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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
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.