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1 | /* Harden conditionals. |
2 | Copyright (C) 2021 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 | You should have received a copy of the GNU General Public License | |
18 | along with GCC; see the file COPYING3. If not see | |
19 | <http://www.gnu.org/licenses/>. */ | |
20 | ||
21 | #include "config.h" | |
22 | #include "system.h" | |
23 | #include "coretypes.h" | |
24 | #include "backend.h" | |
25 | #include "tree.h" | |
26 | #include "fold-const.h" | |
27 | #include "gimple.h" | |
28 | #include "gimplify.h" | |
29 | #include "tree-pass.h" | |
30 | #include "ssa.h" | |
31 | #include "gimple-iterator.h" | |
32 | #include "tree-cfg.h" | |
33 | #include "basic-block.h" | |
34 | #include "cfghooks.h" | |
35 | #include "cfgloop.h" | |
36 | #include "diagnostic.h" | |
37 | #include "intl.h" | |
38 | ||
39 | namespace { | |
40 | ||
41 | /* These passes introduces redundant, but reversed conditionals at | |
42 | compares, such as those used in conditional branches, and those | |
43 | that compute boolean results. This doesn't make much sense for | |
44 | abstract CPUs, but this kind of hardening may avoid undesirable | |
45 | execution paths on actual CPUs under such attacks as of power | |
46 | deprivation. */ | |
47 | ||
48 | /* Define a pass to harden conditionals other than branches. */ | |
49 | ||
50 | const pass_data pass_data_harden_compares = { | |
51 | GIMPLE_PASS, | |
52 | "hardcmp", | |
53 | OPTGROUP_NONE, | |
54 | TV_NONE, | |
55 | PROP_cfg | PROP_ssa, // properties_required | |
56 | 0, // properties_provided | |
57 | 0, // properties_destroyed | |
58 | 0, // properties_start | |
59 | TODO_update_ssa | |
60 | | TODO_cleanup_cfg | |
61 | | TODO_verify_il, // properties_finish | |
62 | }; | |
63 | ||
64 | class pass_harden_compares : public gimple_opt_pass | |
65 | { | |
66 | public: | |
67 | pass_harden_compares (gcc::context *ctxt) | |
68 | : gimple_opt_pass (pass_data_harden_compares, ctxt) | |
69 | {} | |
70 | opt_pass *clone () { return new pass_harden_compares (m_ctxt); } | |
71 | virtual bool gate (function *) { | |
72 | return flag_harden_compares; | |
73 | } | |
74 | virtual unsigned int execute (function *); | |
75 | }; | |
76 | ||
77 | /* Define a pass to harden conditionals in branches. This pass must | |
78 | run after the above, otherwise it will re-harden the checks | |
79 | introduced by the above. */ | |
80 | ||
81 | const pass_data pass_data_harden_conditional_branches = { | |
82 | GIMPLE_PASS, | |
83 | "hardcbr", | |
84 | OPTGROUP_NONE, | |
85 | TV_NONE, | |
86 | PROP_cfg | PROP_ssa, // properties_required | |
87 | 0, // properties_provided | |
88 | 0, // properties_destroyed | |
89 | 0, // properties_start | |
90 | TODO_update_ssa | |
91 | | TODO_cleanup_cfg | |
92 | | TODO_verify_il, // properties_finish | |
93 | }; | |
94 | ||
95 | class pass_harden_conditional_branches : public gimple_opt_pass | |
96 | { | |
97 | public: | |
98 | pass_harden_conditional_branches (gcc::context *ctxt) | |
99 | : gimple_opt_pass (pass_data_harden_conditional_branches, ctxt) | |
100 | {} | |
101 | opt_pass *clone () { return new pass_harden_conditional_branches (m_ctxt); } | |
102 | virtual bool gate (function *) { | |
103 | return flag_harden_conditional_branches; | |
104 | } | |
105 | virtual unsigned int execute (function *); | |
106 | }; | |
107 | ||
108 | } | |
109 | ||
110 | /* If VAL is an SSA name, return an SSA name holding the same value, | |
111 | but without the compiler's knowing that it holds the same value, so | |
112 | that uses thereof can't be optimized the way VAL might. Insert | |
113 | stmts that initialize it before *GSIP, with LOC. | |
114 | ||
115 | Otherwise, VAL must be an invariant, returned unchanged. */ | |
116 | ||
117 | static inline tree | |
118 | detach_value (location_t loc, gimple_stmt_iterator *gsip, tree val) | |
119 | { | |
120 | if (TREE_CONSTANT (val) || TREE_CODE (val) != SSA_NAME) | |
121 | { | |
122 | gcc_checking_assert (is_gimple_min_invariant (val)); | |
123 | return val; | |
124 | } | |
125 | ||
126 | tree ret = copy_ssa_name (val); | |
127 | ||
128 | /* Output asm ("" : "=g" (ret) : "0" (val)); */ | |
129 | vec<tree, va_gc> *inputs = NULL; | |
130 | vec<tree, va_gc> *outputs = NULL; | |
131 | vec_safe_push (outputs, | |
132 | build_tree_list | |
133 | (build_tree_list | |
134 | (NULL_TREE, build_string (2, "=g")), | |
135 | ret)); | |
136 | vec_safe_push (inputs, | |
137 | build_tree_list | |
138 | (build_tree_list | |
139 | (NULL_TREE, build_string (1, "0")), | |
140 | val)); | |
141 | gasm *detach = gimple_build_asm_vec ("", inputs, outputs, | |
142 | NULL, NULL); | |
143 | gimple_set_location (detach, loc); | |
144 | gsi_insert_before (gsip, detach, GSI_SAME_STMT); | |
145 | ||
146 | SSA_NAME_DEF_STMT (ret) = detach; | |
147 | ||
148 | return ret; | |
149 | } | |
150 | ||
151 | /* Build a cond stmt out of COP, LHS, RHS, insert it before *GSIP with | |
152 | location LOC. *GSIP must be at the end of a basic block. The succ | |
153 | edge out of the block becomes the true or false edge opposite to | |
154 | that in FLAGS. Create a new block with a single trap stmt, in the | |
155 | cold partition if the function is partitioned,, and a new edge to | |
156 | it as the other edge for the cond. */ | |
157 | ||
158 | static inline void | |
159 | insert_check_and_trap (location_t loc, gimple_stmt_iterator *gsip, | |
160 | int flags, enum tree_code cop, tree lhs, tree rhs) | |
161 | { | |
162 | basic_block chk = gsi_bb (*gsip); | |
163 | ||
164 | gcond *cond = gimple_build_cond (cop, lhs, rhs, NULL, NULL); | |
165 | gimple_set_location (cond, loc); | |
166 | gsi_insert_before (gsip, cond, GSI_SAME_STMT); | |
167 | ||
168 | basic_block trp = create_empty_bb (chk); | |
169 | ||
170 | gimple_stmt_iterator gsit = gsi_after_labels (trp); | |
171 | gcall *trap = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0); | |
172 | gimple_set_location (trap, loc); | |
173 | gsi_insert_before (&gsit, trap, GSI_SAME_STMT); | |
174 | ||
175 | if (dump_file) | |
176 | fprintf (dump_file, | |
177 | "Adding reversed compare to block %i, and trap to block %i\n", | |
178 | chk->index, trp->index); | |
179 | ||
180 | if (BB_PARTITION (chk)) | |
181 | BB_SET_PARTITION (trp, BB_COLD_PARTITION); | |
182 | ||
183 | int true_false_flag = flags & (EDGE_TRUE_VALUE | EDGE_FALSE_VALUE); | |
184 | gcc_assert (true_false_flag); | |
185 | int neg_true_false_flag = (~flags) & (EDGE_TRUE_VALUE | EDGE_FALSE_VALUE); | |
186 | ||
187 | /* Remove the fallthru bit, and set the truth value for the | |
188 | preexisting edge and for the newly-created one. In hardcbr, | |
189 | FLAGS is taken from the edge of the original cond expr that we're | |
190 | dealing with, so the reversed compare is expected to yield the | |
191 | negated result, and the same result calls for a trap. In | |
192 | hardcmp, we're comparing the boolean results of the original and | |
193 | of the reversed compare, so we're passed FLAGS to trap on | |
194 | equality. */ | |
195 | single_succ_edge (chk)->flags &= ~EDGE_FALLTHRU; | |
196 | single_succ_edge (chk)->flags |= neg_true_false_flag; | |
197 | edge e = make_edge (chk, trp, true_false_flag); | |
198 | e->goto_locus = loc; | |
199 | ||
200 | if (dom_info_available_p (CDI_DOMINATORS)) | |
201 | set_immediate_dominator (CDI_DOMINATORS, trp, chk); | |
202 | if (current_loops) | |
203 | add_bb_to_loop (trp, current_loops->tree_root); | |
204 | } | |
205 | ||
206 | /* Split edge E, and insert_check_and_trap (see above) in the | |
207 | newly-created block, using detached copies of LHS's and RHS's | |
208 | values (see detach_value above) for the COP compare. */ | |
209 | ||
210 | static inline void | |
211 | insert_edge_check_and_trap (location_t loc, edge e, | |
212 | enum tree_code cop, tree lhs, tree rhs) | |
213 | { | |
214 | int flags = e->flags; | |
215 | basic_block src = e->src; | |
216 | basic_block dest = e->dest; | |
217 | location_t eloc = e->goto_locus; | |
218 | ||
219 | basic_block chk = split_edge (e); | |
220 | e = NULL; | |
221 | ||
222 | single_pred_edge (chk)->goto_locus = loc; | |
223 | single_succ_edge (chk)->goto_locus = eloc; | |
224 | ||
225 | if (dump_file) | |
226 | fprintf (dump_file, | |
227 | "Splitting edge %i->%i into block %i\n", | |
228 | src->index, dest->index, chk->index); | |
229 | ||
230 | gimple_stmt_iterator gsik = gsi_after_labels (chk); | |
231 | ||
232 | bool same_p = (lhs == rhs); | |
233 | lhs = detach_value (loc, &gsik, lhs); | |
234 | rhs = same_p ? lhs : detach_value (loc, &gsik, rhs); | |
235 | ||
236 | insert_check_and_trap (loc, &gsik, flags, cop, lhs, rhs); | |
237 | } | |
238 | ||
239 | /* Harden cond stmts at the end of FUN's blocks. */ | |
240 | ||
241 | unsigned int | |
242 | pass_harden_conditional_branches::execute (function *fun) | |
243 | { | |
244 | basic_block bb; | |
245 | FOR_EACH_BB_REVERSE_FN (bb, fun) | |
246 | { | |
247 | gimple_stmt_iterator gsi = gsi_last_bb (bb); | |
248 | ||
249 | if (gsi_end_p (gsi)) | |
250 | continue; | |
251 | ||
252 | gcond *cond = dyn_cast <gcond *> (gsi_stmt (gsi)); | |
253 | if (!cond) | |
254 | continue; | |
255 | ||
256 | /* Turn: | |
257 | ||
258 | if (x op y) goto l1; else goto l2; | |
259 | ||
260 | into: | |
261 | ||
262 | if (x op y) goto l1'; else goto l2'; | |
263 | l1': if (x' cop y') goto l1'trap; else goto l1; | |
264 | l1'trap: __builtin_trap (); | |
265 | l2': if (x' cop y') goto l2; else goto l2'trap; | |
266 | l2'trap: __builtin_trap (); | |
267 | ||
268 | where cop is a complementary boolean operation to op; l1', l1'trap, | |
269 | l2' and l2'trap are newly-created labels; and x' and y' hold the same | |
270 | value as x and y, but in a way that does not enable the compiler to | |
271 | optimize the redundant compare away. | |
272 | */ | |
273 | ||
274 | enum tree_code op = gimple_cond_code (cond); | |
275 | tree lhs = gimple_cond_lhs (cond); | |
276 | tree rhs = gimple_cond_rhs (cond); | |
277 | location_t loc = gimple_location (cond); | |
278 | ||
279 | enum tree_code cop = invert_tree_comparison (op, HONOR_NANS (lhs)); | |
280 | ||
281 | if (cop == ERROR_MARK) | |
282 | /* ??? Can we do better? */ | |
283 | continue; | |
284 | ||
285 | insert_edge_check_and_trap (loc, EDGE_SUCC (bb, 0), cop, lhs, rhs); | |
286 | insert_edge_check_and_trap (loc, EDGE_SUCC (bb, 1), cop, lhs, rhs); | |
287 | } | |
288 | ||
289 | return 0; | |
290 | } | |
291 | ||
292 | /* Instantiate a hardcbr pass. */ | |
293 | ||
294 | gimple_opt_pass * | |
295 | make_pass_harden_conditional_branches (gcc::context *ctxt) | |
296 | { | |
297 | return new pass_harden_conditional_branches (ctxt); | |
298 | } | |
299 | ||
300 | /* Harden boolean-yielding compares in FUN. */ | |
301 | ||
302 | unsigned int | |
303 | pass_harden_compares::execute (function *fun) | |
304 | { | |
305 | basic_block bb; | |
306 | /* Go backwards over BBs and stmts, so that, even if we split the | |
307 | block multiple times to insert a cond_expr after each compare we | |
308 | find, we remain in the same block, visiting every preexisting | |
309 | stmt exactly once, and not visiting newly-added blocks or | |
310 | stmts. */ | |
311 | FOR_EACH_BB_REVERSE_FN (bb, fun) | |
312 | for (gimple_stmt_iterator gsi = gsi_last_bb (bb); | |
313 | !gsi_end_p (gsi); gsi_prev (&gsi)) | |
314 | { | |
315 | gassign *asgn = dyn_cast <gassign *> (gsi_stmt (gsi)); | |
316 | if (!asgn) | |
317 | continue; | |
318 | ||
319 | /* Turn: | |
320 | ||
321 | z = x op y; | |
322 | ||
323 | into: | |
324 | ||
325 | z = x op y; | |
326 | z' = x' cop y'; | |
327 | if (z == z') __builtin_trap (); | |
328 | ||
329 | where cop is a complementary boolean operation to op; and x' | |
330 | and y' hold the same value as x and y, but in a way that does | |
331 | not enable the compiler to optimize the redundant compare | |
332 | away. | |
333 | */ | |
334 | ||
335 | enum tree_code op = gimple_assign_rhs_code (asgn); | |
336 | ||
337 | enum tree_code cop; | |
338 | ||
339 | switch (op) | |
340 | { | |
341 | case EQ_EXPR: | |
342 | case NE_EXPR: | |
343 | case GT_EXPR: | |
344 | case GE_EXPR: | |
345 | case LT_EXPR: | |
346 | case LE_EXPR: | |
347 | case LTGT_EXPR: | |
348 | case UNEQ_EXPR: | |
349 | case UNGT_EXPR: | |
350 | case UNGE_EXPR: | |
351 | case UNLT_EXPR: | |
352 | case UNLE_EXPR: | |
353 | case ORDERED_EXPR: | |
354 | case UNORDERED_EXPR: | |
355 | cop = invert_tree_comparison (op, | |
356 | HONOR_NANS | |
357 | (gimple_assign_rhs1 (asgn))); | |
358 | ||
359 | if (cop == ERROR_MARK) | |
360 | /* ??? Can we do better? */ | |
361 | continue; | |
362 | ||
363 | break; | |
364 | ||
365 | /* ??? Maybe handle these too? */ | |
366 | case TRUTH_NOT_EXPR: | |
367 | /* ??? The code below assumes binary ops, it would have to | |
368 | be adjusted for TRUTH_NOT_EXPR, since it's unary. */ | |
369 | case TRUTH_ANDIF_EXPR: | |
370 | case TRUTH_ORIF_EXPR: | |
371 | case TRUTH_AND_EXPR: | |
372 | case TRUTH_OR_EXPR: | |
373 | case TRUTH_XOR_EXPR: | |
374 | default: | |
375 | continue; | |
376 | } | |
377 | ||
378 | /* These are the operands for the verification. */ | |
379 | tree lhs = gimple_assign_lhs (asgn); | |
380 | tree op1 = gimple_assign_rhs1 (asgn); | |
381 | tree op2 = gimple_assign_rhs2 (asgn); | |
382 | location_t loc = gimple_location (asgn); | |
383 | ||
384 | /* Vector booleans can't be used in conditional branches. ??? | |
385 | Can we do better? How to reduce compare and | |
386 | reversed-compare result vectors to a single boolean? */ | |
387 | if (VECTOR_TYPE_P (TREE_TYPE (op1))) | |
388 | continue; | |
389 | ||
390 | gcc_checking_assert (TREE_CODE (TREE_TYPE (lhs)) == BOOLEAN_TYPE); | |
391 | ||
392 | tree rhs = copy_ssa_name (lhs); | |
393 | ||
394 | gimple_stmt_iterator gsi_split = gsi; | |
395 | /* Don't separate the original assignment from debug stmts | |
396 | that might be associated with it, and arrange to split the | |
397 | block after debug stmts, so as to make sure the split block | |
398 | won't be debug stmts only. */ | |
399 | gsi_next_nondebug (&gsi_split); | |
400 | ||
401 | bool same_p = (op1 == op2); | |
402 | op1 = detach_value (loc, &gsi_split, op1); | |
403 | op2 = same_p ? op1 : detach_value (loc, &gsi_split, op2); | |
404 | ||
405 | gassign *asgnck = gimple_build_assign (rhs, cop, op1, op2); | |
406 | gimple_set_location (asgnck, loc); | |
407 | gsi_insert_before (&gsi_split, asgnck, GSI_SAME_STMT); | |
408 | ||
409 | /* We wish to insert a cond_expr after the compare, so arrange | |
410 | for it to be at the end of a block if it isn't. */ | |
411 | if (!gsi_end_p (gsi_split)) | |
412 | { | |
413 | gsi_prev (&gsi_split); | |
414 | split_block (bb, gsi_stmt (gsi_split)); | |
415 | gsi_next (&gsi_split); | |
416 | gcc_checking_assert (gsi_end_p (gsi_split)); | |
417 | ||
418 | single_succ_edge (bb)->goto_locus = loc; | |
419 | ||
420 | if (dump_file) | |
421 | fprintf (dump_file, "Splitting block %i\n", bb->index); | |
422 | } | |
423 | ||
424 | gcc_checking_assert (single_succ_p (bb)); | |
425 | ||
426 | insert_check_and_trap (loc, &gsi_split, EDGE_TRUE_VALUE, | |
427 | EQ_EXPR, lhs, rhs); | |
428 | } | |
429 | ||
430 | return 0; | |
431 | } | |
432 | ||
433 | /* Instantiate a hardcmp pass. */ | |
434 | ||
435 | gimple_opt_pass * | |
436 | make_pass_harden_compares (gcc::context *ctxt) | |
437 | { | |
438 | return new pass_harden_compares (ctxt); | |
439 | } |