[Bug threads/18006] New: internal error if threaded program calls clone(CLONE_VM)

["palves at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18006

            Bug ID: 18006
           Summary: internal error if threaded program calls
                    clone(CLONE_VM)
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: threads
          Assignee: unassigned at sourceware dot org
          Reporter: palves at redhat dot com

On GNU/Linux, if a pthreaded program has a thread call clone(CLONE_VM)
directly,
and then that clone LWP hits a debug event (breakpoint, etc.) GDB internal
errors.  Threaded programs shouldn't really be calling clone directly, but GDB
shouldn't crash either.

On GDB 7.9, the crash looks like:

 (gdb) break clone_fn
 Breakpoint 2 at 0x4007d8: file clone-thread_db.c, line 35.
 (gdb) r
 ...
 [Thread debugging using libthread_db enabled]
 ...
 /home/pedro/gdb/mygit/src/gdb/linux-nat.c:1030: internal-error:
lin_lwp_attach_lwp: Assertion `lwpid > 0' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.

On earlier releases, it looked like this:

 (gdb) b clone_fn 
 Breakpoint 1 at 0x4007d8: file clone-thread_db.c, line 35.
 (gdb) r
 ...
 [Thread debugging using libthread_db enabled]
 ...
 [New Thread 0x7ffff7fc2700 (LWP 3886)]
 ../../gdb/linux-thread-db.c:437: internal-error: thread_get_info_callback:
Assertion `inout->thread_info != NULL' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.

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