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Hannes filed a bug showing a crash, where a pretty-printer written in Python could cause a use-after-free. He sent a patch, but I thought a different approach was needed. In a much earlier patch (see bug #12533), we changed the Python code to release new values from the value chain when constructing a gdb.Value. The rationale for this is that if you write a command that does a lot of computations in a loop, all the values will be kept live by the value chain, resulting in gdb using a large amount of memory. However, suppose a value is passed to Python from some code in gdb that needs to use the value after the call into Python. In this scenario, value_to_value_object will still release the value -- and because gdb code doesn't generally keep strong references to values (a consequence of the ancient decision to use the value chain to avoid memory management), this will result in a use-after-free. This scenario can happen, as it turns out, when a value is passed to Python for pretty-printing. Now, normally this route boxes the value via value_to_value_object_no_release, avoiding the problematic release from the value chain. However, if you then call Value.cast, the underlying value API might return the same value, when is then released from the chain. This patch fixes the problem by changing how value boxing is done. value_to_value_object no longer removes a value from the chain. Instead, every spot in gdb that might construct new values uses a scoped_value_mark to ensure that the requirements of bug #12533 are met. And, because incoming values aren't ever released from the chain (the Value.cast one comes earlier on the chain than the scoped_value_mark), the bug can no longer occur. (Note that many spots in the Python layer already take this approach, so not many places needed to be touched.) In the future I think we should replace the use of raw "value *" with value_ref_ptr pretty much everywhere. This will ensure lifetime safety throughout gdb. The test case in this patch comes from Hannes' original patch. I only made a trivial ("require") change to it. However, while this fails for him, I can't make it fail on this machine; nevertheless, he tried my patch and reported the bug as being fixed. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30044
41 lines
1.2 KiB
Plaintext
41 lines
1.2 KiB
Plaintext
# Copyright (C) 2023 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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# Test casting of a gdb.Value inside a pretty printer.
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load_lib gdb-python.exp
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require allow_python_tests
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standard_testfile
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if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
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return -1
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}
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if ![runto break_function] {
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return -1
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}
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set remote_python_file [gdb_remote_download host \
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${srcdir}/${subdir}/${testfile}.py]
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gdb_test_no_output "source ${remote_python_file}" \
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"source ${testfile}.py"
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gdb_test "up" "#1.*main.*"
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gdb_test "info locals" "c = {p_i = 10p, i = 5}"
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