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

Tue Apr 17 11:12:12 PDT 2018

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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