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      1 /* _Unwind_Frames_Extra with shadow stack for x86-64 and x86.
      2    Copyright (C) 2017-2022 Free Software Foundation, Inc.
      3 
      4 This file is part of GCC.
      5 
      6 GCC is free software; you can redistribute it and/or modify
      7 it under the terms of the GNU General Public License as published by
      8 the Free Software Foundation; either version 3, or (at your option)
      9 any later version.
     10 
     11 GCC is distributed in the hope that it will be useful,
     12 but WITHOUT ANY WARRANTY; without even the implied warranty of
     13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     14 GNU General Public License for more details.
     15 
     16 Under Section 7 of GPL version 3, you are granted additional
     17 permissions described in the GCC Runtime Library Exception, version
     18 3.1, as published by the Free Software Foundation.
     19 
     20 You should have received a copy of the GNU General Public License and
     21 a copy of the GCC Runtime Library Exception along with this program;
     22 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
     23 <http://www.gnu.org/licenses/>.  */
     24 
     25 #include <x86gprintrin.h>
     26 
     27 /* Unwind the shadow stack for EH.  */
     28 #undef _Unwind_Frames_Extra
     29 #define _Unwind_Frames_Extra(x)			\
     30   do						\
     31     {						\
     32       _Unwind_Word ssp = _get_ssp ();		\
     33       if (ssp != 0)				\
     34 	{					\
     35 	  _Unwind_Word tmp = (x);		\
     36 	  while (tmp > 255)			\
     37 	    {					\
     38 	      _inc_ssp (255);			\
     39 	      tmp -= 255;			\
     40 	    }					\
     41 	  _inc_ssp (tmp);			\
     42 	}					\
     43     }						\
     44     while (0)
     45 
     46 /* Linux CET kernel places a restore token on shadow stack for signal
     47    handler to enhance security.  The restore token is 8 byte and aligned
     48    to 8 bytes.  It is usually transparent to user programs since kernel
     49    will pop the restore token when signal handler returns.  But when an
     50    exception is thrown from a signal handler, now we need to pop the
     51    restore token from shadow stack.  For x86-64, we just need to treat
     52    the signal frame as normal frame.  For i386, we need to search for
     53    the restore token to check if the original shadow stack is 8 byte
     54    aligned.  If the original shadow stack is 8 byte aligned, we just
     55    need to pop 2 slots, one restore token, from shadow stack.  Otherwise,
     56    we need to pop 3 slots, one restore token + 4 byte padding, from
     57    shadow stack.  */
     58 #ifndef __x86_64__
     59 #undef _Unwind_Frames_Increment
     60 #define _Unwind_Frames_Increment(context, frames)	\
     61   if (_Unwind_IsSignalFrame (context))			\
     62     do							\
     63       {							\
     64 	_Unwind_Word ssp, prev_ssp, token;		\
     65 	ssp = _get_ssp ();				\
     66 	if (ssp != 0)					\
     67 	  {						\
     68 	    /* Align shadow stack pointer to the next	\
     69 	       8 byte aligned boundary.  */		\
     70 	    ssp = (ssp + 4) & ~7;			\
     71 	    do						\
     72 	      {						\
     73 		/* Look for a restore token.  */	\
     74 		token = (*(_Unwind_Word *) (ssp - 8));	\
     75 		prev_ssp = token & ~7;			\
     76 		if (prev_ssp == ssp)			\
     77 		  break;				\
     78 		ssp += 8;				\
     79 	      }						\
     80 	    while (1);					\
     81 	    frames += (token & 0x4) ? 3 : 2;		\
     82 	  }						\
     83       }							\
     84     while (0);						\
     85   else							\
     86     frames++;
     87 #endif
     88