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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.
99 lines
4.1 KiB
Plaintext
99 lines
4.1 KiB
Plaintext
# 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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# Setup an unwinder that uses gdb.UnwindInfo.add_saved_register with
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# the register's 'pc' and 'sp'. On some (all?) targets, these
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# registers are implemented as user-registers, and so can't normally
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# be written to directly.
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#
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# The Python unwinder now includes code similar to how the expression
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# evaluator would handle something like 'set $pc=0x1234', we fetch the
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# value of '$pc', and then use the value's location to tell us which
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# register to write to.
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#
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# The unwinder defined here deliberately breaks the unwind by setting
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# the unwound $pc and $sp to be equal to the current frame's $pc and
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# $sp. GDB will spot this as a loop in the backtrace and terminate
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# the unwind.
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#
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# However, by the time the unwind terminates we have already shown
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# that it is possible to call add_saved_register with a user-register,
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# so the test is considered passed.
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#
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# For completeness this test checks two cases, calling
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# add_saved_register with a gdb.RegisterDescriptor and calling
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# add_saved_register with a string containing the register name.
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load_lib gdb-python.exp
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standard_testfile
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if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
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return -1
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}
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# Skip all tests if Python scripting is not enabled.
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if { [skip_python_tests] } { continue }
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if ![runto_main] then {
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fail "can't run to main"
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return 0
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}
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set pyfile [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
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gdb_breakpoint [gdb_get_line_number "Break here"]
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gdb_continue_to_breakpoint "stop at test breakpoint"
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# Load the script containing the unwinders. There are actually two
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# unwinders defined here that will catch the same function, so we
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# immediately disable one of the unwinders.
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gdb_test_no_output "source ${pyfile}"\
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"import python scripts"
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gdb_test "disable unwinder global \"break unwinding using strings\"" \
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"1 unwinder disabled" "disable the unwinder that uses strings"
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# At this point we are using the unwinder that passes a
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# gdb.RegisterDescriptor to add_saved_register.
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gdb_test_sequence "bt" "Backtrace corrupted by descriptor based unwinder" {
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"\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
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"\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
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"Backtrace stopped: previous frame inner to this frame \\(corrupt stack\\?\\)"
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}
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# Disable the unwinder that calls add_saved_register with a
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# gdb.RegisterDescriptor, and enable the unwinder that calls
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# add_saved_register with a string (containing the register name).
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gdb_test "disable unwinder global \"break unwinding using descriptors\"" \
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"1 unwinder disabled" "disable the unwinder that uses descriptors"
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gdb_test "enable unwinder global \"break unwinding using strings\"" \
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"1 unwinder enabled" "enable the unwinder that uses strings"
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gdb_test_sequence "bt" "Backtrace corrupted by string based unwinder" {
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"\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
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"\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
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"Backtrace stopped: previous frame inner to this frame \\(corrupt stack\\?\\)"
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}
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# Just for completeness, disable the string unwinder again (neither of
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# our special unwinders are now enabled), and check the backtrace. We
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# now get the complete stack back to main.
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gdb_test "disable unwinder global \"break unwinding using strings\"" \
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"1 unwinder disabled" "disable the unwinder that uses strings again"
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gdb_test_sequence "bt" "Backtrace not corrupted when using no unwinder" {
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"\\r\\n#0 \[^\r\n\]* foo \\(\\) at "
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"\\r\\n#1 \[^\r\n\]* bar \\(\\) at "
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"\\r\\n#2 \[^\r\n\]* main \\(\\) at "
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}
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