> On Oct 15, 2014, at 1:43 PM, Ryan Brown <[email protected]> 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?
Yes it does make sense. > > 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. Yeah, you will need to trace through and see what is going wrong by debugging this. > > 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 > > [email protected] > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev > > > > _______________________________________________ > lldb-dev mailing list > [email protected] > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev _______________________________________________ lldb-dev mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
