1 | /* Warn on problematic uses of alloca and variable length arrays. |
2 | Copyright (C) 2016-2023 Free Software Foundation, Inc. |
3 | Contributed by Aldy Hernandez <aldyh@redhat.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 "gimple.h" |
27 | #include "tree-pass.h" |
28 | #include "ssa.h" |
29 | #include "gimple-pretty-print.h" |
30 | #include "diagnostic-core.h" |
31 | #include "fold-const.h" |
32 | #include "gimple-iterator.h" |
33 | #include "tree-ssa.h" |
34 | #include "tree-cfg.h" |
35 | #include "builtins.h" |
36 | #include "calls.h" |
37 | #include "cfgloop.h" |
38 | #include "intl.h" |
39 | #include "gimple-range.h" |
40 | |
41 | static unsigned HOST_WIDE_INT adjusted_warn_limit (bool); |
42 | |
43 | const pass_data pass_data_walloca = { |
44 | .type: GIMPLE_PASS, |
45 | .name: "walloca" , |
46 | .optinfo_flags: OPTGROUP_NONE, |
47 | .tv_id: TV_NONE, |
48 | PROP_cfg, // properties_required |
49 | .properties_provided: 0, // properties_provided |
50 | .properties_destroyed: 0, // properties_destroyed |
51 | .todo_flags_start: 0, // properties_start |
52 | .todo_flags_finish: 0, // properties_finish |
53 | }; |
54 | |
55 | class pass_walloca : public gimple_opt_pass |
56 | { |
57 | public: |
58 | pass_walloca (gcc::context *ctxt) |
59 | : gimple_opt_pass(pass_data_walloca, ctxt), xlimit_certain_p (false) |
60 | {} |
61 | opt_pass *clone () final override { return new pass_walloca (m_ctxt); } |
62 | void set_pass_param (unsigned int n, bool param) final override |
63 | { |
64 | gcc_assert (n == 0); |
65 | // Set to true to enable only warnings for alloca calls that |
66 | // are certainly in excess of the limit. This includes calls |
67 | // with constant arguments but excludes those in ranges (that |
68 | // can only be determined by range analysis) as well as |
69 | // the "may be too large" kind. |
70 | xlimit_certain_p = param; |
71 | } |
72 | bool gate (function *) final override; |
73 | unsigned int execute (function *) final override; |
74 | |
75 | private: |
76 | // Set to TRUE the first time we run this pass on a function. |
77 | bool xlimit_certain_p; |
78 | }; |
79 | |
80 | bool |
81 | pass_walloca::gate (function *fun ATTRIBUTE_UNUSED) |
82 | { |
83 | // Warning is disabled when its size limit is greater than PTRDIFF_MAX |
84 | // for the target maximum, which makes the limit negative since when |
85 | // represented in signed HOST_WIDE_INT. |
86 | unsigned HOST_WIDE_INT max = tree_to_uhwi (TYPE_MAX_VALUE (ptrdiff_type_node)); |
87 | return (adjusted_warn_limit (false) <= max |
88 | || adjusted_warn_limit (true) <= max); |
89 | } |
90 | |
91 | // Possible problematic uses of alloca. |
92 | enum alloca_type { |
93 | // Alloca argument is within known bounds that are appropriate. |
94 | ALLOCA_OK, |
95 | |
96 | // Alloca argument is KNOWN to have a value that is too large. |
97 | ALLOCA_BOUND_DEFINITELY_LARGE, |
98 | |
99 | // Alloca argument may be too large. |
100 | ALLOCA_BOUND_MAYBE_LARGE, |
101 | |
102 | // Alloca appears in a loop. |
103 | ALLOCA_IN_LOOP, |
104 | |
105 | // Alloca argument is 0. |
106 | ALLOCA_ARG_IS_ZERO, |
107 | |
108 | // Alloca call is unbounded. That is, there is no controlling |
109 | // predicate for its argument. |
110 | ALLOCA_UNBOUNDED |
111 | }; |
112 | |
113 | // Type of an alloca call with its corresponding limit, if applicable. |
114 | class alloca_type_and_limit { |
115 | public: |
116 | enum alloca_type type; |
117 | // For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE |
118 | // types, this field indicates the assumed limit if known or |
119 | // integer_zero_node if unknown. For any other alloca types, this |
120 | // field is undefined. |
121 | wide_int limit; |
122 | alloca_type_and_limit (); |
123 | alloca_type_and_limit (enum alloca_type type, |
124 | wide_int i) : type(type), limit(i) { } |
125 | alloca_type_and_limit (enum alloca_type type) : type(type) |
126 | { |
127 | limit = wi::to_wide (integer_zero_node); |
128 | } |
129 | }; |
130 | |
131 | /* Return TRUE if the user specified a limit for either VLAs or ALLOCAs. */ |
132 | |
133 | static bool |
134 | warn_limit_specified_p (bool is_vla) |
135 | { |
136 | unsigned HOST_WIDE_INT max = is_vla ? warn_vla_limit : warn_alloca_limit; |
137 | return max != HOST_WIDE_INT_MAX; |
138 | } |
139 | |
140 | /* Return the value of the argument N to -Walloca-larger-than= or |
141 | -Wvla-larger-than= adjusted for the target data model so that |
142 | when N == HOST_WIDE_INT_MAX, the adjusted value is set to |
143 | PTRDIFF_MAX on the target. This is done to prevent warnings |
144 | for unknown/unbounded allocations in the "permissive mode" |
145 | while still diagnosing excessive and necessarily invalid |
146 | allocations. */ |
147 | |
148 | static unsigned HOST_WIDE_INT |
149 | adjusted_warn_limit (bool idx) |
150 | { |
151 | static HOST_WIDE_INT limits[2]; |
152 | if (limits[idx]) |
153 | return limits[idx]; |
154 | |
155 | limits[idx] = idx ? warn_vla_limit : warn_alloca_limit; |
156 | if (limits[idx] != HOST_WIDE_INT_MAX) |
157 | return limits[idx]; |
158 | |
159 | limits[idx] = tree_to_shwi (TYPE_MAX_VALUE (ptrdiff_type_node)); |
160 | return limits[idx]; |
161 | } |
162 | |
163 | // Analyze the alloca call in STMT and return the alloca type with its |
164 | // corresponding limit (if applicable). IS_VLA is set if the alloca |
165 | // call was created by the gimplifier for a VLA. |
166 | |
167 | static class alloca_type_and_limit |
168 | alloca_call_type (gimple *stmt, bool is_vla) |
169 | { |
170 | gcc_assert (gimple_alloca_call_p (stmt)); |
171 | tree len = gimple_call_arg (gs: stmt, index: 0); |
172 | |
173 | gcc_assert (!is_vla || warn_vla_limit >= 0); |
174 | gcc_assert (is_vla || warn_alloca_limit >= 0); |
175 | |
176 | // Adjust warn_alloca_max_size for VLAs, by taking the underlying |
177 | // type into account. |
178 | unsigned HOST_WIDE_INT max_size = adjusted_warn_limit (idx: is_vla); |
179 | |
180 | // Check for the obviously bounded case. |
181 | if (TREE_CODE (len) == INTEGER_CST) |
182 | { |
183 | if (tree_to_uhwi (len) > max_size) |
184 | return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE, |
185 | wi::to_wide (t: len)); |
186 | if (integer_zerop (len)) |
187 | { |
188 | const offset_int maxobjsize |
189 | = wi::to_offset (t: max_object_size ()); |
190 | alloca_type result = (max_size < maxobjsize |
191 | ? ALLOCA_ARG_IS_ZERO : ALLOCA_OK); |
192 | return alloca_type_and_limit (result); |
193 | } |
194 | |
195 | return alloca_type_and_limit (ALLOCA_OK); |
196 | } |
197 | |
198 | struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_OK); |
199 | // If we have a declared maximum size, we can take it into account. |
200 | if (gimple_call_builtin_p (stmt, BUILT_IN_ALLOCA_WITH_ALIGN_AND_MAX)) |
201 | { |
202 | tree arg = gimple_call_arg (gs: stmt, index: 2); |
203 | if (compare_tree_int (arg, max_size) <= 0) |
204 | ret = alloca_type_and_limit (ALLOCA_OK); |
205 | else |
206 | { |
207 | const offset_int maxobjsize |
208 | = wi::to_offset (t: max_object_size ()); |
209 | alloca_type result = (max_size < maxobjsize |
210 | ? ALLOCA_BOUND_MAYBE_LARGE : ALLOCA_OK); |
211 | ret = alloca_type_and_limit (result, wi::to_wide (t: arg)); |
212 | } |
213 | return ret; |
214 | } |
215 | |
216 | // If the user specified a limit, use it. |
217 | int_range_max r; |
218 | if (warn_limit_specified_p (is_vla) |
219 | && TREE_CODE (len) == SSA_NAME |
220 | && types_compatible_p (TREE_TYPE (len), size_type_node) |
221 | && get_range_query (cfun)->range_of_expr (r, expr: len, stmt) |
222 | && !r.varying_p ()) |
223 | { |
224 | // The invalid bits are anything outside of [0, MAX_SIZE]. |
225 | int_range<2> invalid_range (size_type_node, |
226 | wi::shwi (val: 0, TYPE_PRECISION (size_type_node)), |
227 | wi::shwi (val: max_size, TYPE_PRECISION (size_type_node)), |
228 | VR_ANTI_RANGE); |
229 | |
230 | r.intersect (invalid_range); |
231 | if (r.undefined_p ()) |
232 | return alloca_type_and_limit (ALLOCA_OK); |
233 | |
234 | return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, |
235 | wi::to_wide (integer_zero_node)); |
236 | } |
237 | |
238 | const offset_int maxobjsize = tree_to_shwi (max_object_size ()); |
239 | /* When MAX_SIZE is greater than or equal to PTRDIFF_MAX treat |
240 | allocations that aren't visibly constrained as OK, otherwise |
241 | report them as (potentially) unbounded. */ |
242 | alloca_type unbounded_result = (max_size < maxobjsize.to_uhwi () |
243 | ? ALLOCA_UNBOUNDED : ALLOCA_OK); |
244 | return alloca_type_and_limit (unbounded_result); |
245 | } |
246 | |
247 | // Return TRUE if STMT is in a loop, otherwise return FALSE. |
248 | |
249 | static bool |
250 | in_loop_p (gimple *stmt) |
251 | { |
252 | basic_block bb = gimple_bb (g: stmt); |
253 | return |
254 | bb->loop_father && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun); |
255 | } |
256 | |
257 | unsigned int |
258 | pass_walloca::execute (function *fun) |
259 | { |
260 | enable_ranger (m: fun); |
261 | basic_block bb; |
262 | FOR_EACH_BB_FN (bb, fun) |
263 | { |
264 | for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (i: si); |
265 | gsi_next (i: &si)) |
266 | { |
267 | gimple *stmt = gsi_stmt (i: si); |
268 | if (!gimple_alloca_call_p (stmt)) |
269 | continue; |
270 | |
271 | location_t loc = gimple_nonartificial_location (g: stmt); |
272 | loc = expansion_point_location_if_in_system_header (loc); |
273 | |
274 | const bool is_vla |
275 | = gimple_call_alloca_for_var_p (s: as_a <gcall *> (p: stmt)); |
276 | |
277 | // Strict mode whining for VLAs is handled by the front-end, |
278 | // so we can safely ignore this case. Also, ignore VLAs if |
279 | // the user doesn't care about them. |
280 | if (is_vla) |
281 | { |
282 | if (warn_vla > 0 || warn_vla_limit < 0) |
283 | continue; |
284 | } |
285 | else if (warn_alloca) |
286 | { |
287 | warning_at (loc, OPT_Walloca, "use of %<alloca%>" ); |
288 | continue; |
289 | } |
290 | else if (warn_alloca_limit < 0) |
291 | continue; |
292 | |
293 | class alloca_type_and_limit t |
294 | = alloca_call_type (stmt, is_vla); |
295 | |
296 | unsigned HOST_WIDE_INT adjusted_alloca_limit |
297 | = adjusted_warn_limit (idx: false); |
298 | // Even if we think the alloca call is OK, make sure it's not in a |
299 | // loop, except for a VLA, since VLAs are guaranteed to be cleaned |
300 | // up when they go out of scope, including in a loop. |
301 | if (t.type == ALLOCA_OK && !is_vla && in_loop_p (stmt)) |
302 | { |
303 | /* As in other instances, only diagnose this when the limit |
304 | is less than the maximum valid object size. */ |
305 | const offset_int maxobjsize |
306 | = wi::to_offset (t: max_object_size ()); |
307 | if (adjusted_alloca_limit < maxobjsize.to_uhwi ()) |
308 | t = alloca_type_and_limit (ALLOCA_IN_LOOP); |
309 | } |
310 | |
311 | enum opt_code wcode |
312 | = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_; |
313 | char buff[WIDE_INT_MAX_INL_PRECISION / 4 + 4]; |
314 | switch (t.type) |
315 | { |
316 | case ALLOCA_OK: |
317 | break; |
318 | case ALLOCA_BOUND_MAYBE_LARGE: |
319 | { |
320 | if (xlimit_certain_p) |
321 | break; |
322 | |
323 | auto_diagnostic_group d; |
324 | if (warning_at (loc, wcode, |
325 | (is_vla |
326 | ? G_("argument to variable-length " |
327 | "array may be too large" ) |
328 | : G_("argument to %<alloca%> may be too " |
329 | "large" ))) |
330 | && t.limit != 0) |
331 | { |
332 | gcc_assert (t.limit.get_len () < WIDE_INT_MAX_INL_ELTS); |
333 | print_decu (wi: t.limit, buf: buff); |
334 | inform (loc, "limit is %wu bytes, but argument " |
335 | "may be as large as %s" , |
336 | is_vla ? warn_vla_limit : adjusted_alloca_limit, |
337 | buff); |
338 | } |
339 | } |
340 | break; |
341 | case ALLOCA_BOUND_DEFINITELY_LARGE: |
342 | { |
343 | auto_diagnostic_group d; |
344 | if (warning_at (loc, wcode, |
345 | (is_vla |
346 | ? G_("argument to variable-length" |
347 | " array is too large" ) |
348 | : G_("argument to %<alloca%> is too large" ))) |
349 | && t.limit != 0) |
350 | { |
351 | gcc_assert (t.limit.get_len () < WIDE_INT_MAX_INL_ELTS); |
352 | print_decu (wi: t.limit, buf: buff); |
353 | inform (loc, "limit is %wu bytes, but argument is %s" , |
354 | is_vla ? warn_vla_limit : adjusted_alloca_limit, |
355 | buff); |
356 | } |
357 | } |
358 | break; |
359 | case ALLOCA_UNBOUNDED: |
360 | if (xlimit_certain_p) |
361 | break; |
362 | |
363 | warning_at (loc, wcode, |
364 | (is_vla |
365 | ? G_("unbounded use of variable-length array" ) |
366 | : G_("unbounded use of %<alloca%>" ))); |
367 | break; |
368 | case ALLOCA_IN_LOOP: |
369 | gcc_assert (!is_vla); |
370 | warning_at (loc, wcode, |
371 | "use of %<alloca%> within a loop" ); |
372 | break; |
373 | case ALLOCA_ARG_IS_ZERO: |
374 | warning_at (loc, wcode, |
375 | (is_vla |
376 | ? G_("argument to variable-length array " |
377 | "is zero" ) |
378 | : G_("argument to %<alloca%> is zero" ))); |
379 | break; |
380 | default: |
381 | gcc_unreachable (); |
382 | } |
383 | } |
384 | } |
385 | disable_ranger (fun); |
386 | return 0; |
387 | } |
388 | |
389 | gimple_opt_pass * |
390 | make_pass_walloca (gcc::context *ctxt) |
391 | { |
392 | return new pass_walloca (ctxt); |
393 | } |
394 | |