Reading signal handler frame in AIX

In AIX if gdb is debugging an application which has a signal handler
and reaches the signal handler frame, then we need to read the back
chain address from sigcontext saved on the stack, similarly the LR.

As backchain at an offset 0 will be 0, because we will have a
sigconext saved after the minimum stack size. So the correct
backchain will be at an offset after minimum stack and the LR at
an offset 8 will be of the signal millicode address.
If the back chain pointer is NULL and the LR field is in the kernel
segment(ex. 0x00004a14) then we can probably assume we are in a
signal  handler.

sample output

(gdb) bt
0  sig_handle_aix (signo=11) at aix-sighandle.c:7
1  0x0000000000004a94 in ?? ()
(gdb)

expected output

(gdb) bt
0  sig_handle_aix (signo=11) at aix-sighandle.c:7
1  <signal handler called>
2  0x0000000100000748 in foo () at aix-sighandle.c:14
3  0x000000010000079c in main () at aix-sighandle.c:19

gdb/ChangeLog:

2018-11-01  Sangamesh Mallayya <sangamesh.swamy@in.ibm.com>

	* rs6000-aix-tdep.c: Include "trad-frame.h" and "frame-unwind.h".
	(SIG_FRAME_LR_OFFSET64): New define.
	(SIG_FRAME_FP_OFFSET64): New define.
	(aix_sighandle_frame_cache): New Function.
	(aix_sighandle_frame_this_id): New Function.
	(aix_sighandle_frame_prev_register): New Function.
	(aix_sighandle_frame_sniffer): New Function.
	(aix_sighandle_frame_unwind): New global variable.
	(rs6000_aix_init_osabi): Install new frame unwinder.

gdb/testsuite/ChangeLog:

2018-11-01  Sangamesh Mallayya <sangamesh.swamy@in.ibm.com>

	* gdb.arch/aix-sighandle.c: New file.
	* gdb.arch/aix-sighandle.exp: New file.

1 files changed, 115 insertions, 1 deletions

diff --git a/gdb/rs6000-aix-tdep.c b/gdb/rs6000-aix-tdep.c
index 50a146a..4528cb2 100644
--- a/gdb/rs6000-aix-tdep.c
+++ b/gdb/rs6000-aix-tdep.c
@@ -38,6 +38,8 @@
 #include "solib-aix.h"
 #include "target-float.h"
 #include "xml-utils.h"
+#include "trad-frame.h"
+#include "frame-unwind.h"
 
 /* If the kernel has to deliver a signal, it pushes a sigcontext
    structure on the stack and then calls the signal handler, passing
@@ -45,11 +47,122 @@
    the signal handler doesn't save this register, so we have to
    access the sigcontext structure via an offset from the signal handler
    frame.
-   The following constants were determined by experimentation on AIX 3.2.  */
+   The following constants were determined by experimentation on AIX 3.2.
+
+   sigcontext structure have the mstsave saved under the
+   sc_jmpbuf.jmp_context. STKMIN(minimum stack size) is 56 for 32-bit
+   processes, and iar offset under sc_jmpbuf.jmp_context is 40.
+   ie offsetof(struct sigcontext, sc_jmpbuf.jmp_context.iar).
+   so PC offset in this case is STKMIN+iar offset, which is 96. */
+
 #define SIG_FRAME_PC_OFFSET 96
 #define SIG_FRAME_LR_OFFSET 108
+/* STKMIN+grp1 offset, which is 56+228=284 */
 #define SIG_FRAME_FP_OFFSET 284
 
+/* 64 bit process.
+   STKMIN64  is 112 and iar offset is 312. So 112+312=424 */
+#define SIG_FRAME_LR_OFFSET64 424
+/* STKMIN64+grp1 offset. 112+56=168 */
+#define SIG_FRAME_FP_OFFSET64 168
+
+static struct trad_frame_cache *
+aix_sighandle_frame_cache (struct frame_info *this_frame,
+			   void **this_cache)
+{
+  LONGEST backchain;
+  CORE_ADDR base, base_orig, func;
+  struct gdbarch *gdbarch = get_frame_arch (this_frame);
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+  enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
+  struct trad_frame_cache *this_trad_cache;
+
+  if ((*this_cache) != NULL)
+    return (struct trad_frame_cache *) (*this_cache);
+
+  this_trad_cache = trad_frame_cache_zalloc (this_frame);
+  (*this_cache) = this_trad_cache;
+
+  base = get_frame_register_unsigned (this_frame,
+                                      gdbarch_sp_regnum (gdbarch));
+  base_orig = base;
+
+  if (tdep->wordsize == 4)
+    {
+      func = read_memory_unsigned_integer (base_orig +
+					   SIG_FRAME_PC_OFFSET + 8,
+					   tdep->wordsize, byte_order);
+      safe_read_memory_integer (base_orig + SIG_FRAME_FP_OFFSET + 8,
+				tdep->wordsize, byte_order, &backchain);
+      base = (CORE_ADDR)backchain;
+    }
+  else
+    {
+      func = read_memory_unsigned_integer (base_orig +
+					   SIG_FRAME_LR_OFFSET64,
+					   tdep->wordsize, byte_order);
+      safe_read_memory_integer (base_orig + SIG_FRAME_FP_OFFSET64,
+				tdep->wordsize, byte_order, &backchain);
+      base = (CORE_ADDR)backchain;
+    }
+
+  trad_frame_set_reg_value (this_trad_cache, gdbarch_pc_regnum (gdbarch), func);
+  trad_frame_set_reg_value (this_trad_cache, gdbarch_sp_regnum (gdbarch), base);
+
+  if (tdep->wordsize == 4)
+    trad_frame_set_reg_addr (this_trad_cache, tdep->ppc_lr_regnum,
+                             base_orig + 0x38 + 52 + 8);
+  else
+    trad_frame_set_reg_addr (this_trad_cache, tdep->ppc_lr_regnum,
+                             base_orig + 0x70 + 320);
+
+  trad_frame_set_id (this_trad_cache, frame_id_build (base, func));
+  trad_frame_set_this_base (this_trad_cache, base);
+
+  return this_trad_cache;
+}
+
+static void
+aix_sighandle_frame_this_id (struct frame_info *this_frame,
+			     void **this_prologue_cache,
+			     struct frame_id *this_id)
+{
+  struct trad_frame_cache *this_trad_cache
+    = aix_sighandle_frame_cache (this_frame, this_prologue_cache);
+  trad_frame_get_id (this_trad_cache, this_id);
+}
+
+static struct value *
+aix_sighandle_frame_prev_register (struct frame_info *this_frame,
+				   void **this_prologue_cache, int regnum)
+{
+  struct trad_frame_cache *this_trad_cache
+    = aix_sighandle_frame_cache (this_frame, this_prologue_cache);
+  return trad_frame_get_register (this_trad_cache, this_frame, regnum);
+}
+
+int
+aix_sighandle_frame_sniffer (const struct frame_unwind *self,
+			     struct frame_info *this_frame,
+			     void **this_prologue_cache)
+{
+  CORE_ADDR pc = get_frame_pc (this_frame);
+  if (pc && pc < AIX_TEXT_SEGMENT_BASE)
+    return 1;
+
+  return 0;
+}
+
+/* AIX signal handler frame unwinder */
+
+static const struct frame_unwind aix_sighandle_frame_unwind = {
+  SIGTRAMP_FRAME,
+  default_frame_unwind_stop_reason,
+  aix_sighandle_frame_this_id,
+  aix_sighandle_frame_prev_register,
+  NULL,
+  aix_sighandle_frame_sniffer
+};
 
 /* Core file support.  */
 
@@ -1061,6 +1174,7 @@ rs6000_aix_init_osabi (struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_auto_wide_charset (gdbarch, rs6000_aix_auto_wide_charset);
 
   set_solib_ops (gdbarch, &solib_aix_so_ops);
+  frame_unwind_append_unwinder (gdbarch, &aix_sighandle_frame_unwind);
 }
 
 void