH 1/5] gdb/python: handle saving user registers in a frame unwinder

Andrew Burgess
2021-05-29T20:57:27Z

This patch came about because I wanted to write a frame unwinder that
would corrupt the backtrace in a particular way. In order to achieve
what I wanted I ended up trying to write an unwinder like this:

class FrameId(object):
.... snip class definition ....

class TestUnwinder(Unwinder):
def __init__(self):
Unwinder.__init__(self, "some name")

def __call__(self, pending_frame):
pc_desc = pending_frame.architecture().registers().find("pc")
pc = pending_frame.read_register(pc_desc)

sp_desc = pending_frame.architecture().registers().find("sp")
sp = pending_frame.read_register(sp_desc)

# ... snip code to decide if this unwinder applies or not.

fid = FrameId(pc, sp)
unwinder = pending_frame.create_unwind_info(fid)
unwinder.add_saved_register(pc_desc, pc)
unwinder.add_saved_register(sp_desc, sp)
return unwinder

The important things here are the two calls:

unwinder.add_saved_register(pc_desc, pc)
unwinder.add_saved_register(sp_desc, sp)

On x86-64 these would fail with an assertion error:

gdb/regcache.c:168: internal-error: int register_size(gdbarch*, int): Assertion `regnum >= 0 && regnum < gdbarch_num_cooked_regs (gdbarch)' failed.

What happens is that in unwind_infopy_add_saved_register (py-unwind.c)
we call register_size, as register_size should only be called on
cooked (real or pseudo) registers, and 'pc' and 'sp' are implemented
as user registers (at least on x86-64), we trigger the assertion.

A simple fix would be to check in unwind_infopy_add_saved_register if
the register number we are handling is a cooked register or not, if
not we can throw a 'Bad register' error back to the Python code.

However, I think we can do better.

Consider that at the CLI we can do this:

(gdb) set $pc=0x1234

This works because GDB first evaluates '$pc' to get a register value,
then evaluates '0x1234' to create a value encapsulating the
immediate. The contents of the immediate value are then copied back
to the location of the register value representing '$pc'.

The value location for a user-register will (usually) be the location
of the real register that was accessed, so on x86-64 we'd expect this
to be $rip.

So, in this patch I propose that in the unwinder code, when
add_saved_register is called, if it is passed a
user-register (i.e. non-cooked) then we first fetch the register,
extract the real register number from the value's location, and use
that new register number when handling the add_saved_register call.

If either the value location that we get for the user-register is not
a cooked register then we can throw a 'Bad register' error back to the
Python code, but in most cases this will not happen.

gdb/ChangeLog:

* python/py-unwind.c (unwind_infopy_add_saved_register): Handle
saving user registers.

gdb/testsuite/ChangeLog:

* gdb.python/py-unwind-user-regs.c: New file.
* gdb.python/py-unwind-user-regs.exp: New file.
* gdb.python/py-unwind-user-regs.py: New file.
---
gdb/ChangeLog | 5 +
gdb/python/py-unwind.c | 21 ++++
gdb/testsuite/ChangeLog | 6 ++
.../gdb.python/py-unwind-user-regs.c | 37 +++++++
.../gdb.python/py-unwind-user-regs.exp | 98 +++++++++++++++++++
.../gdb.python/py-unwind-user-regs.py | 72 ++++++++++++++
6 files changed, 239 insertions(+)
create mode 100644 gdb/testsuite/gdb.python/py-unwind-user-regs.c
create mode 100644 gdb/testsuite/gdb.python/py-unwind-user-regs.exp
create mode 100644 gdb/testsuite/gdb.python/py-unwind-user-regs.py

diff --git a/gdb/python/py-unwind.c b/gdb/python/py-unwind.c
index 7c195eb539d..d6e2f85dbc1 100644
--- a/gdb/python/py-unwind.c
+++ b/gdb/python/py-unwind.c
@@ -27,6 +27,7 @@
#include "python-internal.h"
#include "regcache.h"
#include "valprint.h"
+#include "user-regs.h"

/* Debugging of Python unwinders. */

@@ -265,6 +266,26 @@ unwind_infopy_add_saved_register (PyObject *self, PyObject *args)
PyErr_SetString (PyExc_ValueError, "Bad register");
return NULL;
}
+
+ /* If REGNUM identifies a user register then *maybe* we can convert this
+ to a real (i.e. non-user) register. The maybe qualifier is because we
+ don't know what user registers each target might add, however, the
+ following logic should work for the usual style of user registers,
+ where the read function just forwards the register read on to some
+ other register with no adjusting the value. */
+ if (regnum >= gdbarch_num_cooked_regs (pending_frame->gdbarch))
+ {
+ struct value *user_reg_value
+ = value_of_user_reg (regnum, pending_frame->frame_info);
+ if (VALUE_LVAL (user_reg_value) == lval_register)
+ regnum = VALUE_REGNUM (user_reg_value);
+ if (regnum >= gdbarch_num_cooked_regs (pending_frame->gdbarch))
+ {
+ PyErr_SetString (PyExc_ValueError, "Bad register");
+ return NULL;
+ }
+ }
+
{
struct value *value;
size_t data_size;
diff --git a/gdb/testsuite/gdb.python/py-unwind-user-regs.c b/gdb/testsuite/gdb.python/py-unwind-user-regs.c
new file mode 100644
index 00000000000..8d1efd1a85d
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-unwind-user-regs.c
@@ -0,0 +1,37 @@
+/* This test program is part of GDB, the GNU debugger.
+
+ Copyright 2021 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;
+
+void __attribute__ ((noinline))
+foo (void)
+{
+ ++global_var; /* Break here. */
+}
+
+void __attribute__ ((noinline))
+bar (void)
+{
+ foo ();
+}
+
+int
+main (void)
+{
+ bar ();
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.python/py-unwind-user-regs.exp b/gdb/testsuite/gdb.python/py-unwind-user-regs.exp
new file mode 100644
index 00000000000..7ae3a5bb19f
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-unwind-user-regs.exp
@@ -0,0 +1,98 @@
+# Copyright (C) 2021 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/>.
+
+# Setup an unwinder that uses gdb.UnwindInfo.add_saved_register with
+# the register's 'pc' and 'sp'. On some (all?) targets, these
+# registers are implemented as user-registers, and so can't normally
+# be written to directly.
+#
+# The Python unwinder now includes code similar to how the expression
+# evaluator would handle something like 'set $pc=0x1234', we fetch the
+# value of '$pc', and then use the value's location to tell us which
+# register to write to.
+#
+# The unwinder defined here deliberately breaks the unwind by setting
+# the unwound $pc and $sp to be equal to the current frame's $pc and
+# $sp. GDB will spot this as a loop in the backtrace and terminate
+# the unwind.
+#
+# However, by the time the unwind terminates we have already shown
+# that it is possible to call add_saved_register with a user-register,
+# so the test is considered passed.
+#
+# For completeness this test checks two cases, calling
+# add_saved_register with a gdb.RegisterDescriptor and calling
+# add_saved_register with a string containing the register name.
+
+load_lib gdb-python.exp
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
+ return -1
+}
+
+# Skip all tests if Python scripting is not enabled.
+if { [skip_python_tests] } { continue }
+
+if ![runto_main] then {
+ fail "can't run to main"
+ return 0
+}
+
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
+
+gdb_breakpoint [gdb_get_line_number "Break here"]
+gdb_continue_to_breakpoint "stop at test breakpoint"
+
+# Load the script containing the unwinders. There are actually two
+# unwinders defined here that will catch the same function, so we
+# immediately disable one of the unwinders.
+gdb_test_no_output "source ${pyfile}"\
+ "import python scripts"
+gdb_test "disable unwinder global \"break unwinding using strings\"" \
+ "1 unwinder disabled" "disable the unwinder that uses strings"
+
+# At this point we are using the unwinder that passes a
+# gdb.RegisterDescriptor to add_saved_register.
+gdb_test_sequence "bt" "Backtrace corrupted by descriptor based unwinder" {
+ "\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
+ "\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
+ "Backtrace stopped: previous frame inner to this frame \\(corrupt stack\\?\\)"
+}
+
+# Disable the unwinder that calls add_saved_register with a
+# gdb.RegisterDescriptor, and enable the unwinder that calls
+# add_saved_register with a string (containing the register name).
+gdb_test "disable unwinder global \"break unwinding using descriptors\"" \
+ "1 unwinder disabled" "disable the unwinder that uses descriptors"
+gdb_test "enable unwinder global \"break unwinding using strings\"" \
+ "1 unwinder enabled" "enable the unwinder that uses strings"
+gdb_test_sequence "bt" "Backtrace corrupted by string based unwinder" {
+ "\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
+ "\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
+ "Backtrace stopped: previous frame inner to this frame \\(corrupt stack\\?\\)"
+}
+
+# Just for completeness, disable the string unwinder again (neither of
+# our special unwinders are now enabled), and check the backtrace. We
+# now get the complete stack back to main.
+gdb_test "disable unwinder global \"break unwinding using strings\"" \
+ "1 unwinder disabled" "disable the unwinder that uses strings again"
+gdb_test_sequence "bt" "Backtrace not corrupted when using no unwinder" {
+ "\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
+ "\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
+ "\\r\\n#2 \[^\r\n\]* main \\(\\) at "
+}
diff --git a/gdb/testsuite/gdb.python/py-unwind-user-regs.py b/gdb/testsuite/gdb.python/py-unwind-user-regs.py
new file mode 100644
index 00000000000..e5edd7cbd9c
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-unwind-user-regs.py
@@ -0,0 +1,72 @@
+# Copyright (C) 2021 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
+
+
+class FrameId(object):
+ def __init__(self, sp, pc):
+ self._sp = sp
+ self._pc = pc
+
+ @property
+ def sp(self):
+ return self._sp
+
+ @property
+ def pc(self):
+ return self._pc
+
+
+class TestUnwinder(Unwinder):
+ def __init__(self, use_descriptors):
+ if use_descriptors:
+ tag = "using descriptors"
+ else:
+ tag = "using strings"
+
+ Unwinder.__init__(self, "break unwinding %s" % tag)
+ self._use_descriptors = use_descriptors
+
+ def __call__(self, pending_frame):
+ pc_desc = pending_frame.architecture().registers().find("pc")
+ pc = pending_frame.read_register(pc_desc)
+
+ sp_desc = pending_frame.architecture().registers().find("sp")
+ sp = pending_frame.read_register(sp_desc)
+
+ block = gdb.block_for_pc(int(pc))
+ if block == None:
+ return None
+ func = block.function
+ if func == None:
+ return None
+ if str(func) != "bar":
+ return None
+
+ fid = FrameId(pc, sp)
+ unwinder = pending_frame.create_unwind_info(fid)
+ if self._use_descriptors:
+ unwinder.add_saved_register(pc_desc, pc)
+ unwinder.add_saved_register(sp_desc, sp)
+ else:
+ unwinder.add_saved_register("pc", pc)
+ unwinder.add_saved_register("sp", sp)
+ return unwinder
+
+
+gdb.unwinder.register_unwinder(None, TestUnwinder(True), True)
+gdb.unwinder.register_unwinder(None, TestUnwinder(False), True)
--
2.25.4