Re: [PATCH] Fix PR 21337 (v3): segfault when re-reading symbols with remote debugging.

[Doug Gilmore <Doug dot Gilmore at imgtec dot com>]

On 05/11/2017 08:29 PM, Simon Marchi wrote:
> Hi Doug,
> 
> On 2017-05-10 19:26, Doug Gilmore wrote:
>> The basic issue is that section data referenced through an objfile
>> ....
> 
> Thanks for the commit log.
> ...
> 
> Thanks for adding a test case!
> 
> Simon

Hi Simon,

I attached an updated patch, which also fails with
--target_board=native-extended-gdbserver, though I haven't come up
with a test for --target_board=native-gdbserver, so the test is
basically just a NOOP in that situation.

I think I resolved all of the issues.  Does this look OK?

Thanks,

Doug

Fix PR 21337: segfault when re-reading symbols.

Fix issue exposed by commit 3e29f34.

The basic issue is that section data referenced through an objfile
pointer can also be referenced via the program-space data pointer,
although via a separate mapping mechanism, which is set up by
update_section_map.  Thus once section data attached to an objfile
pointer is released, the section map associated with the program-space
data pointer must be marked dirty to ensure that update_section_map is
called to prevent stale data being referenced.  For the matter at hand
this marking is being done via a call to objfiles_changed.

Before commit 3e29f34 objfiles_changed could be called after all of
the objfile pointers were processed in reread_symbols since section
data references via the program-space data pointer would not occur in
the calls of read_symbols performed by reread_symbols.

With commit 3e29f34 MIPS target specific calls to find_pc_section were
added to the code for DWARF information processing, which is called
via read_symbols.  Thus in reread_symbols the call to objfiles_changed
needs to be called before calling read_symbols, otherwise stale
section data can be referenced.

Thanks to Luis Machado for providing text for the main comment
associated with the change.

gdb/
2017-??-??  Doug Gilmore <Doug.Gilmore@imgtec.com>
    PR gdb/21337.
    * symfile.c (reread_symbols): Call objfiles_changed just before
    read_symbols.

gdb/testsuite
2017-??-??  Doug Gilmore <Doug Gilmore@imgtec.com>
    PR gdb/21337
    * gdb.base/reread-readsym.exp: New file.
    * gdb.base/reread-readsym.c: New file.
---
 gdb/symfile.c                             | 20 ++++++++--
 gdb/testsuite/gdb.base/reread-readsym.c   | 22 +++++++++++
 gdb/testsuite/gdb.base/reread-readsym.exp | 61 +++++++++++++++++++++++++++++++
 3 files changed, 100 insertions(+), 3 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/reread-readsym.c
 create mode 100644 gdb/testsuite/gdb.base/reread-readsym.exp

diff --git a/gdb/symfile.c b/gdb/symfile.c
index 846aabe..0492f56 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -2576,6 +2576,9 @@ reread_symbols (void)
 	  /* Free the obstacks for non-reusable objfiles.  */
 	  psymbol_bcache_free (objfile->psymbol_cache);
 	  objfile->psymbol_cache = psymbol_bcache_init ();
+
+	  /* NB: after this call to obstack_free, objfiles_changed
+	     will need to be called (see discussion below).  */
 	  obstack_free (&objfile->objfile_obstack, 0);
 	  objfile->sections = NULL;
 	  objfile->compunit_symtabs = NULL;
@@ -2628,6 +2631,20 @@ reread_symbols (void)
 	  clear_complaints (&symfile_complaints, 1, 1);
 
 	  objfile->flags &= ~OBJF_PSYMTABS_READ;
+
+	  /* We are about to read new symbols and potentially also DWARF
+	     information.  Some targets may want to pass addresses read from
+	     DWARF DIE's through an adjustment function before saving them, like
+	     MIPS, which may call into "find_pc_section".  When called, that
+	     function will make use of per-objfile program space data.
+
+	     Since we discarded our section information above, we have dangling
+	     pointers in the per-objfile program space data structure.  Force
+	     GDB to update the section mapping information by letting it know
+	     the objfile has changed, making the dangling pointers point to
+	     correct data again.  */
+	  objfiles_changed ();
+
 	  read_symbols (objfile, 0);
 
 	  if (!objfile_has_symbols (objfile))
@@ -2660,9 +2677,6 @@ reread_symbols (void)
 
   if (!new_objfiles.empty ())
     {
-      /* Notify objfiles that we've modified objfile sections.  */
-      objfiles_changed ();
-
       clear_symtab_users (0);
 
       /* clear_objfile_data for each objfile was called before freeing it and
diff --git a/gdb/testsuite/gdb.base/reread-readsym.c b/gdb/testsuite/gdb.base/reread-readsym.c
new file mode 100644
index 0000000..2fee696
--- /dev/null
+++ b/gdb/testsuite/gdb.base/reread-readsym.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/reread-readsym.exp b/gdb/testsuite/gdb.base/reread-readsym.exp
new file mode 100644
index 0000000..6cdb541
--- /dev/null
+++ b/gdb/testsuite/gdb.base/reread-readsym.exp
@@ -0,0 +1,61 @@
+#   Copyright 2017 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+standard_testfile
+
+set gdbfile [standard_output_file ${testfile}.gdb]
+
+# Test rereading executable.  See PR gdb/21337.
+
+proc generate_cmd_file {gdbfile binfile} {
+    set ofd [open $gdbfile w]
+
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "p \"source-command-completed\""
+    close $ofd
+}
+
+if [use_gdb_stub] {
+    return 0
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+# Start with a fresh gdb.
+clean_restart ${testfile}
+
+# Using the source command to read commands from a file is important,
+# otherwise section data is freed and reallocated using the same
+# memory locations and the bug is not exposed.
+generate_cmd_file $gdbfile $binfile
+
+gdb_test "source $gdbfile" ".*source-command-completed.*" \
+    "source $testfile.gdb"
+# Sometimes the failure only occurs on the second invocation.
+gdb_test "source $gdbfile" ".*source-command-completed.*" \
+    "source $testfile.gdb"
-- 
1.9.1