[Bug gdb/19675] GDB doesn't set PC correctly with displaced stepping over clone syscall

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

https://sourceware.org/bugzilla/show_bug.cgi?id=19675

--- Comment #4 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Pedro Alves <palves@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=0d36baa9af0d9929c96b89a184a469c432c68b0d

commit 0d36baa9af0d9929c96b89a184a469c432c68b0d
Author: Pedro Alves <pedro@palves.net>
Date:   Fri Nov 12 20:50:29 2021 +0000

    Step over clone syscall w/ breakpoint, TARGET_WAITKIND_THREAD_CLONED

    (A good chunk of the problem statement in the commit log below is
    Andrew's, adjusted for a different solution, and for covering
    displaced stepping too.  The testcase is mostly Andrew's too.)

    This commit addresses bugs gdb/19675 and gdb/27830, which are about
    stepping over a breakpoint set at a clone syscall instruction, one is
    about displaced stepping, and the other about in-line stepping.

    Currently, when a new thread is created through a clone syscall, GDB
    sets the new thread running.  With 'continue' this makes sense
    (assuming no schedlock):

     - all-stop mode, user issues 'continue', all threads are set running,
       a newly created thread should also be set running.

     - non-stop mode, user issues 'continue', other pre-existing threads
       are not affected, but as the new thread is (sort-of) a child of the
       thread the user asked to run, it makes sense that the new threads
       should be created in the running state.

    Similarly, if we are stopped at the clone syscall, and there's no
    software breakpoint at this address, then the current behaviour is
    fine:

     - all-stop mode, user issues 'stepi', stepping will be done in place
       (as there's no breakpoint to step over).  While stepping the thread
       of interest all the other threads will be allowed to continue.  A
       newly created thread will be set running, and then stopped once the
       thread of interest has completed its step.

     - non-stop mode, user issues 'stepi', stepping will be done in place
       (as there's no breakpoint to step over).  Other threads might be
       running or stopped, but as with the continue case above, the new
       thread will be created running.  The only possible issue here is
       that the new thread will be left running after the initial thread
       has completed its stepi.  The user would need to manually select
       the thread and interrupt it, this might not be what the user
       expects.  However, this is not something this commit tries to
       change.

    The problem then is what happens when we try to step over a clone
    syscall if there is a breakpoint at the syscall address.

    - For both all-stop and non-stop modes, with in-line stepping:

       + user issues 'stepi',
       + [non-stop mode only] GDB stops all threads.  In all-stop mode all
         threads are already stopped.
       + GDB removes s/w breakpoint at syscall address,
       + GDB single steps just the thread of interest, all other threads
         are left stopped,
       + New thread is created running,
       + Initial thread completes its step,
       + [non-stop mode only] GDB resumes all threads that it previously
         stopped.

    There are two problems in the in-line stepping scenario above:

      1. The new thread might pass through the same code that the initial
         thread is in (i.e. the clone syscall code), in which case it will
         fail to hit the breakpoint in clone as this was removed so the
         first thread can single step,

      2. The new thread might trigger some other stop event before the
         initial thread reports its step completion.  If this happens we
         end up triggering an assertion as GDB assumes that only the
         thread being stepped should stop.  The assert looks like this:

         infrun.c:5899: internal-error: int
finish_step_over(execution_control_state*): Assertion
`ecs->event_thread->control.trap_expected' failed.

    - For both all-stop and non-stop modes, with displaced stepping:

       + user issues 'stepi',
       + GDB starts the displaced step, moves thread's PC to the
         out-of-line scratch pad, maybe adjusts registers,
       + GDB single steps the thread of interest, [non-stop mode only] all
         other threads are left as they were, either running or stopped.
         In all-stop, all other threads are left stopped.
       + New thread is created running,
       + Initial thread completes its step, GDB re-adjusts its PC,
         restores/releases scratchpad,
       + [non-stop mode only] GDB resumes the thread, now past its
         breakpoint.
       + [all-stop mode only] GDB resumes all threads.

    There is one problem with the displaced stepping scenario above:

      3. When the parent thread completed its step, GDB adjusted its PC,
         but did not adjust the child's PC, thus that new child thread
         will continue execution in the scratch pad, invoking undefined
         behavior.  If you're lucky, you see a crash.  If unlucky, the
         inferior gets silently corrupted.

    What is needed is for GDB to have more control over whether the new
    thread is created running or not.  Issue #1 above requires that the
    new thread not be allowed to run until the breakpoint has been
    reinserted.  The only way to guarantee this is if the new thread is
    held in a stopped state until the single step has completed.  Issue #3
    above requires that GDB is informed of when a thread clones itself,
    and of what is the child's ptid, so that GDB can fixup both the parent
    and the child.

    When looking for solutions to this problem I considered how GDB
    handles fork/vfork as these have some of the same issues.  The main
    difference between fork/vfork and clone is that the clone events are
    not reported back to core GDB.  Instead, the clone event is handled
    automatically in the target code and the child thread is immediately
    set running.

    Note we have support for requesting thread creation events out of the
    target (TARGET_WAITKIND_THREAD_CREATED).  However, those are reported
    for the new/child thread.  That would be sufficient to address in-line
    stepping (issue #1), but not for displaced-stepping (issue #3).  To
    handle displaced-stepping, we need an event that is reported to the
    _parent_ of the clone, as the information about the displaced step is
    associated with the clone parent.  TARGET_WAITKIND_THREAD_CREATED
    includes no indication of which thread is the parent that spawned the
    new child.  In fact, for some targets, like e.g., Windows, it would be
    impossible to know which thread that was, as thread creation there
    doesn't work by "cloning".

    The solution implemented here is to model clone on fork/vfork, and
    introduce a new TARGET_WAITKIND_THREAD_CLONED event.  This event is
    similar to TARGET_WAITKIND_FORKED and TARGET_WAITKIND_VFORKED, except
    that we end up with a new thread in the same process, instead of a new
    thread of a new process.  Like FORKED and VFORKED, THREAD_CLONED
    waitstatuses have a child_ptid property, and the child is held stopped
    until GDB explicitly resumes it.  This addresses the in-line stepping
    case (issues #1 and #2).

    The infrun code that handles displaced stepping fixup for the child
    after a fork/vfork event is thus reused for THREAD_CLONE, with some
    minimal conditions added, addressing the displaced stepping case
    (issue #3).

    The native Linux backend is adjusted to unconditionally report
    TARGET_WAITKIND_THREAD_CLONED events to the core.

    Following the follow_fork model in core GDB, we introduce a
    target_follow_clone target method, which is responsible for making the
    new clone child visible to the rest of GDB.

    Subsequent patches will add clone events support to the remote
    protocol and gdbserver.

    displaced_step_in_progress_thread becomes unused with this patch, but
    a new use will reappear later in the series.  To avoid deleting it and
    readding it back, this patch marks it with attribute unused, and the
    latter patch removes the attribute again.  We need to do this because
    the function is static, and with no callers, the compiler would warn,
    (error with -Werror), breaking the build.

    This adds a new gdb.threads/stepi-over-clone.exp testcase, which
    exercises stepping over a clone syscall, with displaced stepping vs
    inline stepping, and all-stop vs non-stop.  We already test stepping
    over clone syscalls with gdb.base/step-over-syscall.exp, but this test
    uses pthreads, while the other test uses raw clone, and this one is
    more thorough.  The testcase passes on native GNU/Linux, but fails
    against GDBserver.  GDBserver will be fixed by a later patch in the
    series.

    Co-authored-by: Andrew Burgess <aburgess@redhat.com>
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19675
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27830
    Change-Id: I95c06024736384ae8542a67ed9fdf6534c325c8e
    Reviewed-By: Andrew Burgess <aburgess@redhat.com>

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