[Bug gdb/30872] Assertion `buffer.copy_insn_closure.get () != nullptr' failed

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30872

--- Comment #13 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Luis Machado <luisgpm@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=5d4a870e05ac45e3f5a301c672a4079995b5db7a

commit 5d4a870e05ac45e3f5a301c672a4079995b5db7a
Author: Luis Machado <luis.machado@arm.com>
Date:   Thu Sep 28 11:08:29 2023 +0100

    Only allow closure lookup by address if there are threads
displaced-stepping

    Since commit 1e5ccb9c5ff4fd8ade4a8694676f99f4abf2d679, we have an assertion
in
    displaced_step_buffers::copy_insn_closure_by_addr that makes sure a closure
    is available whenever we have a match between the provided address argument
and
    the buffer address.

    That is fine, but the report in PR30872 shows this assertion triggering
when
    it really shouldn't. After some investigation, here's what I found out.

    The 32-bit Arm architecture is the only one that calls
    gdbarch_displaced_step_copy_insn_closure_by_addr directly, and that's
because
    32-bit Arm needs to figure out the thumb state of the original instruction
    that we displaced-stepped through the displaced-step buffer.

    Before the assertion was put in place by commit
    1e5ccb9c5ff4fd8ade4a8694676f99f4abf2d679, there was the possibility of
    getting nullptr back, which meant we were not doing a displaced-stepping
    operation.

    Now, with the assertion in place, this is running into issues.

    It looks like displaced_step_buffers::copy_insn_closure_by_addr is
    being used to return a couple different answers depending on the
    state we're in:

    1 - If we are actively displaced-stepping, then copy_insn_closure_by_addr
    is supposed to return a valid closure for us, so we can determine the
    thumb mode.

    2 - If we are not actively displaced-stepping, then
copy_insn_closure_by_addr
    should return nullptr to signal that there isn't any displaced-step buffers
    in use, because we don't have a valid closure (but we should always have
    this).

    Since the displaced-step buffers are always allocated, but not always used,
    that means the buffers will always contain data. In particular, the buffer
    addr field cannot be used to determine if the buffer is active or not.

    For instance, we cannot set the buffer addr field to 0x0, as that can be a
    valid PC in some cases.

    My understanding is that the current_thread field should be a good
candidate
    to signal that a particular displaced-step buffer is active or not. If it
is
    nullptr, we have no threads using that buffer to displaced-step. 
Otherwise,
    it is an active buffer in use by a particular thread.

    The following fix modifies the
displaced_step_buffers::copy_insn_closure_by_addr
    function so we only attempt to return a closure if the buffer has an
assigned
    current_thread and if the buffer address matches the address argument.

    Alternatively, I think we could use a function to answer the question of
    whether we're actively displaced-stepping (so we have an active buffer) or
    not.

    I've also added a testcase that exercises the problem. It should reproduce
    reliably on Arm, as that is the only architecture that faces this problem
    at the moment.

    Regression-tested on Ubuntu 20.04. OK?

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30872
    Approved-By: Simon Marchi <simon.marchi@efficios.com>

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