patch: fix stack unwind through uClibc syscall() on mips

["Ján Stanček" <jan.stancek@gmail.com>]

uClibc syscall() is macro which modifies stack before syscall
instruction, gdb is only looking at function prologue and misses the
stack modification made in syscall(). Because of this unwind doesn't
work. Attached is a patch, which is looking at actual $pc and $pc-4,
and in case of syscall it modifies $sp, so mip32_scan_prologue finds
correct values.

Description of bug is also available here:
http://www.listware.net/201003/gnu-gdb/26893.html

2010-03-27  Jan Stancek  <jan.stancek@gmail.com>
        * mips-tdep.c: fix stack unwind through uClibc syscall() on mips

--- cut ---

--- mips-tdep.c.orig    2010-03-27 17:04:57.000000000 +0100
+++ mips-tdep.c 2010-03-27 18:47:46.000000000 +0100
@@ -1895,6 +1895,44 @@ reset_saved_regs (struct gdbarch *gdbarc
   }
 }

+/*
+ * fix the $sp by looking around actual $pc
+ * Currently this handles only uClibc syscalls,
+ * which adjust $sp before syscall itsels
+ */
+int mips32_get_sp_adjustment(struct frame_info *this_frame, CORE_ADDR start_pc)
+{
+  CORE_ADDR pc = get_frame_address_in_block (this_frame);
+  struct gdbarch *gdbarch = get_frame_arch (this_frame);
+  unsigned long inst, high_word, low_word, ret;
+  int reg;
+
+  inst = (unsigned long) mips_fetch_instruction (gdbarch, pc);
+
+  ret = 0;
+  high_word = (inst >> 16) & 0xffff;
+  low_word = inst & 0xffff;
+  reg = high_word & 0x1f;
+
+  if (high_word == 0x27bd               /* addiu $sp,$sp,-i */
+      || high_word == 0x23bd            /* addi $sp,$sp,-i */
+      || high_word == 0x67bd)           /* daddiu $sp,$sp,-i */
+    {
+      if ( reg == MIPS_SP_REGNUM
+          && (low_word & 0x8000) == 0   /* positive stack adjustment */
+          && (pc-4) > start_pc )
+        {
+          pc = pc - 4;
+          inst = (unsigned long) mips_fetch_instruction (gdbarch, pc);
+          if (inst==0x0000000c)         /* syscall */
+            {
+              ret = low_word;
+            }
+        }
+    }
+  return ret;
+}
+
 /* Analyze the function prologue from START_PC to LIMIT_PC. Builds
    the associated FRAME_CACHE if not null.
    Return the address of the first instruction past the prologue.  */
@@ -1920,9 +1958,12 @@ mips32_scan_prologue (struct gdbarch *gd
   /* Can be called when there's no process, and hence when there's no
      THIS_FRAME.  */
   if (this_frame != NULL)
-    sp = get_frame_register_signed (this_frame,
-                                   gdbarch_num_regs (gdbarch)
-                                   + MIPS_SP_REGNUM);
+    {
+      sp = get_frame_register_signed (this_frame,
+              gdbarch_num_regs (gdbarch)
+              + MIPS_SP_REGNUM);
+      sp += mips32_get_sp_adjustment(this_frame, start_pc);
+    }
   else
     sp = 0;