61e2dde2db
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.
72 lines
2.2 KiB
Python
72 lines
2.2 KiB
Python
# Copyright (C) 2021 Free Software Foundation, Inc.
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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import gdb
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from gdb.unwinder import Unwinder
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class FrameId(object):
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def __init__(self, sp, pc):
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self._sp = sp
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self._pc = pc
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@property
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def sp(self):
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return self._sp
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@property
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def pc(self):
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return self._pc
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class TestUnwinder(Unwinder):
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def __init__(self, use_descriptors):
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if use_descriptors:
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tag = "using descriptors"
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else:
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tag = "using strings"
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Unwinder.__init__(self, "break unwinding %s" % tag)
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self._use_descriptors = use_descriptors
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def __call__(self, pending_frame):
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pc_desc = pending_frame.architecture().registers().find("pc")
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pc = pending_frame.read_register(pc_desc)
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sp_desc = pending_frame.architecture().registers().find("sp")
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sp = pending_frame.read_register(sp_desc)
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block = gdb.block_for_pc(int(pc))
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if block is None:
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return None
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func = block.function
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if func is None:
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return None
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if str(func) != "bar":
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return None
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fid = FrameId(pc, sp)
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unwinder = pending_frame.create_unwind_info(fid)
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if self._use_descriptors:
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unwinder.add_saved_register(pc_desc, pc)
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unwinder.add_saved_register(sp_desc, sp)
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else:
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unwinder.add_saved_register("pc", pc)
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unwinder.add_saved_register("sp", sp)
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return unwinder
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gdb.unwinder.register_unwinder(None, TestUnwinder(True), True)
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gdb.unwinder.register_unwinder(None, TestUnwinder(False), True)
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