[lldb-dev] Pending breakpoints to dlsym()ed functions

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Thu Feb 15 03:21:16 PST 2018

+ eugene as the "most recent person who worked on the DYLD plugin" :D

Hi Dmitry,

I've tried your sample, and I was indeed able to reproduce the
problem. What makes your case special is that "sin" and "cos" are
indirect functions (STT_GNU_IFUNC), so we have to do some extra work
(call the resolver function) to resolve them. Doing that while we're
in the process of loading a module seems to be going south. There seem
to two things going wrong here which contribute to the overall effect
of "hanging":
1: We resolve the address of the resolver function as 0xfff...,
presumably because the module is not fully initialized yet.
2: Calling that address results in an inferior SEGV-ing, but for some
reason InferiorCall function does not detect that. (Probably also has
something to do with the "in the middle of module load" context)

A trivial fix would be to avoid calling an obviously wrong address,
but that's not going to solve your immediate problem (just prevent the
hang). May I suggest you file a bug with this information and we'll
see what we can do about that.

As a workaround, you can try setting the breakpoint on the symbol that
the IFUNC will eventually resolve to (in my case that would be
__sin_avx). Not an ideal solution, but I can't think of anything
better now.

On 15 February 2018 at 07:39, Dmitry Antipov via lldb-dev
<lldb-dev at lists.llvm.org> wrote:
> I'm trying to setup a pending breakpoint for sin() and cos() which are
> dlsym()ed from libm.so
> (sample attached), and an attempt to continue execution seems just hangs the
> debugger. For example:
> (lldb) attach 17043
> Process 17043 stopped
> * thread #1, name = 't-dlopen', stop reason = signal SIGSTOP
>     frame #0: 0x0000000000400728 t-dlopen`main(argc=1,
> argv=0x00007ffd2b0a00c8) at t-dlopen.c:21
>    18     for (a = 0; a < DELAY + argc; a++)
>    19       for (b = 0; b < DELAY + argc; b++)
>    20         for (c = 0; c < DELAY + argc; c++)
> -> 21           z += a + b + c;
>    22     while (1)
>    23       {
>    24         void *handle = dlopen (LIBM_SO, RTLD_LAZY);
> Executable module set to "/home/dantipov/tmp/t-dlopen".
> Architecture set to: x86_64--linux.
> (lldb) breakpoint set -n sin
> Breakpoint 1: no locations (pending).
> WARNING:  Unable to resolve breakpoint to any actual locations.
> (lldb) breakpoint set -n cos
> Breakpoint 2: no locations (pending).
> WARNING:  Unable to resolve breakpoint to any actual locations.
> (lldb) process continue                              ;; After this, nothing
> happens for a long time
> Process 17043 resuming
> (lldb) process status                                ;; After this, lldb
> hangs and have to be killed
> I've tried 6.0.0-rc2 as well as 7.0.0 svn trunk 325127, with the same
> disappointing results.
> Dmitry
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