gdb: make it possible to restore selected user-created frames

I would like to improve frame_info_ptr to automatically grab the
information needed to reinflate a frame, and automatically reinflate it
as needed.  One thing that is in the way is the fact that some frames
can be created out of thin air by the create_new_frame function.  These
frames are not the fruit of unwinding from the target's current frame.
These frames are created by the "select-frame view" command.

These frames are not correctly handled by the frame save/restore
functions, save_selected_frame, restore_selected_frame and
lookup_selected_frame.  This can be observed here, using the test
included in this patch:

    $ ./gdb --data-directory=data-directory -nx -q testsuite/outputs/gdb.base/frame-view/frame-view
    Reading symbols from testsuite/outputs/gdb.base/frame-view/frame-view...
    (gdb) break thread_func
    Breakpoint 1 at 0x11a2: file /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/frame-view.c, line 42.
    (gdb) run
    Starting program: /home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/frame-view/frame-view

    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
    [New Thread 0x7ffff7cc46c0 (LWP 4171134)]
    [Switching to Thread 0x7ffff7cc46c0 (LWP 4171134)]

    Thread 2 "frame-view" hit Breakpoint 1, thread_func (p=0x0) at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/frame-view.c:42
    42        foo (11);
    (gdb) info frame
    Stack level 0, frame at 0x7ffff7cc3ee0:
     rip = 0x5555555551a2 in thread_func (/home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/frame-view.c:42); saved rip = 0x7ffff7d4e8fd
     called by frame at 0x7ffff7cc3f80
     source language c.
     Arglist at 0x7ffff7cc3ed0, args: p=0x0
     Locals at 0x7ffff7cc3ed0, Previous frame's sp is 0x7ffff7cc3ee0
     Saved registers:
      rbp at 0x7ffff7cc3ed0, rip at 0x7ffff7cc3ed8
    (gdb) thread 1
    [Switching to thread 1 (Thread 0x7ffff7cc5740 (LWP 4171122))]
    #0  0x00007ffff7d4b4b6 in ?? () from /usr/lib/libc.so.6

Here, we create a custom frame for thread 1 (using the stack from thread
2, for convenience):

    (gdb) select-frame view 0x7ffff7cc3f80 0x5555555551a2

The first calls to "frame" looks good:

    (gdb) frame
    #0  thread_func (p=0x7ffff7d4e630) at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/frame-view.c:42
    42        foo (11);

But not the second one:

    (gdb) frame
    #0  0x00007ffff7d4b4b6 in ?? () from /usr/lib/libc.so.6

This second "frame" command shows the current target frame instead of
the user-created frame.

It's not totally clear how the "select-frame view" feature is expected
to behave, especially since it's not tested.  I heard accounts that it
used to be possible to select a frame like this and do "up" and "down"
to navigate the backtrace starting from that frame.  The fact that
create_new_frame calls frame_unwind_find_by_frame to install the right
unwinder suggest that it used to be possible.  But that doesn't work
today:

    (gdb) select-frame view 0x7ffff7cc3f80 0x5555555551a2
    (gdb) up
    Initial frame selected; you cannot go up.
    (gdb) down
    Bottom (innermost) frame selected; you cannot go down.

and "backtrace" always shows the actual thread's backtrace, it ignores
the user-created frame:

    (gdb) bt
    #0  0x00007ffff7d4b4b6 in ?? () from /usr/lib/libc.so.6
    #1  0x00007ffff7d50403 in ?? () from /usr/lib/libc.so.6
    #2  0x000055555555521a in main () at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/frame-view.c:56

I don't want to address all the `select-frame view` issues , but I think
we can agree that the "frame" command changing the selected frame, as
shown above, is a bug.  I would expect that command to show the
currently selected frame and not change it.

This happens because of the scoped_restore_selected_frame object in
print_frame_args.  The frame information is saved in the constructor
(the backtrace below), and restored in the destructor.

    #0  save_selected_frame (frame_id=0x7ffdc0020ad0, frame_level=0x7ffdc0020af0) at /home/simark/src/binutils-gdb/gdb/frame.c:1682
    #1  0x00005631390242f0 in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7ffdc0020ad0) at /home/simark/src/binutils-gdb/gdb/frame.c:324
    #2  0x000056313993581e in print_frame_args (fp_opts=..., func=0x62100023bde0, frame=..., num=-1, stream=0x60b000000300) at /home/simark/src/binutils-gdb/gdb/stack.c:755
    #3  0x000056313993ad49 in print_frame (fp_opts=..., frame=..., print_level=1, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/simark/src/binutils-gdb/gdb/stack.c:1401
    #4  0x000056313993835d in print_frame_info (fp_opts=..., frame=..., print_level=1, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:1126
    #5  0x0000563139932e0b in print_stack_frame (frame=..., print_level=1, print_what=SRC_AND_LOC, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:368
    #6  0x0000563139932bbe in print_stack_frame_to_uiout (uiout=0x611000016840, frame=..., print_level=1, print_what=SRC_AND_LOC, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:346
    #7  0x0000563139b0641e in print_selected_thread_frame (uiout=0x611000016840, selection=...) at /home/simark/src/binutils-gdb/gdb/thread.c:1993
    #8  0x0000563139940b7f in frame_command_core (fi=..., ignored=true) at /home/simark/src/binutils-gdb/gdb/stack.c:1871
    #9  0x000056313994db9e in frame_command_helper<frame_command_core>::base_command (arg=0x0, from_tty=1) at /home/simark/src/binutils-gdb/gdb/stack.c:1976

Since the user-created frame has level 0 (identified by the saved level
-1), lookup_selected_frame just reselects the target's current frame,
and the user-created frame is lost.

My goal here is to fix this particular problem.

Currently, select_frame does not set selected_frame_id and
selected_frame_level for frames with level 0.  It leaves them at
null_frame_id / -1, indicating to restore_selected_frame to use the
target's current frame.  User-created frames also have level 0, so add a
special case them such that select_frame saves their selected id and
level.

save_selected_frame does not need any change.

Change the assertion in restore_selected_frame that checks `frame_level
!= 0` to account for the fact that we can restore user-created frames,
which have level 0.

Finally, change lookup_selected_frame to make it able to re-create
user-created frame_info objects from selected_frame_level and
selected_frame_id.

Add a minimal test case for the case described above, that is the
"select-frame view" command followed by the "frame" command twice.  In
order to have a known stack frame to switch to, the test spawns a second
thread, and tells the first thread to use the other thread's top frame.

Change-Id: Ifc77848dc465fbd21324b9d44670833e09fe98c7
Reviewed-By: Bruno Larsen <blarsen@redhat.com>

1 files changed, 23 insertions, 7 deletions

diff --git a/gdb/frame.c b/gdb/frame.c
index 0909109c1f4..9ab8fa0310e 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -69,6 +69,7 @@ set_backtrace_options user_set_backtrace_options;
 
 static frame_info_ptr get_prev_frame_raw (frame_info_ptr this_frame);
 static const char *frame_stop_reason_symbol_string (enum unwind_stop_reason reason);
+static frame_info_ptr create_new_frame (frame_id id);
 
 /* Status of some values cached in the frame_info object.  */
 
@@ -1669,9 +1670,12 @@ get_current_frame (void)
 
    If SELECTED_FRAME_ID / SELECTED_FRAME_LEVEL are null_frame_id / -1,
    and the target has stack and is stopped, the selected frame is the
-   current (innermost) frame.  This means that SELECTED_FRAME_LEVEL is
-   never 0 and SELECTED_FRAME_ID is never the ID of the innermost
-   frame.
+   current (innermost) target frame.  SELECTED_FRAME_ID is never the ID
+   of the current (innermost) target frame.  SELECTED_FRAME_LEVEL may
+   only be 0 if the selected frame is a user-created one (created and
+   selected through the "select-frame view" command), in which case
+   SELECTED_FRAME_ID is the frame id derived from the user-provided
+   addresses.
 
    If SELECTED_FRAME_ID / SELECTED_FRAME_LEVEL are null_frame_id / -1,
    and the target has no stack or is executing, then there's no
@@ -1699,9 +1703,9 @@ void
 restore_selected_frame (frame_id frame_id, int frame_level)
   noexcept
 {
-  /* save_selected_frame never returns level == 0, so we shouldn't see
-     it here either.  */
-  gdb_assert (frame_level != 0);
+  /* Unless it is a user-created frame, save_selected_frame never returns
+     level == 0, so we shouldn't see it here either.  */
+  gdb_assert (frame_level != 0 || frame_id.user_created_p);
 
   /* FRAME_ID can be null_frame_id only IFF frame_level is -1.  */
   gdb_assert ((frame_level == -1 && !frame_id_p (frame_id))
@@ -1735,6 +1739,15 @@ lookup_selected_frame (struct frame_id a_frame_id, int frame_level)
       return;
     }
 
+  /* This means the selected frame was a user-created one.  Create a new one
+     using the user-provided addresses, which happen to be in the frame id.  */
+  if (frame_level == 0)
+    {
+      gdb_assert (a_frame_id.user_created_p);
+      select_frame (create_new_frame (a_frame_id));
+      return;
+    }
+
   /* select_frame never saves 0 in SELECTED_FRAME_LEVEL, so we
      shouldn't see it here.  */
   gdb_assert (frame_level > 0);
@@ -1859,7 +1872,10 @@ select_frame (frame_info_ptr fi)
 
   selected_frame = fi;
   selected_frame_level = frame_relative_level (fi);
-  if (selected_frame_level == 0)
+
+  /* If the frame is a user-created one, save its level and frame id just like
+     any other non-level-0 frame.  */
+  if (selected_frame_level == 0 && !fi->this_id.value.user_created_p)
     {
       /* Treat the current frame especially -- we want to always
 	 save/restore it without warning, even if the frame ID changes