libgo/runtime/go-recover.c

   1 /* go-recover.c -- support for the go recover function.
   2
   3    Copyright 2010 The Go Authors. All rights reserved.
   4    Use of this source code is governed by a BSD-style
   5    license that can be found in the LICENSE file.  */
   6
   7 #include "runtime.h"
   8 #include "interface.h"
   9 #include "go-panic.h"
  10 #include "go-defer.h"
  11
  12 /* This is called by a thunk to see if the real function should be
  13    permitted to recover a panic value.  Recovering a value is
  14    permitted if the thunk was called directly by defer.  RETADDR is
  15    the return address of the function which is calling
  16    __go_can_recover--this is, the thunk.  */
  17
  18 _Bool
  19 __go_can_recover (const void *retaddr)
  20 {
  21   G *g;
  22   struct __go_defer_stack *d;
  23   const char* ret;
  24   const char* dret;
  25   Location loc;
  26   const byte *name;
  27
  28   g = runtime_g ();
  29
  30   d = g->defer;
  31   if (d == NULL)
  32     return 0;
  33
  34   /* The panic which this function would recover is the one on the top
  35      of the panic stack.  We do not want to recover it if that panic
  36      was on the top of the panic stack when this function was
  37      deferred.  */
  38   if (d->__panic == g->panic)
  39     return 0;
  40
  41   /* D->__RETADDR is the address of a label immediately following the
  42      call to the thunk.  We can recover a panic if that is the same as
  43      the return address of the thunk.  We permit a bit of slack in
  44      case there is any code between the function return and the label,
  45      such as an instruction to adjust the stack pointer.  */
  46
  47   ret = (const char *) retaddr;
  48
  49 #ifdef __sparc__
  50   /* On SPARC the address we get, from __builtin_return_address, is
  51      the address of the call instruction.  Adjust forward, also
  52      skipping the delayed instruction following the call.  */
  53   ret += 8;
  54 #endif
  55
  56   dret = (const char *) d->__retaddr;
  57   if (ret <= dret && ret + 16 >= dret)
  58     return 1;
  59
  60   /* If the function calling recover was created by reflect.MakeFunc,
  61      then RETADDR will be somewhere in libffi.  Our caller is
  62      permitted to recover if it was called from libffi.  */
  63   if (!d->__makefunc_can_recover)
  64     return 0;
  65
  66   if (runtime_callers (2, &loc, 1) < 1)
  67     return 0;
  68
  69   /* If we have no function name, then we weren't called by Go code.
  70      Guess that we were called by libffi.  */
  71   if (loc.function.len == 0)
  72     return 1;
  73
  74   if (loc.function.len < 4)
  75     return 0;
  76   name = loc.function.str;
  77   if (*name == '_')
  78     {
  79       if (loc.function.len < 5)
  80         return 0;
  81       ++name;
  82     }
  83
  84   if (name[0] == 'f' && name[1] == 'f' && name[2] == 'i' && name[3] == '_')
  85     return 1;
  86
  87   /* We may also be called by reflect.makeFuncImpl.call, for a
  88      function created by reflect.MakeFunc.  */
  89   if (__builtin_strstr ((const char *) name, "makeFuncImpl") != NULL)
  90     return 1;
  91
  92   return 0;
  93 }
  94
  95 /* This function is called when code is about to enter a function
  96    created by reflect.MakeFunc.  It is called by the function stub
  97    used by MakeFunc.  If the stub is permitted to call recover, then a
  98    real MakeFunc function is permitted to call recover.  */
  99
 100 void
 101 __go_makefunc_can_recover (const void *retaddr)
 102 {
 103   struct __go_defer_stack *d;
 104
 105   d = runtime_g ()->defer;
 106   if (d != NULL
 107       && !d->__makefunc_can_recover
 108       && __go_can_recover (retaddr))
 109     d->__makefunc_can_recover = 1;
 110 }
 111
 112 /* This function is called when code is about to exit a function
 113    created by reflect.MakeFunc.  It is called by the function stub
 114    used by MakeFunc.  It clears the __makefunc_can_recover field.
 115    It's OK to always clear this field, because __go_can_recover will
 116    only be called by a stub created for a function that calls recover.
 117    That stub will not call a function created by reflect.MakeFunc, so
 118    by the time we get here any caller higher up on the call stack no
 119    longer needs the information.  */
 120
 121 void
 122 __go_makefunc_returning (void)
 123 {
 124   struct __go_defer_stack *d;
 125
 126   d = runtime_g ()->defer;
 127   if (d != NULL)
 128     d->__makefunc_can_recover = 0;
 129 }
 130
 131 /* This is only called when it is valid for the caller to recover the
 132    value on top of the panic stack, if there is one.  */
 133
 134 struct __go_empty_interface
 135 __go_recover ()
 136 {
 137   G *g;
 138   struct __go_panic_stack *p;
 139
 140   g = runtime_g ();
 141
 142   if (g->panic == NULL || g->panic->__was_recovered)
 143     {
 144       struct __go_empty_interface ret;
 145
 146       ret.__type_descriptor = NULL;
 147       ret.__object = NULL;
 148       return ret;
 149     }
 150   p = g->panic;
 151   p->__was_recovered = 1;
 152   return p->__arg;
 153 }