[lldb-dev] Problem unwinding from inside of a CRT function

Zachary Turner zturner at google.com
Fri Jan 16 12:14:22 PST 2015

I'm still trying to wrap my head around the way LLDB does things.

If I understand correctly, when you run thread step-over, it enters single
step mode (by setting the trap flag on the CPU).  Every time the CPU traps,
LLDB picks this up and asks the ThreadPlan "should i stop now?"  "should i
stop now?" until the ThreadPlan returns true.  And part of this "should i
stop now" involves generating an unwind.

If my understanding is correct, then I have some questions:

1) Isn't this extremely slow?  What if I'm in main(), and the program I'm
debugging is, say, clang, and I say "step over the entire compilation"?  It
seems like this would take a decade to return.

2) What if one of the instructions is a pushf / popf?   You step over a
pushf, then later you try to continue, and it continues over the popf,
which restores the trap flag.  Now LLDB is confused because it doesn't
think it's single stepping, but the CPU does.  How does this work?

On Fri Jan 16 2015 at 11:50:21 AM Greg Clayton <gclayton at apple.com> wrote:

> One important thing to get right before proceeding is getting the correct
> address bounds of all functions so that the disassembly unwinder can do its
> job. You said you are stopped at a function, but don't know the function
> bounds. You will want to modify your object file reader (COFF?) to create a
> viable symbol table that can be used. On MacOSX we use the actual symbol
> table from the object file and supplement it with all sorts of goodies:
> 1 - LC_FUNCTION_STARTS load command which tells us all function bounds
> even if their symbols have been stripped
> 2 - the PLT entries are made into symbols
> 3 - more data from the __LINKEDIT is used to create other symbols
> Can you modify your COFF plug-in to get the symbols bounds for every
> function somehow? Then we can rely on the unwind plan that manually
> disassembles the functions and makes its own unwind info.
> Greg
> > On Jan 15, 2015, at 5:01 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > Btw, I'm still a little uncomfortable that not having unwind/ symbol
> info at any point no matter how deep in a function call chain, has the
> possibility to mess up a step over. In my original example, i had symbols
> for main but not printf. Is that not sufficient to step over a call to
> printf? It should be able to know from that a) the bounds of main(), b) the
> pc corresponding to the next line of source after printf, and c) the value
> of esp. Aren't those 3 pieces of information enough to step over any line
> of source, regardless of whether you have unwind information for the code
> inside the function you're stepping over?
> > On Thu, Jan 15, 2015 at 4:36 PM <jingham at apple.com> wrote:
> >
> > > On Jan 15, 2015, at 4:18 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > Which is unfortunate, because it seems to be needed even for basic
> stepping to work, like step over.  Originally I was just trying to
> implement stepping, and that's how I ran into this issue.  So that brings
> me to a related question.  Why is step over as complicated as it is?  It
> seems to me like step over can be implemented by disassembling 1 opcode,
> adding the size of the opcode to the current pc, and having the
> ThreadPlan::ShouldStop always return false unless the pc is equal to old_pc
> + size_of_opcode.
> > >
> >
> > You are describing "thread step-inst".  That should pretty much always
> work regardless of unwinder, etc.
> >
> > Source step over, as Jason said, is much more complicated.
> >
> > Jim
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
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