http://sourceware.org/ml/gdb-patches/2016-03/msg00013.html
Subject: [PATCH 1/2] Fix PR gdb/19676: Disable displaced stepping if /proc not mounted

On GNU/Linux archs that support displaced stepping, if /proc is not
mounted, GDB gets stuck not able to step past breakpoints:

 (gdb) c
 Continuing.
 dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:2163
 2163      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot find AT_ENTRY auxiliary vector entry.
 (gdb) c
 Continuing.
 dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:2163
 2163      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot find AT_ENTRY auxiliary vector entry.
 (gdb)

That's because GDB can't figure out where the scratch pad is.

This is a regression introduced by the earlier changes to make the
Linux native target always work in non-stop mode.

This commit makes GDB detect the case and fallback to stepping over
breakpoints in-line.

gdb/ChangeLog:
2016-03-01  Pedro Alves  <pedro@cascais.lan>

	PR gdb/19676
	* infrun.c (displaced_step_prepare): Also disable displaced
	stepping on NOT_SUPPORTED_ERROR.
	* linux-tdep.c (linux_displaced_step_location): If reading auxv
	fails, throw NOT_SUPPORTED_ERROR instead of generic error.
---
 gdb/infrun.c     | 3 ++-
 gdb/linux-tdep.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 3e8c9e0..696105d 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1894,7 +1894,8 @@ displaced_step_prepare (ptid_t ptid)
     {
       struct displaced_step_inferior_state *displaced_state;
 
-      if (ex.error != MEMORY_ERROR)
+      if (ex.error != MEMORY_ERROR
+	  && ex.error != NOT_SUPPORTED_ERROR)
 	throw_exception (ex);
 
       if (debug_infrun)
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 555c302..f197aa7 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -2426,7 +2426,8 @@ linux_displaced_step_location (struct gdbarch *gdbarch)
      location.  The auxiliary vector gets us the PowerPC-side entry
      point address instead.  */
   if (target_auxv_search (&current_target, AT_ENTRY, &addr) <= 0)
-    error (_("Cannot find AT_ENTRY auxiliary vector entry."));
+    throw_error (NOT_SUPPORTED_ERROR,
+		 _("Cannot find AT_ENTRY auxiliary vector entry."));
 
   /* Make certain that the address points at real code, and not a
      function descriptor.  */
-- 
2.5.0



http://sourceware.org/ml/gdb-patches/2016-03/msg00014.html
Subject: [PATCH 2/2] Fix PR gdb/19676: Internal error in linux-thread.db.c if /proc not mounted

If /proc is not mounted, GDB fails an assertion in find_new_threads_once:

 Continuing.
 /home/pedro/gdb/mygit/src/gdb/linux-thread-db.c:1249: internal-error: find_new_threads_once: Assertion `!target_has_execution' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)

That was supposed to catch misuses of td_ta_thr_iter, which is unsafe
for live debugging.  However, if /proc is not mounted, we still
fallback to using it.

I didn't bother with a warning, because GDB already prints several
others related to failing to open /proc files.

gdb/ChangeLog:
2016-03-01  Pedro Alves  <pedro@cascais.lan>

	PR gdb/19676
	* linux-thread-db.c (try_thread_db_load_1): Leave
	info->td_ta_thr_iter_p NULL iff debugging a live process and we
	have /proc access.
	(find_new_threads_once): Assert that we have a non-NULL
	info->td_ta_thr_iter_p instead of checking whether the target has
	execution.
---
 gdb/linux-thread-db.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c
index 1eb457d..ce60beb 100644
--- a/gdb/linux-thread-db.c
+++ b/gdb/linux-thread-db.c
@@ -564,7 +564,6 @@ try_thread_db_load_1 (struct thread_db_info *info)
 
   /* These are essential.  */
   CHK (TDB_VERBOSE_DLSYM (info, td_ta_map_lwp2thr));
-  CHK (TDB_VERBOSE_DLSYM (info, td_ta_thr_iter));
   CHK (TDB_VERBOSE_DLSYM (info, td_thr_validate));
   CHK (TDB_VERBOSE_DLSYM (info, td_thr_get_info));
 
@@ -572,10 +571,6 @@ try_thread_db_load_1 (struct thread_db_info *info)
   TDB_DLSYM (info, td_thr_tls_get_addr);
   TDB_DLSYM (info, td_thr_tlsbase);
 
-#undef TDB_VERBOSE_DLSYM
-#undef TDB_DLSYM
-#undef CHK
-
   /* It's best to avoid td_ta_thr_iter if possible.  That walks data
      structures in the inferior's address space that may be corrupted,
      or, if the target is running, may change while we walk them.  If
@@ -587,6 +582,15 @@ try_thread_db_load_1 (struct thread_db_info *info)
      currently on core targets, as it uses ptrace directly.  */
   if (target_has_execution
       && linux_proc_task_list_dir_exists (ptid_get_pid (inferior_ptid)))
+    info->td_ta_thr_iter_p = NULL;
+  else
+    CHK (TDB_VERBOSE_DLSYM (info, td_ta_thr_iter));
+
+#undef TDB_VERBOSE_DLSYM
+#undef TDB_DLSYM
+#undef CHK
+
+  if (info->td_ta_thr_iter_p == NULL)
     {
       struct lwp_info *lp;
       int pid = ptid_get_pid (inferior_ptid);
@@ -1246,7 +1250,7 @@ find_new_threads_once (struct thread_db_info *info, int iteration,
   data.new_threads = 0;
 
   /* See comment in thread_db_update_thread_list.  */
-  gdb_assert (!target_has_execution);
+  gdb_assert (info->td_ta_thr_iter_p != NULL);
 
   TRY
     {
-- 
2.5.0