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111
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1 /* Warn on problematic uses of alloca and variable length arrays.
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2 Copyright (C) 2016-2017 Free Software Foundation, Inc.
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3 Contributed by Aldy Hernandez <aldyh@redhat.com>.
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4
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5 This file is part of GCC.
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6
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7 GCC is free software; you can redistribute it and/or modify it under
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8 the terms of the GNU General Public License as published by the Free
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9 Software Foundation; either version 3, or (at your option) any later
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10 version.
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11
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12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
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14 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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15 for more details.
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16
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17 You should have received a copy of the GNU General Public License
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18 along with GCC; see the file COPYING3. If not see
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19 <http://www.gnu.org/licenses/>. */
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20
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21 #include "config.h"
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22 #include "system.h"
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23 #include "coretypes.h"
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24 #include "backend.h"
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25 #include "tree.h"
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26 #include "gimple.h"
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27 #include "tree-pass.h"
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28 #include "ssa.h"
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29 #include "gimple-pretty-print.h"
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30 #include "diagnostic-core.h"
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31 #include "fold-const.h"
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32 #include "gimple-iterator.h"
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33 #include "tree-ssa.h"
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34 #include "params.h"
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35 #include "tree-cfg.h"
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36 #include "calls.h"
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37 #include "cfgloop.h"
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38 #include "intl.h"
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39
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40 const pass_data pass_data_walloca = {
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41 GIMPLE_PASS,
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42 "walloca",
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43 OPTGROUP_NONE,
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44 TV_NONE,
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45 PROP_cfg, // properties_required
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46 0, // properties_provided
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47 0, // properties_destroyed
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48 0, // properties_start
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49 0, // properties_finish
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50 };
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51
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52 class pass_walloca : public gimple_opt_pass
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53 {
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54 public:
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55 pass_walloca (gcc::context *ctxt)
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56 : gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false)
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57 {}
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58 opt_pass *clone () { return new pass_walloca (m_ctxt); }
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59 void set_pass_param (unsigned int n, bool param)
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60 {
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61 gcc_assert (n == 0);
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62 first_time_p = param;
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63 }
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64 virtual bool gate (function *);
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65 virtual unsigned int execute (function *);
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66
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67 private:
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68 // Set to TRUE the first time we run this pass on a function.
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69 bool first_time_p;
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70 };
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71
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72 bool
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73 pass_walloca::gate (function *fun ATTRIBUTE_UNUSED)
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74 {
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75 // The first time this pass is called, it is called before
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76 // optimizations have been run and range information is unavailable,
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77 // so we can only perform strict alloca checking.
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78 if (first_time_p)
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79 return warn_alloca != 0;
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80
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81 return ((unsigned HOST_WIDE_INT) warn_alloca_limit > 0
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82 || (unsigned HOST_WIDE_INT) warn_vla_limit > 0);
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83 }
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84
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85 // Possible problematic uses of alloca.
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86 enum alloca_type {
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87 // Alloca argument is within known bounds that are appropriate.
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88 ALLOCA_OK,
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89
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90 // Alloca argument is KNOWN to have a value that is too large.
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91 ALLOCA_BOUND_DEFINITELY_LARGE,
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92
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93 // Alloca argument may be too large.
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94 ALLOCA_BOUND_MAYBE_LARGE,
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95
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96 // Alloca argument is bounded but of an indeterminate size.
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97 ALLOCA_BOUND_UNKNOWN,
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98
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99 // Alloca argument was casted from a signed integer.
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100 ALLOCA_CAST_FROM_SIGNED,
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101
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102 // Alloca appears in a loop.
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103 ALLOCA_IN_LOOP,
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104
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105 // Alloca argument is 0.
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106 ALLOCA_ARG_IS_ZERO,
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107
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108 // Alloca call is unbounded. That is, there is no controlling
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109 // predicate for its argument.
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110 ALLOCA_UNBOUNDED
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111 };
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112
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113 // Type of an alloca call with its corresponding limit, if applicable.
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114 struct alloca_type_and_limit {
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115 enum alloca_type type;
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116 // For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE
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117 // types, this field indicates the assumed limit if known or
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118 // integer_zero_node if unknown. For any other alloca types, this
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119 // field is undefined.
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120 wide_int limit;
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121 alloca_type_and_limit ();
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122 alloca_type_and_limit (enum alloca_type type,
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123 wide_int i) : type(type), limit(i) { }
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124 alloca_type_and_limit (enum alloca_type type) : type(type) { }
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125 };
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126
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127 // NOTE: When we get better range info, this entire function becomes
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128 // irrelevant, as it should be possible to get range info for an SSA
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129 // name at any point in the program.
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130 //
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131 // We have a few heuristics up our sleeve to determine if a call to
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132 // alloca() is within bounds. Try them out and return the type of
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133 // alloca call with its assumed limit (if applicable).
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134 //
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135 // Given a known argument (ARG) to alloca() and an EDGE (E)
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136 // calculating said argument, verify that the last statement in the BB
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137 // in E->SRC is a gate comparing ARG to an acceptable bound for
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138 // alloca(). See examples below.
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139 //
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140 // If set, ARG_CASTED is the possible unsigned argument to which ARG
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141 // was casted to. This is to handle cases where the controlling
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142 // predicate is looking at a casted value, not the argument itself.
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143 // arg_casted = (size_t) arg;
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144 // if (arg_casted < N)
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145 // goto bb3;
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146 // else
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147 // goto bb5;
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148 //
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149 // MAX_SIZE is WARN_ALLOCA= adjusted for VLAs. It is the maximum size
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150 // in bytes we allow for arg.
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151
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152 static struct alloca_type_and_limit
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153 alloca_call_type_by_arg (tree arg, tree arg_casted, edge e, unsigned max_size)
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154 {
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155 basic_block bb = e->src;
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156 gimple_stmt_iterator gsi = gsi_last_bb (bb);
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157 gimple *last = gsi_stmt (gsi);
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158 if (!last || gimple_code (last) != GIMPLE_COND)
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159 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
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160
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161 enum tree_code cond_code = gimple_cond_code (last);
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162 if (e->flags & EDGE_TRUE_VALUE)
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163 ;
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164 else if (e->flags & EDGE_FALSE_VALUE)
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165 cond_code = invert_tree_comparison (cond_code, false);
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166 else
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167 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
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168
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169 // Check for:
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170 // if (ARG .COND. N)
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171 // goto <bb 3>;
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172 // else
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173 // goto <bb 4>;
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174 // <bb 3>:
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175 // alloca(ARG);
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176 if ((cond_code == LE_EXPR
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177 || cond_code == LT_EXPR
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178 || cond_code == GT_EXPR
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179 || cond_code == GE_EXPR)
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180 && (gimple_cond_lhs (last) == arg
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181 || gimple_cond_lhs (last) == arg_casted))
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182 {
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183 if (TREE_CODE (gimple_cond_rhs (last)) == INTEGER_CST)
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184 {
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185 tree rhs = gimple_cond_rhs (last);
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186 int tst = wi::cmpu (wi::to_widest (rhs), max_size);
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187 if ((cond_code == LT_EXPR && tst == -1)
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188 || (cond_code == LE_EXPR && (tst == -1 || tst == 0)))
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189 return alloca_type_and_limit (ALLOCA_OK);
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190 else
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191 {
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192 // Let's not get too specific as to how large the limit
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193 // may be. Someone's clearly an idiot when things
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194 // degrade into "if (N > Y) alloca(N)".
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195 if (cond_code == GT_EXPR || cond_code == GE_EXPR)
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196 rhs = integer_zero_node;
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197 return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE,
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198 wi::to_wide (rhs));
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199 }
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200 }
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201 else
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202 return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);
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203 }
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204
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205 // Similarly, but check for a comparison with an unknown LIMIT.
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206 // if (LIMIT .COND. ARG)
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207 // alloca(arg);
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208 //
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209 // Where LIMIT has a bound of unknown range.
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210 //
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211 // Note: All conditions of the form (ARG .COND. XXXX) where covered
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212 // by the previous check above, so we only need to look for (LIMIT
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213 // .COND. ARG) here.
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214 tree limit = gimple_cond_lhs (last);
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215 if ((gimple_cond_rhs (last) == arg
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216 || gimple_cond_rhs (last) == arg_casted)
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217 && TREE_CODE (limit) == SSA_NAME)
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218 {
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219 wide_int min, max;
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220 value_range_type range_type = get_range_info (limit, &min, &max);
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221
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222 if (range_type == VR_UNDEFINED || range_type == VR_VARYING)
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223 return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);
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224
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225 // ?? It looks like the above `if' is unnecessary, as we never
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226 // get any VR_RANGE or VR_ANTI_RANGE here. If we had a range
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227 // for LIMIT, I suppose we would have taken care of it in
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228 // alloca_call_type(), or handled above where we handle (ARG .COND. N).
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229 //
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230 // If this ever triggers, we should probably figure out why and
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231 // handle it, though it is likely to be just an ALLOCA_UNBOUNDED.
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232 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
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233 }
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234
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235 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
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236 }
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237
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238 // Return TRUE if SSA's definition is a cast from a signed type.
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239 // If so, set *INVALID_CASTED_TYPE to the signed type.
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240
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241 static bool
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242 cast_from_signed_p (tree ssa, tree *invalid_casted_type)
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243 {
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244 gimple *def = SSA_NAME_DEF_STMT (ssa);
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245 if (def
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246 && !gimple_nop_p (def)
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247 && gimple_assign_cast_p (def)
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248 && !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def))))
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249 {
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250 *invalid_casted_type = TREE_TYPE (gimple_assign_rhs1 (def));
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251 return true;
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252 }
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253 return false;
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254 }
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255
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256 // Return TRUE if X has a maximum range of MAX, basically covering the
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257 // entire domain, in which case it's no range at all.
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258
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259 static bool
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260 is_max (tree x, wide_int max)
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261 {
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262 return wi::max_value (TREE_TYPE (x)) == max;
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263 }
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264
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265 // Analyze the alloca call in STMT and return the alloca type with its
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266 // corresponding limit (if applicable). IS_VLA is set if the alloca
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267 // call was created by the gimplifier for a VLA.
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268 //
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269 // If the alloca call may be too large because of a cast from a signed
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270 // type to an unsigned type, set *INVALID_CASTED_TYPE to the
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271 // problematic signed type.
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272
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273 static struct alloca_type_and_limit
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274 alloca_call_type (gimple *stmt, bool is_vla, tree *invalid_casted_type)
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275 {
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276 gcc_assert (gimple_alloca_call_p (stmt));
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277 bool tentative_cast_from_signed = false;
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278 tree len = gimple_call_arg (stmt, 0);
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279 tree len_casted = NULL;
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280 wide_int min, max;
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281 edge_iterator ei;
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282 edge e;
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283
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284 gcc_assert (!is_vla || (unsigned HOST_WIDE_INT) warn_vla_limit > 0);
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285 gcc_assert (is_vla || (unsigned HOST_WIDE_INT) warn_alloca_limit > 0);
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286
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287 // Adjust warn_alloca_max_size for VLAs, by taking the underlying
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288 // type into account.
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289 unsigned HOST_WIDE_INT max_size;
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290 if (is_vla)
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291 max_size = (unsigned HOST_WIDE_INT) warn_vla_limit;
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292 else
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293 max_size = (unsigned HOST_WIDE_INT) warn_alloca_limit;
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294
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295 // Check for the obviously bounded case.
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296 if (TREE_CODE (len) == INTEGER_CST)
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297 {
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298 if (tree_to_uhwi (len) > max_size)
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299 return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE,
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300 wi::to_wide (len));
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301 if (integer_zerop (len))
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302 return alloca_type_and_limit (ALLOCA_ARG_IS_ZERO);
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303
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304 return alloca_type_and_limit (ALLOCA_OK);
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305 }
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306
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307 // Check the range info if available.
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308 if (TREE_CODE (len) == SSA_NAME)
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309 {
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310 value_range_type range_type = get_range_info (len, &min, &max);
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311 if (range_type == VR_RANGE)
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312 {
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313 if (wi::leu_p (max, max_size))
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314 return alloca_type_and_limit (ALLOCA_OK);
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315 else
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316 {
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317 // A cast may have created a range we don't care
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318 // about. For instance, a cast from 16-bit to
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319 // 32-bit creates a range of 0..65535, even if there
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320 // is not really a determinable range in the
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321 // underlying code. In this case, look through the
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322 // cast at the original argument, and fall through
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323 // to look at other alternatives.
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324 //
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325 // We only look at through the cast when its from
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326 // unsigned to unsigned, otherwise we may risk
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327 // looking at SIGNED_INT < N, which is clearly not
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328 // what we want. In this case, we'd be interested
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329 // in a VR_RANGE of [0..N].
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330 //
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331 // Note: None of this is perfect, and should all go
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332 // away with better range information. But it gets
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333 // most of the cases.
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334 gimple *def = SSA_NAME_DEF_STMT (len);
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335 if (gimple_assign_cast_p (def))
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336 {
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337 tree rhs1 = gimple_assign_rhs1 (def);
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338 tree rhs1type = TREE_TYPE (rhs1);
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339
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340 // Bail if the argument type is not valid.
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341 if (!INTEGRAL_TYPE_P (rhs1type))
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342 return alloca_type_and_limit (ALLOCA_OK);
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343
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344 if (TYPE_UNSIGNED (rhs1type))
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345 {
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346 len_casted = rhs1;
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347 range_type = get_range_info (len_casted, &min, &max);
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348 }
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349 }
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350 // An unknown range or a range of the entire domain is
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351 // really no range at all.
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352 if (range_type == VR_VARYING
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353 || (!len_casted && is_max (len, max))
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354 || (len_casted && is_max (len_casted, max)))
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355 {
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356 // Fall through.
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357 }
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358 else if (range_type == VR_ANTI_RANGE)
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359 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
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360 else if (range_type != VR_VARYING)
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361 return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, max);
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362 }
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363 }
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364 else if (range_type == VR_ANTI_RANGE)
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365 {
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366 // There may be some wrapping around going on. Catch it
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367 // with this heuristic. Hopefully, this VR_ANTI_RANGE
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368 // nonsense will go away, and we won't have to catch the
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369 // sign conversion problems with this crap.
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370 //
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371 // This is here to catch things like:
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372 // void foo(signed int n) {
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373 // if (n < 100)
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374 // alloca(n);
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375 // ...
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376 // }
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377 if (cast_from_signed_p (len, invalid_casted_type))
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378 {
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379 // Unfortunately this also triggers:
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380 //
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381 // __SIZE_TYPE__ n = (__SIZE_TYPE__)blah;
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382 // if (n < 100)
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383 // alloca(n);
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384 //
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385 // ...which is clearly bounded. So, double check that
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386 // the paths leading up to the size definitely don't
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387 // have a bound.
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388 tentative_cast_from_signed = true;
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389 }
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390 }
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391 // No easily determined range and try other things.
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392 }
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393
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394 // If we couldn't find anything, try a few heuristics for things we
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395 // can easily determine. Check these misc cases but only accept
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396 // them if all predecessors have a known bound.
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397 struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_OK);
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398 FOR_EACH_EDGE (e, ei, gimple_bb (stmt)->preds)
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399 {
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400 gcc_assert (!len_casted || TYPE_UNSIGNED (TREE_TYPE (len_casted)));
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401 ret = alloca_call_type_by_arg (len, len_casted, e, max_size);
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402 if (ret.type != ALLOCA_OK)
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403 break;
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404 }
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405
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406 if (ret.type != ALLOCA_OK && tentative_cast_from_signed)
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407 ret = alloca_type_and_limit (ALLOCA_CAST_FROM_SIGNED);
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408
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409 // If we have a declared maximum size, we can take it into account.
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410 if (ret.type != ALLOCA_OK
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411 && gimple_call_builtin_p (stmt, BUILT_IN_ALLOCA_WITH_ALIGN_AND_MAX))
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412 {
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|
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413 tree arg = gimple_call_arg (stmt, 2);
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414 if (compare_tree_int (arg, max_size) <= 0)
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415 ret = alloca_type_and_limit (ALLOCA_OK);
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416 else
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417 ret = alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE,
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|
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418 wi::to_wide (arg));
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|
|
419 }
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|
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420
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421 return ret;
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422 }
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423
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|
424 // Return TRUE if STMT is in a loop, otherwise return FALSE.
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425
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426 static bool
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|
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427 in_loop_p (gimple *stmt)
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|
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428 {
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|
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429 basic_block bb = gimple_bb (stmt);
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|
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430 return
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|
|
431 bb->loop_father && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun);
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|
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432 }
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433
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434 unsigned int
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|
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435 pass_walloca::execute (function *fun)
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436 {
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|
|
437 basic_block bb;
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|
|
438 FOR_EACH_BB_FN (bb, fun)
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|
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439 {
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|
|
440 for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si);
|
|
|
441 gsi_next (&si))
|
|
|
442 {
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|
|
443 gimple *stmt = gsi_stmt (si);
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|
|
444 location_t loc = gimple_location (stmt);
|
|
|
445
|
|
|
446 if (!gimple_alloca_call_p (stmt))
|
|
|
447 continue;
|
|
|
448 gcc_assert (gimple_call_num_args (stmt) >= 1);
|
|
|
449
|
|
|
450 const bool is_vla
|
|
|
451 = gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));
|
|
|
452
|
|
|
453 // Strict mode whining for VLAs is handled by the front-end,
|
|
|
454 // so we can safely ignore this case. Also, ignore VLAs if
|
|
|
455 // the user doesn't care about them.
|
|
|
456 if (is_vla
|
|
|
457 && (warn_vla > 0 || !warn_vla_limit))
|
|
|
458 continue;
|
|
|
459
|
|
|
460 if (!is_vla && (warn_alloca || !warn_alloca_limit))
|
|
|
461 {
|
|
|
462 if (warn_alloca)
|
|
|
463 warning_at (loc, OPT_Walloca, G_("use of %<alloca%>"));
|
|
|
464 continue;
|
|
|
465 }
|
|
|
466
|
|
|
467 tree invalid_casted_type = NULL;
|
|
|
468 struct alloca_type_and_limit t
|
|
|
469 = alloca_call_type (stmt, is_vla, &invalid_casted_type);
|
|
|
470
|
|
|
471 // Even if we think the alloca call is OK, make sure it's not in a
|
|
|
472 // loop, except for a VLA, since VLAs are guaranteed to be cleaned
|
|
|
473 // up when they go out of scope, including in a loop.
|
|
|
474 if (t.type == ALLOCA_OK && !is_vla && in_loop_p (stmt))
|
|
|
475 t = alloca_type_and_limit (ALLOCA_IN_LOOP);
|
|
|
476
|
|
|
477 enum opt_code wcode
|
|
|
478 = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_;
|
|
|
479 char buff[WIDE_INT_MAX_PRECISION / 4 + 4];
|
|
|
480 switch (t.type)
|
|
|
481 {
|
|
|
482 case ALLOCA_OK:
|
|
|
483 break;
|
|
|
484 case ALLOCA_BOUND_MAYBE_LARGE:
|
|
|
485 if (warning_at (loc, wcode,
|
|
|
486 is_vla ? G_("argument to variable-length array "
|
|
|
487 "may be too large")
|
|
|
488 : G_("argument to %<alloca%> may be too large"))
|
|
|
489 && t.limit != 0)
|
|
|
490 {
|
|
|
491 print_decu (t.limit, buff);
|
|
|
492 inform (loc, G_("limit is %u bytes, but argument "
|
|
|
493 "may be as large as %s"),
|
|
|
494 is_vla ? warn_vla_limit : warn_alloca_limit, buff);
|
|
|
495 }
|
|
|
496 break;
|
|
|
497 case ALLOCA_BOUND_DEFINITELY_LARGE:
|
|
|
498 if (warning_at (loc, wcode,
|
|
|
499 is_vla ? G_("argument to variable-length array "
|
|
|
500 "is too large")
|
|
|
501 : G_("argument to %<alloca%> is too large"))
|
|
|
502 && t.limit != 0)
|
|
|
503 {
|
|
|
504 print_decu (t.limit, buff);
|
|
|
505 inform (loc, G_("limit is %u bytes, but argument is %s"),
|
|
|
506 is_vla ? warn_vla_limit : warn_alloca_limit, buff);
|
|
|
507 }
|
|
|
508 break;
|
|
|
509 case ALLOCA_BOUND_UNKNOWN:
|
|
|
510 warning_at (loc, wcode,
|
|
|
511 is_vla ? G_("variable-length array bound is unknown")
|
|
|
512 : G_("%<alloca%> bound is unknown"));
|
|
|
513 break;
|
|
|
514 case ALLOCA_UNBOUNDED:
|
|
|
515 warning_at (loc, wcode,
|
|
|
516 is_vla ? G_("unbounded use of variable-length array")
|
|
|
517 : G_("unbounded use of %<alloca%>"));
|
|
|
518 break;
|
|
|
519 case ALLOCA_IN_LOOP:
|
|
|
520 gcc_assert (!is_vla);
|
|
|
521 warning_at (loc, wcode, G_("use of %<alloca%> within a loop"));
|
|
|
522 break;
|
|
|
523 case ALLOCA_CAST_FROM_SIGNED:
|
|
|
524 gcc_assert (invalid_casted_type != NULL_TREE);
|
|
|
525 warning_at (loc, wcode,
|
|
|
526 is_vla ? G_("argument to variable-length array "
|
|
|
527 "may be too large due to "
|
|
|
528 "conversion from %qT to %qT")
|
|
|
529 : G_("argument to %<alloca%> may be too large "
|
|
|
530 "due to conversion from %qT to %qT"),
|
|
|
531 invalid_casted_type, size_type_node);
|
|
|
532 break;
|
|
|
533 case ALLOCA_ARG_IS_ZERO:
|
|
|
534 warning_at (loc, wcode,
|
|
|
535 is_vla ? G_("argument to variable-length array "
|
|
|
536 "is zero")
|
|
|
537 : G_("argument to %<alloca%> is zero"));
|
|
|
538 break;
|
|
|
539 default:
|
|
|
540 gcc_unreachable ();
|
|
|
541 }
|
|
|
542 }
|
|
|
543 }
|
|
|
544 return 0;
|
|
|
545 }
|
|
|
546
|
|
|
547 gimple_opt_pass *
|
|
|
548 make_pass_walloca (gcc::context *ctxt)
|
|
|
549 {
|
|
|
550 return new pass_walloca (ctxt);
|
|
|
551 }
|
