[haiku-bugs] Re: [Haiku] #6751: gdb won't single step
- From: "grahamh" <trac@xxxxxxxxxxxx>
- Date: Thu, 28 Oct 2010 18:07:16 -0000
#6751: gdb won't single step
----------------------------------------------+----------------------------
Reporter: grahamh | Owner: bonefish
Type: bug | Status: assigned
Priority: normal | Milestone: R1
Component: Applications/Command Line Tools | Version:
Resolution: | R1/Development
Blocked By: | Keywords:
Platform: x86 | Has a Patch: 0
| Blocking:
----------------------------------------------+----------------------------
Comment (by grahamh):
After some more investigation, here is what I think is happening.
Asking for a single-step in gdb sends a message to the in-kernel debug
facility, which sets the CPU's single-step flag in the context of the
debugged thread. When the thread is resumed, it executes one instruction,
then traps with an INT1 debug exception. The INT1 handler passes control
to the generic trap handler, which calls gInterruptHandlerTable[1], which
is x86_handle_debug_exception. So far, so good.
The first that x86_handle_debug_exception() does is look at DR6 and DR7 to
work out what kind of debug event this is. However, if the exception was
from user space ( IFRAME_IS_USER(frame) ), it uses supposedly cached
copies of the DRs from the thread's cpu info struct. The comment above
that line says that they should have been saved by
x86_exit_user_debug_at_kernel_entry - but as we have seen, that function
isn't in the execution chain here.
So the exception handler sees a bogus DR6=0, DR7=0, it can't understand
the trap type, and it just returns. Future instructions are then all
trapped in the same way, up until the process makes a system call (or
possibly a context switch). At that point,
x86_exit_user_debug_at_kernel_entry happens and DR6 and DR7 are stored in
the thread struct. Single stepping is resumed when the system call ends
and the thread's user mode context is restored. Now having correct DR
values, the trap handler stops the process and returns to GDB after the
next instruction - which is in the syscall trampoline in the commpage.
Changing the top of x86_handle_debug_exception to always use DR6 and DR7
directly, rather than sometimes using thread->cpu->arch.dr6, fixes the
problem with gdb single stepping. However, the behaviour looks like it was
put in to fix a debugger crash, so it probably needs a more subtle fix.
Just to finish, I noticed that this issue was also mentioned in bug #5742,
and looks to have been introduced with changeset 36340, which was the fix
for that one.
--
Ticket URL: <http://dev.haiku-os.org/ticket/6751#comment:8>
Haiku <http://dev.haiku-os.org>
Haiku - the operating system.
Other related posts:
