[lldb-dev] eh_frame or debug_frame
Ryan Brown
ribrdb at google.com
Wed Oct 15 17:33:18 PDT 2014
Yes, I'm using os x. You could try using the binary I uploaded in
http://llvm.org/bugs/show_bug.cgi?id=21118
That will only have one go thread though.
-- Ryan Brown
On Wed, Oct 15, 2014 at 5:22 PM, Jason Molenda <jmolenda at apple.com> wrote:
> Yeah, I was afraid of that.
>
> What I'm trying to do with this code is say "unwind using your super-super
> smart techniques ... but if you hit a wall, try the simplistic unwind
> method and see if you can get further."
>
> The problem here is that lldb is doing the full stack walk as far as it
> can be walked ... but it thinks maybe switching to the architecture default
> unwind plan might get it further (which it does not). The switch to the
> arch default unwind plan is destructive - it replaces the assembly profile
> unwind instructions for that function - and is remembered for future stack
> walks. That's why your threads get progressively fewer backtraces.
>
> I'll need to look into this and come up with a fix. I don't suppose your
> go binary runs on mac os x, does it? It would be great if I had a failing
> test program in front of me while I try to come up with a fix.
>
>
> > On Oct 15, 2014, at 5:13 PM, Ryan Brown <ribrdb at google.com> wrote:
> >
> > So adding "return false" to the top of TryFallbackUnwindPlan() fixes the
> problem.
> > The call at UnwindLLDB:177, when !reg_ctx_sp->IsValid() seems to be the
> only one I'm hitting.
> >
> > -- Ryan Brown
> >
> > On Wed, Oct 15, 2014 at 3:20 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > urgh, sorry, I wasn't paying attention to the svn log output when I copy
> & pasted the rev. It's this change I wanted to mention - r219247. It's
> going to be someone calling TryFallbackUnwindPlan(), I just added some new
> cases where that could be called. It may not be my most recent change
> (219247) but it's going to be that method which is causing the problem.
> >
> >
> > > On Oct 15, 2014, at 2:59 PM, Ryan Brown <ribrdb at google.com> wrote:
> > >
> > > rolling back r219772 (Be more consistent about null checks for the
> Process and ABI in GetFullUnwindPlanForFrame) doesn't seem to have any
> effect.
> > >
> > > -- Ryan Brown
> > >
> > > On Wed, Oct 15, 2014 at 2:31 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > >
> > > > On Oct 15, 2014, at 1:43 PM, Ryan Brown <ribrdb at google.com> wrote:
> > > >
> > > > Go doesn't have exception handlers, so it doesn't write .eh_frame.
> Wouldn't it make sense to use .debug_frame if .eh_frame is missing?
> > >
> > >
> > > We could do that. I'm surprised if go is emitting x86_64 code without
> eh_frame. As Joerg points out, debug_frame is great but it may not be
> available when an analysis tool is examining a binary. eh_frame has the
> benefit of always being in the binary.
> > >
> > > >
> > > > With my custom RegisterContext I got backtraces to work for my
> memory threads. But something strange is going on. I have 10 threads that
> should have identical traces, but the first has 5 frames, then 4, 3, 2, and
> the rest only have 1 frame.
> > >
> > >
> > > It's easiest to isolate one thread backtrace in a situation like
> this. For instance, looking at thread 7 in your program. (the unwind
> algorithms have no cross-thread information passing):
> > >
> > >
> > > th7/fr0 initialized frame current pc is 0xdaef cfa is 0x20809feb8
> using assembly insn profiling UnwindPlan
> > >
> > > lldb is using the assembly unwind inspection for frame 0. You said
> that all ten threads should have the same backtrace but thread #2 is at
> 0x2fe8c, #3 is at 0x209a, threads 4-15 are at 0xdaef. You meant threads
> 4-15 should all be the same.
> > >
> > >
> > > th7/fr5 pc = 0x0000000000002078
> > > th7/fr5 fp = 0xffffffffffffffff
> > > th7/fr4 supplying caller's stack pointer (7) value, computed from
> CFA
> > > th7/fr5 sp = 0x000000020809ffc8
> > > th7/fr5 active row: 0x0000000000002050: CFA=rbp+16 => rbp=[rbp]
> rsp=rbp+16 rip=[rbp+8]
> > >
> > > That's the architectural default unwind plan for x86_64 ABIs. Over in
> thread 6, it looks like failed to unwind past frame 5 with the assembly
> unwind, figured the assembly unwind was incorrect, and tried switching over
> to using the architectural default unwind plan:
> > >
> > > th6/fr0 supplying caller's saved reg 6's location, cached
> > > th6/fr5 full unwind plan 'assembly insn profiling' has been
> replaced by architecture default unwind plan 'x86_64 default unwind plan'
> for this function from now on.
> > > th6/fr5 supplying caller's saved reg 16's location using x86_64
> default unwind plan UnwindPlan
> > > th6/fr5 supplying caller's register 16 from the stack, saved at
> CFA plus offset -8
> > > th6/fr6 could not get pc value
> > > Frame 6 invalid RegisterContext for this frame, stopping stack
> walk
> > > th6 Unwind of this thread is complete.
> > >
> > > From this point forward main.okread() will use the arch default unwind
> plan which isn't going to work.
> > >
> > > Can you try rolling back r219772 and seeing if that helps? I suspect
> lldb may be slowly stripping off the last frame of the unwind for each
> thread as it progresses.
> > >
> > > J
> > >
> > > PS- "bt all" works just as well as "thread backtrace all".
> > >
> > >
> > > >
> > > > There's a log here, thread 6 is the one with the complete backtrace.
> https://gist.github.com/ribrdb/386fb0e555e82483d21d
> > > >
> > > > Comparing thread 7 with thread 6, things seem fine up to line 627:
> > > > th7/fr4 supplying caller's stack pointer (7) value, computed
> from CFA
> > > > th7/fr5 sp = 0x000000020809ffc8
> > > > th7/fr5 active row: 0x0000000000002050: CFA=rbp+16 => rbp=[rbp]
> rsp=rbp+16 rip=[rbp+8]
> > > >
> > > > While thread 6 has:
> > > > th6/fr4 supplying caller's stack pointer (7) value, computed
> from CFA
> > > > th6/fr5 sp = 0x000000020809f7c8
> > > > th6/fr5 active row: 0x000000000000206a: CFA=rsp+16 =>
> rsp=rsp+16 rip=[rsp+8]
> > > >
> > > > I don't know where rbp came from, it's not in the function at all:
> > > > 0x2050 <main.okread>: movq %gs:0x8a0, %rcx
> > > > 0x2059 <main.okread+9>: cmpq 0x10(%rcx), %rsp
> > > > 0x205d <main.okread+13>: ja 0x2066 ;
> main.okread + 22 at test.go:9
> > > > 0x205f <main.okread+15>: callq 0x2d510 ;
> runtime.morestack_noctxt at asm_amd64.s:330
> > > > 0x2064 <main.okread+20>: jmp 0x2050 ;
> main.okread at test.go:9
> > > > 0x2066 <main.okread+22>: subq $0x8, %rsp
> > > > 0x206a <main.okread+26>: movq 0x10(%rsp), %rbx
> > > > 0x206f <main.okread+31>: movq %rbx, (%rsp)
> > > > 0x2073 <main.okread+35>: callq 0x2000 ;
> main.doread at test.go:5
> > > > 0x2078 <main.okread+40>: addq $0x8, %rsp
> > > > 0x207c <main.okread+44>: retq
> > > > 0x207d <main.okread+45>: addb %al, (%rax)
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > -- Ryan Brown
> > > >
> > > > On Wed, Oct 15, 2014 at 11:48 AM, Ryan Brown <ribrdb at google.com>
> wrote:
> > > > Yes, I'm writing a class to do that now. It's just not supported by
> any of the existing register contexts.
> > > >
> > > > -- Ryan Brown
> > > >
> > > > On Wed, Oct 15, 2014 at 11:37 AM, Jason Molenda <jason at molenda.com>
> wrote:
> > > > Can't your OS plugin for the goroutines use the same sp and ip
> register numbers as x86_64 (instead of 0 and 1 like you might be using
> right now) when it reports them to lldb, and return all the other registers
> as "unavailable" if they're requested?
> > > >
> > > > The tricky bit about living on eh_frame / debug_frame is that lldb
> doesn't know what kind of unwind info it is being given. Is it just for
> exception handling locations? Does it contain prologue setup? epilogue?
> Is it fully asynchronous - giving unwind details at all locations? There
> aren't any flags in eh_frame/debug_frame that could give us a hint about
> what we're working with.
> > > >
> > > >
> > > >
> > > > On Oct 15, 2014, at 11:24 AM, Ryan Brown <ribrdb at google.com> wrote:
> > > >
> > > > > I'm actually struggling with this right now. I'm trying to
> implement an OS plugin so goroutines show up as threads.
> > > > > The go compiler puts instruction accurate unwind info into
> .debug_frame, I'm not sure what (if anything) goes into eh_frame.
> > > > > However lldb uses the disassembly instead of the dwarf info. The
> x86 unwinder assumes that all threads have the same LLDB register numbers,
> but other parts of the code require that the LLDB register number is <
> (number of registers). Goroutines only store sp and ip, so it seems I'm
> going to have to create a custom RegisterContext subclass to get the
> existing unwinder to work for goroutines.
> > > > >
> > > > > On Tue, Oct 14, 2014 at 5:51 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > > > > > On Oct 13, 2014, at 9:55 AM, Greg Clayton <gclayton at apple.com
> > > > > > wrote:
> > > > >
> > > > > >
> > > > >
> > > > >
> > > > > >
> > > > >
> > > > >
> > > > > >> On Oct 10, 2014, at 1:58 PM, Francois Pichet <pichet2000 at
> gmail.com
> > > > > > wrote:
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >> On Fri, Oct 10, 2014 at 4:20 PM, Greg Clayton <gclayton at
> apple.com
> > > > > > wrote:
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>> On Oct 10, 2014, at 1:05 PM, Philippe Lavoie <philippe.lavoie
> at octasic.com
> > > > > > wrote:
> > > > >
> > > > > >>>
> > > > >
> > > > >
> > > > > >>>
> > > > > Hi,
> > > > >
> > > > > >>>
> > > > >
> > > > >
> > > > > >>>
> > > > > I noticed that by default lldb does not read .debug_frame section
> to unwind frames but relies instead on .eh_frame .
> > > > >
> > > > > >>>
> > > > >
> > > > >
> > > > > >>>
> > > > > Is there a way to fallback to reading .debug_frame?
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>
> > > > > Not currently. Most compilers (gcc _and_ clang) put the same old
> stuff in .debug_frame as they do in .eh_frame, so we haven't had to use
> .debug_frame over .eh_frame yet. What compiler are using that is putting
> different (more complete) info in .debug_frame vs .eh_frame?
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>
> > > > >
> > > > >
> > > > > >>
> > > > > What about about C or C++ program compiled with -fno-exceptions?
> > > > >
> > > > > >>
> > > > > They will fall back to the UnwindAssembly way even if the
> .debug_frame is present right?
> > > > >
> > > > > >
> > > > >
> > > > >
> > > > > >
> > > > > If no EH frame exists for a frame, then we will always fall back
> to UnwindAssembly. We always use UnwindAssembly for the first frame and for
> any frame that is past an async interrupt (sigtramp). We use the EH
> frame/.debug_frame for any non-zero frames, but will always use
> UnwindAssembly if there is no such info.
> > > > >
> > > > >
> > > > >
> > > > > I want to expand on what Greg said earlier about eh_frame versus
> debug_frame.
> > > > >
> > > > > Ideally, eh_frame will be the minimal unwind instructions
> necessary to unwind the stack when exceptions are thrown/caught. eh_frame
> will not include unwind instructions for the prologue instructions or
> epilogue instructions -- because we can't throw an exception there, or have
> an exception thrown from a called function "below" us on the stack. We
> call these unwind instructions "synchronous" because they only describe the
> unwind state from a small set of locations.
> > > > >
> > > > > debug_frame would describe how to unwind the stack at every
> instruction location. Every instruction of the prologue and epilogue. If
> the code is built without a frame pointer, then it would have unwind
> instructions at every place where the stack pointer is modified. We
> describe these unwind instructions as "asynchronous" because they describe
> the unwind state at every instruction location.
> > > > >
> > > > >
> > > > > Instead what we have with gcc and clang is eh_frame instructions
> that describe the prologue (and some versions of gcc, the epilogue) plus
> the unwind state at synchronous unwind locations (where an exception can be
> thrown). We have a half-way blend of asynchronous and synchronous ... it's
> "pretty good" but not "guaranteed" from a debugger's perspective. It would
> be great if eh_frame was genuinely only the unwind instructions for
> exception handling and debug_frame had the full unwind state at every
> instruction and we could depend on debug_frame. But in reality, the same
> unwind instructions are put in both eh_frame and debug_frame -- so there's
> little point in ever reading debug_frame. lldb does not read debug_frame
> today, although it would be easy to do so.
> > > > >
> > > > >
> > > > > As an experiment starting late August (r216406), lldb is now
> trying to use eh_frame for the currently-executing frame. Even though it
> isn't *guaranteed* to be accurate at all instructions, in practice it's
> pretty good -- good enough that gdb seems to be able to live on it. Tong
> Shen's patch in r216406 does augment the eh_frame unwind instructions with
> the epilogue unwind... newer gcc's apparently describe the epilogue in
> eh_frame but few other compilers do.
> > > > >
> > > > > It's an open question how well living off eh_frame unwind
> instructions will work with a non-gcc/non-clang compiler. That's why I say
> this is an "experiment" - we may have to revert to lldb's UnwindAssembly
> profiling code for the currently-executing function if this breaks with
> other compilers.
> > > > >
> > > > > J
> > > > >
> > > > >
> > > > > _______________________________________________
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> > > > > lldb-dev at cs.uiuc.edu
> > > > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > > >
> > > >
> > > >
> > > > _______________________________________________
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> > >
> > >
> >
> >
>
>
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