[lldb-dev] [llvm-dev] lldb stops on every call to dlopen

Tue Apr 17 12:59:54 PDT 2018

Ding!  That’s it.  This program does use SIGALRM.  I wouldn’t have thought it would be enabled at this point, but apparently it is because I got lots of sigalarm halts during the .so loading.

Further, if I run in lldb with 

process handle -n false -p false -s false SIGALRM

then the debugger seems to run fine without stopping during dlopen().  The SIGALRM itself isn’t important to the operation of the program.

With this knowledge I can probably create a simple testcase.  Would this be considered an lldb bug?  If so should I submit a testcase in some way?

thanks!
--steve

 The lover of nature is he whose inward and outward senses are still truly adjusted to each other; who has retained the spirit of infancy even into the era of manhood.

> On Apr 17, 2018, at 1:12 PM, Pavel Labath <labath at google.com> wrote:
> 
> It's a bit of a wild guess, but is it possible that you (or one of the
> libraries you use) are doing anything with signals (SIGALRM or such?). I
> think I remember looking at the code handling the server-side ignored
> signal handling and thinking that it could go wrong if you get a signal
> while doing a instruction-step. I am not sure it fully applies here as the
> last command that lldb client did was a "continue", but i think it has to
> have something to do with signals, as you end up stopped in a signal
> handler.
> 
> Could you try the following sequence of commands?
> (lldb) process launch --stop-at-entry-point
> (lldb) process handle --notify true --stop true #Stop on all signals
> (lldb) continue
> 
> and let us know if you see any extra stops due to signals. If that doesn't
> find anything then I think we'll have start pulling logs from the
> lldb-server side, as there doesn't seem to be anything wrong with the
> client. The easiest way to achieve that is to do a
> export LLDB_SERVER_LOG_CHANNELS="posix all:gdb-remote packets"
> export LLDB_DEBUGSERVER_LOG_FILE=/tmp/server.log
> before launching lldb.
> On Tue, 17 Apr 2018 at 18:28, Steve Ravet <steve.ravet at apple.com> wrote:
> 
>> Pavel asked for a dump of gdb-remote commands.  I got that and ran it
> through the gdbremote decoder, and trimmed to include what looks like the
> last successful continue after breakpoint and then the halt on dlopen.
> Both cases stop on signal 5.
> 
>> After the stop message the debugger issues two binary reads and then
> apparently makes the decision that it should stop rather than continue.
> The stopping case is missing the equivalent of "Element 1: Single stepping
> past breakpoint site 2 at 0x2aaaaaab9eb0” which is in the continuing case.
> I’ve attached the file here:
> 
> 
>> thanks,
>> --steve
> 
> 
>>  From the ashes of disaster grow the roses of success.
> 
> 
> 
>> On Apr 17, 2018, at 11:27 AM, Jim Ingham <jingham at apple.com> wrote:
> 
>> It is interesting that the stop reason on the thread that stopped is
> "trace".  That's what you would expect returning from the single-step to
> step over the breakpoint.  But it looks like we got a signal while
> single-stepping, but the stop reason was misreported by somebody.
> 
>> Jim
> 
> 
>> On Apr 17, 2018, at 6:00 AM, Pavel Labath via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> 
>> [+lldb-dev]
> 
>> Hello Steve,
> 
>> thanks for the report.
> 
>> The fact that you see the rendezvous breakpoint being hit many times is
> not
>> surprising. We get those every time the library is loaded (we need that to
>> load relevant debug info and set potential breakpoints). However, they
>> should generally not be surfaced to the user (unless you have the
>> stop-on-sharedlibrary-events setting set, which you don't).
> 
>> The part that is suspicious to me is that __restore_rt shows up on the top
>> of the backtrace. This is a trampoline used to return from signal
> handlers,
>> and it would seem to indicate that you got some sort of a signal while
>> loading the libraries. I don't know why this would happen, but it could be
>> that this is confusing lldb's auto-resume logic.
> 
>> The interesting part to see here is what lldb thinks are the stop reasons
>> for individual threads in the process (is the process multi-threaded?) for
>> the last couple of stops. The "lldb step" and "gdb-remote packets" log
>> categories are the most interesting to observe here. If you are able to
>> send me the log traces, I can help you interpret them.
> 
>> regards,
>> pavel
> 
> 
> 
> 
> 
>> On Tue, 17 Apr 2018 at 02:27, Steve Ravet via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
> 
>> Hello lldb developers, I am running into a problem with lldb on Linux.  I
> 
>> am currently running llvm 6.0.0.
> 
>> I have an executable that dynamically loads a large number of shared
> 
>> libraries at runtime.  These are explicitly loaded via dlopen (they are
>> specified in a configuration file), and after loading a few (typically a
>> dozen or so, but the number varies) lldb will halt during dlopen.  If I
>> continue, it will load a few more then halt again, which makes debugging
>> from startup impractical since there are so many libraries to be loaded
>> (more than a hundred of them).
> 
>> When I build and debug this same C++ on macOS, the debugger works fine.
> 
>> I have verified that target.process.stop-on-sharedlibrary-events is false.
>> I turned on dyld logging and I see lots of log messages about
>> RendezvousBreakpoint being hit, but I don’t see anything that sheds light
>> on why some libraries load without stopping but others don’t.
> 
>> I have tried to recreate this in a trivial program that calls dlopen in a
> 
>> loop, but haven’t been able to reproduce.
> 
>> Can your offer any suggestions for further debugging this?  More
> 
>> supporting evidence follows.
> 
>> Here is the message when the debugger stops:
> 
> 
>> Process 120004 stopped
>> * thread #1, name = ‘xxxxxxxx', stop reason = trace
>>    frame #0: 0x00002aaaacfca6a0 libc.so.6`__restore_rt
>> libc.so.6`__restore_rt:
>> ->  0x2aaaacfca6a0 <+0>: movq   $0xf, %rax
>>    0x2aaaacfca6a7 <+7>: syscall
>>    0x2aaaacfca6a9 <+9>: nopl   (%rax)
> 
> 
>> libc.so.6`__libc_sigaction:
>>    0x2aaaacfca6b0 <+0>: subq   $0xd0, %rsp
> 
> 
>> I do not have the stop on shared library events setting enabled:
> 
> 
>> (lldb) settings show target.process.stop-on-sharedlibrary-events
>> target.process.stop-on-sharedlibrary-events (boolean) = false
> 
> 
> 
> 
>> The backtrace goes back to dlopen:
> 
> 
>> (lldb) bt
>> * thread #1, name = ‘xxxxx', stop reason = trace
>>  * frame #0: 0x00002aaaacfca6a0 libc.so.6`__restore_rt
>>    frame #1: 0x00002aaaaaab9eb0 ld-linux-x86-64.so.2
>>    frame #2: 0x00002aaaaaabdc53 ld-linux-x86-64.so.2`dl_open_worker + 499
>>    frame #3: 0x00002aaaaaab9286 ld-linux-x86-64.so.2`_dl_catch_error +
> 
>> 102
> 
>>    frame #4: 0x00002aaaaaabd63a ld-linux-x86-64.so.2`_dl_open + 186
>>    frame #5: 0x00002aaaac39df66 libdl.so.2`dlopen_doit + 102
>>    frame #6: 0x00002aaaaaab9286 ld-linux-x86-64.so.2`_dl_catch_error +
> 
>> 102
> 
>>    frame #7: 0x00002aaaac39e29c libdl.so.2`_dlerror_run + 124
>>    frame #8: 0x00002aaaac39dee1 libdl.so.2`__dlopen_check + 49
> 
> 
>> the dyld debug log has a lot of this:
>> 209 intern-state     DynamicLoaderPOSIXDYLD::RendezvousBreakpointHit pid
> 
>> 153501 stop_when_images_change=false
> 
>> 210 intern-state     DynamicLoaderPOSIXDYLD::RendezvousBreakpointHit
> 
>> called for pid 153501
> 
>> 211 intern-state     DYLDRendezvous::Resolve address size: 8, padding 4
>> 212 intern-state     DYLDRendezvous::Resolve cursor = 0x2aaaaaccc160
>> 213 intern-state     DynamicLoaderPOSIXDYLD::RendezvousBreakpointHit pid
> 
>> 153501 stop_when_images_change=false
> 
>> 214 intern-state     DynamicLoaderPOSIXDYLD::RendezvousBreakpointHit
> 
>> called for pid 153501
> 
>> 215 intern-state     DYLDRendezvous::Resolve address size: 8, padding 4
>> 216 intern-state     DYLDRendezvous::Resolve cursor = 0x2aaaaaccc160
> 
> 
> 
> 
>> thanks,
>> --steve
> 
> 
> 
>>  In the woods too, a man casts off his years, as the snake his slough,
> 
>> and at what period soever of life, is always a child.
> 
> 
> 
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