libgcc/hardcfr.c

   1 /* Control flow redundancy hardening
   2    Copyright (C) 2022-2024 Free Software Foundation, Inc.
   3    Contributed by Alexandre Oliva <oliva@adacore.com>
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it under
   8 the terms of the GNU General Public License as published by the Free
   9 Software Foundation; either version 3, or (at your option) any later
  10 version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 Under Section 7 of GPL version 3, you are granted additional
  18 permissions described in the GCC Runtime Library Exception, version
  19 3.1, as published by the Free Software Foundation.
  20
  21 You should have received a copy of the GNU General Public License and
  22 a copy of the GCC Runtime Library Exception along with this program;
  23 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  24 <http://www.gnu.org/licenses/>.  */
  25
  26 /* Avoid infinite recursion.  */
  27 #pragma GCC optimize ("-fno-harden-control-flow-redundancy")
  28
  29 #include <stddef.h>
  30 #include <stdbool.h>
  31
  32 /* This should be kept in sync with gcc/gimple-harden-control-flow.cc.  */
  33 #if __CHAR_BIT__ >= 28
  34 # define VWORDmode __QI__
  35 #elif __CHAR_BIT__ >= 14
  36 # define VWORDmode __HI__
  37 #else
  38 # define VWORDmode __SI__
  39 #endif
  40
  41 typedef unsigned int __attribute__ ((__mode__ (VWORDmode))) vword;
  42
  43 /* This function is optionally called at the end of a function to verify that
  44    the VISITED array represents a sensible execution path in the CFG.  It is
  45    always expected to pass; the purpose is to detect attempts to subvert
  46    execution by taking unexpected paths, or other execution errors.  The
  47    function, instrumented by pass_harden_control_flow_redundancy at a time in
  48    which it had BLOCKS basic blocks (not counting ENTER and EXIT, so block 2
  49    maps to index 0, the first bit of the first VWORD), sets a bit in the bit
  50    array VISITED as it enters the corresponding basic block.  CFG holds a
  51    representation of the control flow graph at the time of the instrumentation:
  52    an array of VWORDs holding, for each block, a sequence of predecessors, and
  53    a sequence of successors.  Each pred and succ sequence is represented as a
  54    sequence of pairs (mask, index), terminated by an index-less all-zero mask.
  55    If the bit corresponding to the block is set, then at least one of the pred
  56    masks, and at least one of the succ masks, must have a bit set in
  57    VISITED[index].  An ENTRY block predecessor and an EXIT block successor are
  58    represented in a (mask, index) pair that tests the block's own bit.  */
  59 extern void __hardcfr_check (size_t blocks,
  60                              vword const *visited,
  61                              vword const *cfg);
  62
  63 /* Compute the MASK for the bit representing BLOCK in WORDIDX's vword in a
  64    visited blocks bit array.  */
  65 static inline void
  66 block2mask (size_t const block, vword *const mask, size_t *const wordidx)
  67 {
  68   size_t wbits = __CHAR_BIT__ * sizeof (vword);
  69   *wordidx = block / wbits;
  70   *mask = (vword)1 << (block % wbits);
  71 }
  72
  73 /* Check whether the bit corresponding to BLOCK is set in VISITED.  */
  74 static inline bool
  75 visited_p (size_t const block, vword const *const visited)
  76 {
  77   vword mask;
  78   size_t wordidx;
  79   block2mask (block, &mask, &wordidx);
  80   vword w = visited[wordidx];
  81   return (w & mask) != 0;
  82 }
  83
  84 /* Check whether any VISITED bits that would correspond to blocks after BLOCKS
  85    are set.  */
  86 static inline bool
  87 excess_bits_set_p (size_t const blocks, vword const *const visited)
  88 {
  89   vword mask;
  90   size_t wordidx;
  91   block2mask (blocks - 1, &mask, &wordidx);
  92   mask = -mask - mask;
  93   vword w = visited[wordidx];
  94   return (w & mask) != 0;
  95 }
  96
  97 /* Read and consume a mask from **CFG_IT.  (Consume meaning advancing the
  98    iterator to the next word).  If the mask is zero, return FALSE.  Otherwise,
  99    also read and consume an index, and set *MASK and/or *WORDIDX, whichever are
 100    nonNULL, to the corresponding read values, and finally return TRUE.  */
 101 static inline bool
 102 next_pair (vword const **const cfg_it,
 103            vword *const mask,
 104            size_t *const wordidx)
 105 {
 106   vword m = **cfg_it;
 107   ++*cfg_it;
 108   if (!m)
 109     return false;
 110
 111   if (mask)
 112     *mask = m;
 113
 114   size_t word = **cfg_it;
 115   ++*cfg_it;
 116
 117   if (wordidx)
 118     *wordidx = word;
 119
 120   return true;
 121 }
 122
 123 /* Return TRUE iff any of the bits in MASK is set in VISITED[WORDIDX].  */
 124 static inline bool
 125 test_mask (vword const *const visited,
 126            vword const mask, size_t const wordidx)
 127 {
 128   return (visited[wordidx] & mask) != 0;
 129 }
 130
 131 /* Scan a sequence of pairs (mask, index) at **CFG_IT until its terminator is
 132    reached and consumed.  */
 133 static inline void
 134 consume_seq (vword const **const cfg_it)
 135 {
 136   while (next_pair (cfg_it, NULL, NULL))
 137     /* Do nothing.  */;
 138 }
 139
 140 /* Check that at least one of the MASK bits in a sequence of pairs (mask,
 141    index) at **CFG_IT is set in the corresponding VISITED[INDEX] word.  Trap if
 142    we reach the terminator without finding any.  Consume the entire sequence
 143    otherwise, so that *CFG_IT points just past the terminator, which may be the
 144    beginning of the next sequence.  */
 145 static inline bool
 146 check_seq (vword const *const visited, vword const **const cfg_it)
 147 {
 148   vword mask;
 149   size_t wordidx;
 150
 151   /* If the block was visited, check that at least one of the
 152      preds/succs was also visited.  */
 153   do
 154     /* If we get to the end of the sequence without finding any
 155        match, something is amiss.  */
 156     if (!next_pair (cfg_it, &mask, &wordidx))
 157       return false;
 158   /* Keep searching until we find a match, at which point the
 159      condition is satisfied.  */
 160   while (!test_mask (visited, mask, wordidx));
 161
 162   /* Consume the remaining entries in the sequence, whether we found a match or
 163      skipped the block, so as to position the iterator at the beginning of the
 164      next .  */
 165   consume_seq (cfg_it);
 166
 167   return true;
 168 }
 169
 170 /* Print out the CFG with BLOCKS blocks, presumed to be associated with CALLER.
 171    This is expected to be optimized out entirely, unless the verbose part of
 172    __hardcfr_check_fail is enabled.  */
 173 static inline void
 174 __hardcfr_debug_cfg (size_t const blocks,
 175                      void const *const caller,
 176                      vword const *const cfg)
 177 {
 178   __builtin_printf ("CFG at %p, for %p", cfg, caller);
 179   vword const *cfg_it = cfg;
 180   for (size_t i = 0; i < blocks; i++)
 181     {
 182       vword mask; size_t wordidx;
 183       block2mask (i, &mask, &wordidx);
 184       __builtin_printf ("\nblock %lu (%lu/0x%lx)\npreds: ",
 185                         (unsigned long)i,
 186                         (unsigned long)wordidx, (unsigned long)mask);
 187       while (next_pair (&cfg_it, &mask, &wordidx))
 188         __builtin_printf (" (%lu/0x%lx)",
 189                           (unsigned long)wordidx, (unsigned long)mask);
 190       __builtin_printf ("\nsuccs: ");
 191       while (next_pair (&cfg_it, &mask, &wordidx))
 192         __builtin_printf (" (%lu/0x%lx)",
 193                           (unsigned long)wordidx, (unsigned long)mask);
 194     }
 195   __builtin_printf ("\n");
 196 }
 197
 198 #ifndef ATTRIBUTE_UNUSED
 199 # define ATTRIBUTE_UNUSED __attribute__ ((__unused__))
 200 #endif
 201
 202 /* This is called when an out-of-line hardcfr check fails.  All the arguments
 203    are ignored, and it just traps, unless HARDCFR_VERBOSE_FAIL is enabled.  IF
 204    it is, it prints the PART of the CFG, expected to have BLOCKS blocks, that
 205    failed at CALLER's BLOCK, and the VISITED bitmap.  When the verbose mode is
 206    enabled, it also forces __hardcfr_debug_cfg (above) to be compiled into an
 207    out-of-line function, that could be called from a debugger.
 208    */
 209
 210 #ifdef __BPF__
 211 __attribute__((__always_inline__))
 212 #endif
 213 static inline void
 214 __hardcfr_check_fail (size_t const blocks ATTRIBUTE_UNUSED,
 215                       vword const *const visited ATTRIBUTE_UNUSED,
 216                       vword const *const cfg ATTRIBUTE_UNUSED,
 217                       size_t const block ATTRIBUTE_UNUSED,
 218                       int const part ATTRIBUTE_UNUSED,
 219                       void const *const caller ATTRIBUTE_UNUSED)
 220 {
 221 #if HARDCFR_VERBOSE_FAIL
 222   static const char *parts[] = { "preds", "succs", "no excess" };
 223
 224   vword mask; size_t wordidx;
 225   block2mask (block, &mask, &wordidx);
 226   if (part == 2)
 227     mask = -mask - mask;
 228   __builtin_printf ("hardcfr fail at %p block %lu (%lu/0x%lx), expected %s:",
 229                     caller, (unsigned long)block,
 230                     (unsigned long)wordidx, (unsigned long)mask,
 231                     parts[part]);
 232
 233   if (part != 2)
 234     {
 235       /* Skip data for previous blocks.  */
 236       vword const *cfg_it = cfg;
 237       for (size_t i = block; i--; )
 238         {
 239           consume_seq (&cfg_it);
 240           consume_seq (&cfg_it);
 241         }
 242       for (size_t i = part; i--; )
 243         consume_seq (&cfg_it);
 244
 245       while (next_pair (&cfg_it, &mask, &wordidx))
 246         __builtin_printf (" (%lu/0x%lx)",
 247                           (unsigned long)wordidx, (unsigned long)mask);
 248     }
 249
 250   __builtin_printf ("\nvisited:");
 251   block2mask (blocks - 1, &mask, &wordidx);
 252   for (size_t i = 0; i <= wordidx; i++)
 253     __builtin_printf (" (%lu/0x%lx)",
 254                       (unsigned long)i, (unsigned long)visited[i]);
 255   __builtin_printf ("\n");
 256
 257   /* Reference __hardcfr_debug_cfg so that it's output out-of-line, so that it
 258      can be called from a debugger.  */
 259   if (!caller || caller == __hardcfr_debug_cfg)
 260     return;
 261 #endif
 262   __builtin_trap ();
 263 }
 264
 265 /* Check that, for each of the BLOCKS basic blocks, if its bit is set in
 266    VISITED, at least one of its predecessors in CFG is also set, and at also
 267    that at least one of its successors in CFG is also set.  */
 268 void
 269 __hardcfr_check (size_t const blocks,
 270                  vword const *const visited,
 271                  vword const *const cfg)
 272 {
 273   vword const *cfg_it = cfg;
 274   for (size_t i = 0; i < blocks; i++)
 275     {
 276       bool v = visited_p (i, visited);
 277
 278       /* For each block, there are two sequences of pairs (mask, index), each
 279          sequence terminated by a single all-zero mask (no index).  The first
 280          sequence is for predecessor blocks, the second is for successors.  At
 281          least one of each must be set.  */
 282       if (!v)
 283         {
 284           /* Consume predecessors.  */
 285           consume_seq (&cfg_it);
 286           /* Consume successors.  */
 287           consume_seq (&cfg_it);
 288         }
 289       else
 290         {
 291           /* Check predecessors.  */
 292           if (!check_seq (visited, &cfg_it))
 293             __hardcfr_check_fail (blocks, visited, cfg, i, 0,
 294                                   __builtin_return_address (0));
 295           /* Check successors.  */
 296           if (!check_seq (visited, &cfg_it))
 297             __hardcfr_check_fail (blocks, visited, cfg, i, 1,
 298                                   __builtin_return_address (0));
 299         }
 300     }
 301   if (excess_bits_set_p (blocks, visited))
 302     __hardcfr_check_fail (blocks, visited, cfg, blocks - 1, 2,
 303                           __builtin_return_address (0));
 304 }