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 <jmole...@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 <rib...@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 <jmole...@apple.com> > wrote: > > > > > On Oct 15, 2014, at 1:43 PM, Ryan Brown <rib...@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 <rib...@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 <ja...@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 <rib...@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 <jmole...@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 > > > > > > > > > > > > _______________________________________________ > > > > lldb-dev mailing list > > > > lldb-dev@cs.uiuc.edu > > > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev > > > > > > > > > > > > _______________________________________________ > > > lldb-dev mailing list > > > lldb-dev@cs.uiuc.edu > > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev > > > > > >
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