[PATCH][AT_ENTRY version] PR13858 - Can't do displaced stepping with no symbols

[Pedro Alves <palves at redhat dot com>]

Following up from:
 https://sourceware.org/ml/gdb-patches/2015-04/msg00331.html

Yao, could you give this a quick try on ARM?  ARM is the only
touched port that I can't easily test.  I pushed it to
the users/palves/disp-step-no-symbols branch.  Just testing the
new gdb.base/step-over-no-symbols.exp test should be sufficient.

Thanks,
Pedro Alves

---
>From 707d8ada790d0187748dfd2f42727f10868377e7 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Thu, 9 Apr 2015 14:59:58 +0100
Subject: [PATCH] PR13858 - Can't do displaced stepping with no symbols

Running break-interp.exp with the target always in non-stop mode trips
on PR13858, as enabling non-stop also enables displaced stepping.

The problem is that when GDB doesn't know where the entry point is, it
doesn't know where to put the displaced stepping scratch pad.  The
test added by this commit exercises this.  Without the fix, we get:

 (gdb) PASS: gdb.base/step-over-no-symbols.exp: displaced=on: break *$pc
 set displaced-stepping on
 (gdb) PASS: gdb.base/step-over-no-symbols.exp: displaced=on: set displaced-stepping on
 stepi
 0x00000000004005be in ?? ()
 Entry point address is not known.
 (gdb) PASS: gdb.base/step-over-no-symbols.exp: displaced=on: stepi
 p /x $pc
 $2 = 0x4005be
 (gdb) PASS: gdb.base/step-over-no-symbols.exp: displaced=on: get after PC
 FAIL: gdb.base/step-over-no-symbols.exp: displaced=on: advanced

The fix switches all GNU/Linux ports to get the entry point from
AT_ENTRY in the target auxiliary vector instead of from symbols.  This
is currently only done by PPC when Cell debugging is enabled, but I
think all archs should be able to do the same.  Note that
ppc_linux_displaced_step_location cached the result, I'm guessing to
avoid constantly re-fetching the auxv out of remote targets, but
that's no longer necessary nowadays, as the auxv blob is itself cached
in the inferior object.  The ppc_linux_entry_point_addr global is
obviously bad for multi-process too nowadays.

Tested on x86-64 (-m64/-m32), PPC64 (-m64/-m32) and S/390 GNU/Linux.

gdb/ChangeLog:
2015-04-09  Pedro Alves  <palves@redhat.com>

	PR gdb/13858
	* amd64-linux-tdep.c (amd64_linux_init_abi_common): Install
	linux_displaced_step_location as gdbarch_displaced_step_location
	hook.
	* arm-linux-tdep.c (arm_linux_init_abi): Likewise.
	* i386-linux-tdep.c (i386_linux_init_abi): Likewise.
	* linux-tdep.c (linux_displaced_step_location): New function,
	based on ppc_linux_displaced_step_location.
	* linux-tdep.h (linux_displaced_step_location): New declaration.
	* ppc-linux-tdep.c (ppc_linux_entry_point_addr): Delete.
	(ppc_linux_inferior_created, ppc_linux_displaced_step_location):
	Delete.
	(ppc_linux_init_abi): Install linux_displaced_step_location as
	gdbarch_displaced_step_location hook, even without Cell/B.E..
	(_initialize_ppc_linux_tdep): Don't install
	ppc_linux_inferior_created as inferior_created observer.
	* s390-linux-tdep.c (s390_gdbarch_init): Install
	linux_displaced_step_location as gdbarch_displaced_step_location
	hook.

gdb/testsuite/
2015-04-09  Pedro Alves  <palves@redhat.com>

	PR gdb/13858
	* gdb.base/step-over-no-symbols.exp: New file.
---
 gdb/amd64-linux-tdep.c                          |  2 +-
 gdb/arm-linux-tdep.c                            |  2 +-
 gdb/i386-linux-tdep.c                           |  2 +-
 gdb/linux-tdep.c                                | 26 ++++++++
 gdb/linux-tdep.h                                |  5 ++
 gdb/ppc-linux-tdep.c                            | 52 +--------------
 gdb/s390-linux-tdep.c                           |  3 +-
 gdb/testsuite/gdb.base/step-over-no-symbols.exp | 87 +++++++++++++++++++++++++
 8 files changed, 125 insertions(+), 54 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/step-over-no-symbols.exp

diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index b126366..2a8de82 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1833,7 +1833,7 @@ amd64_linux_init_abi_common(struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_displaced_step_free_closure (gdbarch,
                                            simple_displaced_step_free_closure);
   set_gdbarch_displaced_step_location (gdbarch,
-                                       displaced_step_at_entry_point);
+                                       linux_displaced_step_location);
 
   set_gdbarch_get_siginfo_type (gdbarch, linux_get_siginfo_type);
 
diff --git a/gdb/arm-linux-tdep.c b/gdb/arm-linux-tdep.c
index 2b5aed7..f58da84 100644
--- a/gdb/arm-linux-tdep.c
+++ b/gdb/arm-linux-tdep.c
@@ -1448,7 +1448,7 @@ arm_linux_init_abi (struct gdbarch_info info,
   set_gdbarch_displaced_step_fixup (gdbarch, arm_displaced_step_fixup);
   set_gdbarch_displaced_step_free_closure (gdbarch,
 					   simple_displaced_step_free_closure);
-  set_gdbarch_displaced_step_location (gdbarch, displaced_step_at_entry_point);
+  set_gdbarch_displaced_step_location (gdbarch, linux_displaced_step_location);
 
   /* Reversible debugging, process record.  */
   set_gdbarch_process_record (gdbarch, arm_process_record);
diff --git a/gdb/i386-linux-tdep.c b/gdb/i386-linux-tdep.c
index 4a0ce60..061ad3a 100644
--- a/gdb/i386-linux-tdep.c
+++ b/gdb/i386-linux-tdep.c
@@ -990,7 +990,7 @@ i386_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_displaced_step_free_closure (gdbarch,
                                            simple_displaced_step_free_closure);
   set_gdbarch_displaced_step_location (gdbarch,
-                                       displaced_step_at_entry_point);
+                                       linux_displaced_step_location);
 
   /* Functions for 'catch syscall'.  */
   set_xml_syscall_file_name (gdbarch, XML_SYSCALL_FILENAME_I386);
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 0b11e58..7a917f4 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -2348,6 +2348,32 @@ linux_infcall_mmap (CORE_ADDR size, unsigned prot)
   return retval;
 }
 
+/* See linux-tdep.h.  */
+
+CORE_ADDR
+linux_displaced_step_location (struct gdbarch *gdbarch)
+{
+  CORE_ADDR addr;
+  int bp_len;
+
+  /* Determine entry point from target auxiliary vector.  */
+  if (target_auxv_search (&current_target, AT_ENTRY, &addr) <= 0)
+    error (_("Cannot find AT_ENTRY auxiliary vector entry."));
+
+  /* Make certain that the address points at real code, and not a
+     function descriptor.  */
+  addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr,
+					     &current_target);
+
+  /* Inferior calls also use the entry point as a breakpoint location.
+     We don't want displaced stepping to interfere with those
+     breakpoints, so leave space.  */
+  gdbarch_breakpoint_from_pc (gdbarch, &addr, &bp_len);
+  addr += bp_len * 2;
+
+  return addr;
+}
+
 /* Display whether the gcore command is using the
    /proc/PID/coredump_filter file.  */
 
diff --git a/gdb/linux-tdep.h b/gdb/linux-tdep.h
index 78fc36b..ff45286 100644
--- a/gdb/linux-tdep.h
+++ b/gdb/linux-tdep.h
@@ -37,6 +37,11 @@ extern enum gdb_signal linux_gdb_signal_from_target (struct gdbarch *gdbarch,
 extern int linux_gdb_signal_to_target (struct gdbarch *gdbarch,
 				       enum gdb_signal signal);
 
+/* Default GNU/Linux implementation of `displaced_step_location', as
+   defined in gdbarch.h.  Determines the entry point from AT_ENTRY in
+   the target auxiliary vector.  */
+extern CORE_ADDR linux_displaced_step_location (struct gdbarch *gdbarch);
+
 extern void linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch);
 
 extern int linux_is_uclinux (void);
diff --git a/gdb/ppc-linux-tdep.c b/gdb/ppc-linux-tdep.c
index 3afbff2..3849ca6 100644
--- a/gdb/ppc-linux-tdep.c
+++ b/gdb/ppc-linux-tdep.c
@@ -703,47 +703,6 @@ static struct tramp_frame ppc64_linux_sighandler_tramp_frame = {
   ppc64_linux_sighandler_cache_init
 };
 
-
-/* Address to use for displaced stepping.  When debugging a stand-alone
-   SPU executable, entry_point_address () will point to an SPU local-store
-   address and is thus not usable as displaced stepping location.  We use
-   the auxiliary vector to determine the PowerPC-side entry point address
-   instead.  */
-
-static CORE_ADDR ppc_linux_entry_point_addr = 0;
-
-static void
-ppc_linux_inferior_created (struct target_ops *target, int from_tty)
-{
-  ppc_linux_entry_point_addr = 0;
-}
-
-static CORE_ADDR
-ppc_linux_displaced_step_location (struct gdbarch *gdbarch)
-{
-  if (ppc_linux_entry_point_addr == 0)
-    {
-      CORE_ADDR addr;
-
-      /* Determine entry point from target auxiliary vector.  */
-      if (target_auxv_search (&current_target, AT_ENTRY, &addr) <= 0)
-	error (_("Cannot find AT_ENTRY auxiliary vector entry."));
-
-      /* Make certain that the address points at real code, and not a
-	 function descriptor.  */
-      addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr,
-						 &current_target);
-
-      /* Inferior calls also use the entry point as a breakpoint location.
-	 We don't want displaced stepping to interfere with those
-	 breakpoints, so leave space.  */
-      ppc_linux_entry_point_addr = addr + 2 * PPC_INSN_SIZE;
-    }
-
-  return ppc_linux_entry_point_addr;
-}
-
-
 /* Return 1 if PPC_ORIG_R3_REGNUM and PPC_TRAP_REGNUM are usable.  */
 int
 ppc_linux_trap_reg_p (struct gdbarch *gdbarch)
@@ -1837,13 +1796,11 @@ ppc_linux_init_abi (struct gdbarch_info info,
 
       /* Cell/B.E. cross-architecture unwinder support.  */
       frame_unwind_prepend_unwinder (gdbarch, &ppu2spu_unwind);
-
-      /* The default displaced_step_at_entry_point doesn't work for
-	 SPU stand-alone executables.  */
-      set_gdbarch_displaced_step_location (gdbarch,
-					   ppc_linux_displaced_step_location);
     }
 
+  set_gdbarch_displaced_step_location (gdbarch,
+				       linux_displaced_step_location);
+
   set_gdbarch_get_siginfo_type (gdbarch, linux_get_siginfo_type);
 
   /* Support reverse debugging.  */
@@ -1870,9 +1827,6 @@ _initialize_ppc_linux_tdep (void)
   gdbarch_register_osabi (bfd_arch_rs6000, bfd_mach_rs6k, GDB_OSABI_LINUX,
                          ppc_linux_init_abi);
 
-  /* Attach to inferior_created observer.  */
-  observer_attach_inferior_created (ppc_linux_inferior_created);
-
   /* Attach to observers to track __spe_current_active_context.  */
   observer_attach_inferior_created (ppc_linux_spe_context_inferior_created);
   observer_attach_solib_loaded (ppc_linux_spe_context_solib_loaded);
diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index cafa57b..b83a264 100644
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -3295,8 +3295,7 @@ s390_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_gdbarch_displaced_step_fixup (gdbarch, s390_displaced_step_fixup);
   set_gdbarch_displaced_step_free_closure (gdbarch,
 					   simple_displaced_step_free_closure);
-  set_gdbarch_displaced_step_location (gdbarch,
-				       displaced_step_at_entry_point);
+  set_gdbarch_displaced_step_location (gdbarch, linux_displaced_step_location);
   set_gdbarch_max_insn_length (gdbarch, S390_MAX_INSTR_SIZE);
 
   /* Note that GNU/Linux is the only OS supported on this
diff --git a/gdb/testsuite/gdb.base/step-over-no-symbols.exp b/gdb/testsuite/gdb.base/step-over-no-symbols.exp
new file mode 100644
index 0000000..0880d66
--- /dev/null
+++ b/gdb/testsuite/gdb.base/step-over-no-symbols.exp
@@ -0,0 +1,87 @@
+# Copyright (C) 2015 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/>.
+
+# Test that GDB can step past a breakpoint even if GDB doesn't have
+# symbols for the main binary.
+
+standard_testfile start.c
+
+if { [build_executable "failed to build" ${testfile} $srcfile] } {
+    return -1
+}
+
+# Get the current PC.  MSG is used as test message.
+
+proc get_pc { msg } {
+    global hex gdb_prompt
+
+    set addr ""
+    gdb_test_multiple "p /x \$pc" "$msg" {
+	-re " = ($hex).*$gdb_prompt $" {
+	    set addr $expect_out(1,string)
+	    pass "$msg"
+	}
+    }
+
+    return $addr
+}
+
+# Test stepping past a breakpoint with no symbols.  DISPLACED is one
+# of the "set displaced-stepping" options.  GDB should be able to fall
+# back to stepping past the breakpoint using an in-line step-over.
+
+proc test_step_over { displaced } {
+    global hex
+    global binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] {
+	fail "couldn't run to main"
+	untested "stepping over breakpoint with displaced=$displaced"
+	return -1
+    }
+
+    delete_breakpoints
+
+    set msg "purging symbols"
+    gdb_test_multiple "symbol-file" "$msg" {
+	-re "Discard symbol table.*y or n. $" {
+	    gdb_test "y" "No symbol file now." "$msg"
+	}
+    }
+
+    set before_addr [get_pc "get before PC"]
+
+    gdb_test "break *\$pc" "Breakpoint .* at $hex"
+
+    gdb_test_no_output "set displaced-stepping $displaced"
+
+    gdb_test "stepi" "$hex in \?\? .*"
+
+    set after_addr [get_pc "get after PC"]
+
+    gdb_assert {$before_addr != $after_addr} "advanced"
+}
+
+foreach displaced { "off" "on" "auto" } {
+    if { $displaced != "off" && ![support_displaced_stepping] } {
+	continue
+    }
+
+    with_test_prefix "displaced=$displaced" {
+	test_step_over $displaced
+    }
+}
-- 
1.9.3