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

[Pavel Labath via llvm-dev]

[+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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