[lldb-dev] eh_frame or debug_frame
Ryan Brown
ribrdb at google.com
Wed Oct 15 13:43:08 PDT 2014
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?
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.
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
>> >
>> >
>> > _______________________________________________
>> > lldb-dev mailing list
>> > lldb-dev at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/lldb-dev/attachments/20141015/bdd86aae/attachment.html>
More information about the lldb-dev
mailing list