gdb/linux: Delete all other LWPs immediately on ptrace exec event

gdb/linux: Delete all other LWPs immediately on ptrace exec event

I noticed that on an Ubuntu 20.04 system, after a following patch
("Step over clone syscall w/ breakpoint,
TARGET_WAITKIND_THREAD_CLONED"), the gdb.threads/step-over-exec.exp
was passing cleanly, but still, we'd end up with four new unexpected
GDB core dumps:

		 === gdb Summary ===

 # of unexpected core files      4
 # of expected passes            48

That said patch is making the pre-existing
gdb.threads/step-over-exec.exp testcase (almost silently) expose a
latent problem in gdb/linux-nat.c, resulting in a GDB crash when:

 #1 - a non-leader thread execs
 #2 - the post-exec program stops somewhere
 #3 - you kill the inferior

Instead of #3 directly, the testcase just returns, which ends up in
gdb_exit, tearing down GDB, which kills the inferior, and is thus
equivalent to #3 above.

Vis (after said patch is applied):

 $ gdb --args ./gdb /home/pedro/gdb/build/gdb/testsuite/outputs/gdb.threads/step-over-exec/step-over-exec-execr-thread-other-diff-text-segs-true
 ...
 (top-gdb) r
 ...
 (gdb) b main
 ...
 (gdb) r
 ...
 Breakpoint 1, main (argc=1, argv=0x7fffffffdb88) at /home/pedro/gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.threads/step-over-exec.c:69
 69        argv0 = argv[0];
 (gdb) c
 Continuing.
 [New Thread 0x7ffff7d89700 (LWP 2506975)]
 Other going in exec.
 Exec-ing /home/pedro/gdb/build/gdb/testsuite/outputs/gdb.threads/step-over-exec/step-over-exec-execr-thread-other-diff-text-segs-true-execd
 process 2506769 is executing new program: /home/pedro/gdb/build/gdb/testsuite/outputs/gdb.threads/step-over-exec/step-over-exec-execr-thread-other-diff-text-segs-true-execd

 Thread 1 "step-over-exec-" hit Breakpoint 1, main () at /home/pedro/gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.threads/step-over-exec-execd.c:28
 28        foo ();
 (gdb) k
 ...
 Thread 1 "gdb" received signal SIGSEGV, Segmentation fault.
 0x000055555574444c in thread_info::has_pending_waitstatus (this=0x0) at ../../src/gdb/gdbthread.h:393
 393         return m_suspend.waitstatus_pending_p;
 (top-gdb) bt
 #0  0x000055555574444c in thread_info::has_pending_waitstatus (this=0x0) at ../../src/gdb/gdbthread.h:393
 #1  0x0000555555a884d1 in get_pending_child_status (lp=0x5555579b8230, ws=0x7fffffffd130) at ../../src/gdb/linux-nat.c:1345
 #2  0x0000555555a8e5e6 in kill_unfollowed_child_callback (lp=0x5555579b8230) at ../../src/gdb/linux-nat.c:3564
 #3  0x0000555555a92a26 in gdb::function_view<int (lwp_info*)>::bind<int, lwp_info*>(int (*)(lwp_info*))::{lambda(gdb::fv_detail::erased_callable, lwp_info*)#1}::operator()(gdb::fv_detail::erased_callable, lwp_info*) const (this=0x0, ecall=..., args#0=0x5555579b8230) at ../../src/gdb/../gdbsupport/function-view.h:284
 #4  0x0000555555a92a51 in gdb::function_view<int (lwp_info*)>::bind<int, lwp_info*>(int (*)(lwp_info*))::{lambda(gdb::fv_detail::erased_callable, lwp_info*)#1}::_FUN(gdb::fv_detail::erased_callable, lwp_info*) () at ../../src/gdb/../gdbsupport/function-view.h:278
 #5  0x0000555555a91f84 in gdb::function_view<int (lwp_info*)>::operator()(lwp_info*) const (this=0x7fffffffd210, args#0=0x5555579b8230) at ../../src/gdb/../gdbsupport/function-view.h:247
 #6  0x0000555555a87072 in iterate_over_lwps(ptid_t, gdb::function_view<int (lwp_info*)>) (filter=..., callback=...) at ../../src/gdb/linux-nat.c:864
 #7  0x0000555555a8e732 in linux_nat_target::kill (this=0x55555653af40 <the_amd64_linux_nat_target>) at ../../src/gdb/linux-nat.c:3590
 #8  0x0000555555cfdc11 in target_kill () at ../../src/gdb/target.c:911
 ...

The root of the problem is that when a non-leader LWP execs, it just
changes its tid to the tgid, replacing the pre-exec leader thread,
becoming the new leader.  There's no thread exit event for the execing
thread.  It's as if the old pre-exec LWP vanishes without trace.  The
ptrace man page says:

 "PTRACE_O_TRACEEXEC (since Linux 2.5.46)
	Stop the tracee at the next execve(2).  A waitpid(2) by the
	tracer will return a status value such that

	  status>>8 == (SIGTRAP | (PTRACE_EVENT_EXEC<<8))

	If the execing thread is not a thread group leader, the thread
	ID is reset to thread group leader's ID before this stop.
	Since Linux 3.0, the former thread ID can be retrieved with
	PTRACE_GETEVENTMSG."

When the core of GDB processes an exec events, it deletes all the
threads of the inferior.  But, that is too late -- deleting the thread
does not delete the corresponding LWP, so we end leaving the pre-exec
non-leader LWP stale in the LWP list.  That's what leads to the crash
above -- linux_nat_target::kill iterates over all LWPs, and after the
patch in question, that code will look for the corresponding
thread_info for each LWP.  For the pre-exec non-leader LWP still
listed, won't find one.

This patch fixes it, by deleting the pre-exec non-leader LWP (and
thread) from the LWP/thread lists as soon as we get an exec event out
of ptrace.

GDBserver does not need an equivalent fix, because it is already doing
this, as side effect of mourning the pre-exec process, in
gdbserver/linux-low.cc:

  else if (event == PTRACE_EVENT_EXEC && cs.report_exec_events)
    {
...
      /* Delete the execing process and all its threads.  */
      mourn (proc);
      switch_to_thread (nullptr);

The crash with gdb.threads/step-over-exec.exp is not observable on
newer systems, which postdate the glibc change to move "libpthread.so"
internals to "libc.so.6", because right after the exec, GDB traps a
load event for "libc.so.6", which leads to GDB trying to open
libthread_db for the post-exec inferior, and, on such systems that
succeeds.  When we load libthread_db, we call
linux_stop_and_wait_all_lwps, which, as the name suggests, stops all
lwps, and then waits to see their stops.  While doing this, GDB
detects that the pre-exec stale LWP is gone, and deletes it.

If we use "catch exec" to stop right at the exec before the
"libc.so.6" load event ever happens, and issue "kill" right there,
then GDB crashes on newer systems as well.  So instead of tweaking
gdb.threads/step-over-exec.exp to cover the fix, add a new
gdb.threads/threads-after-exec.exp testcase that uses "catch exec".
The test also uses the new "maint info linux-lwps" command if testing
on Linux native, which also exposes the stale LWP problem with an
unfixed GDB.

Also tweak a comment in infrun.c:follow_exec referring to how
linux-nat.c used to behave, as it would become stale otherwise.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I21ec18072c7750f3a972160ae6b9e46590376643