[haiku-bugs] Re: [Haiku] #6751: gdb won't single step
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Fri, 29 Oct 2010 18:04:21 -0000
#6751: gdb won't single step
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Reporter: grahamh | Owner: bonefish
Type: bug | Status: in-progress
Priority: normal | Milestone: R1
Component: Applications/Command Line Tools | Version:
Resolution: | R1/Development
Blocked By: | Keywords:
Platform: x86 | Has a Patch: 0
| Blocking:
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Comment (by bonefish):
Replying to [comment:8 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.
You have either omitted them intentionally or you're missing a few details
here. If you have a look at
`src/system/kernel/arch/x86/arch_interrupts.S`, this is where the magic is
supposed to happen. On any exception (save the double fault) `int_bottom`
is entered. There we check whether we came from userland and, if so,
continue in `int_bottom_user`. The `DISABLE_BREAKPOINTS()` macro is where
we call `x86_exit_user_debug_at_kernel_entry()`, when the
`THREAD_FLAGS_BREAKPOINTS_INSTALLED` thread flag is set. I haven't checked
the details yet, but I guess the problem is that this flag is only set,
when there's indeed at least one breakpoint installed, i.e. not when
single-stepping without any breakpoint (this would also perfectly explain
the problem Rene showed me). I'll think about how to solve this without
breaking what r35951/r36340 were trying to fix. Though probably not before
tomorrow.
--
Ticket URL: <http://dev.haiku-os.org/ticket/6751#comment:10>
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