Simon Marchi
bc2cbe815b
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>
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