/* Warn on problematic uses of alloca and variable length arrays. Copyright (C) 2016-2017 Free Software Foundation, Inc. Contributed by Aldy Hernandez . This file is part of GCC. GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version. GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see . */ #include "config.h" #include "system.h" #include "coretypes.h" #include "backend.h" #include "tree.h" #include "gimple.h" #include "tree-pass.h" #include "ssa.h" #include "gimple-pretty-print.h" #include "diagnostic-core.h" #include "fold-const.h" #include "gimple-iterator.h" #include "tree-ssa.h" #include "params.h" #include "tree-cfg.h" #include "calls.h" #include "cfgloop.h" #include "intl.h" const pass_data pass_data_walloca = { GIMPLE_PASS, "walloca", OPTGROUP_NONE, TV_NONE, PROP_cfg, // properties_required 0, // properties_provided 0, // properties_destroyed 0, // properties_start 0, // properties_finish }; class pass_walloca : public gimple_opt_pass { public: pass_walloca (gcc::context *ctxt) : gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false) {} opt_pass *clone () { return new pass_walloca (m_ctxt); } void set_pass_param (unsigned int n, bool param) { gcc_assert (n == 0); first_time_p = param; } virtual bool gate (function *); virtual unsigned int execute (function *); private: // Set to TRUE the first time we run this pass on a function. bool first_time_p; }; bool pass_walloca::gate (function *fun ATTRIBUTE_UNUSED) { // The first time this pass is called, it is called before // optimizations have been run and range information is unavailable, // so we can only perform strict alloca checking. if (first_time_p) return warn_alloca != 0; return ((unsigned HOST_WIDE_INT) warn_alloca_limit > 0 || (unsigned HOST_WIDE_INT) warn_vla_limit > 0); } // Possible problematic uses of alloca. enum alloca_type { // Alloca argument is within known bounds that are appropriate. ALLOCA_OK, // Alloca argument is KNOWN to have a value that is too large. ALLOCA_BOUND_DEFINITELY_LARGE, // Alloca argument may be too large. ALLOCA_BOUND_MAYBE_LARGE, // Alloca argument is bounded but of an indeterminate size. ALLOCA_BOUND_UNKNOWN, // Alloca argument was casted from a signed integer. ALLOCA_CAST_FROM_SIGNED, // Alloca appears in a loop. ALLOCA_IN_LOOP, // Alloca argument is 0. ALLOCA_ARG_IS_ZERO, // Alloca call is unbounded. That is, there is no controlling // predicate for its argument. ALLOCA_UNBOUNDED }; // Type of an alloca call with its corresponding limit, if applicable. struct alloca_type_and_limit { enum alloca_type type; // For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE // types, this field indicates the assumed limit if known or // integer_zero_node if unknown. For any other alloca types, this // field is undefined. wide_int limit; alloca_type_and_limit (); alloca_type_and_limit (enum alloca_type type, wide_int i) : type(type), limit(i) { } alloca_type_and_limit (enum alloca_type type) : type(type) { } }; // NOTE: When we get better range info, this entire function becomes // irrelevant, as it should be possible to get range info for an SSA // name at any point in the program. // // We have a few heuristics up our sleeve to determine if a call to // alloca() is within bounds. Try them out and return the type of // alloca call with its assumed limit (if applicable). // // Given a known argument (ARG) to alloca() and an EDGE (E) // calculating said argument, verify that the last statement in the BB // in E->SRC is a gate comparing ARG to an acceptable bound for // alloca(). See examples below. // // If set, ARG_CASTED is the possible unsigned argument to which ARG // was casted to. This is to handle cases where the controlling // predicate is looking at a casted value, not the argument itself. // arg_casted = (size_t) arg; // if (arg_casted < N) // goto bb3; // else // goto bb5; // // MAX_SIZE is WARN_ALLOCA= adjusted for VLAs. It is the maximum size // in bytes we allow for arg. static struct alloca_type_and_limit alloca_call_type_by_arg (tree arg, tree arg_casted, edge e, unsigned max_size) { basic_block bb = e->src; gimple_stmt_iterator gsi = gsi_last_bb (bb); gimple *last = gsi_stmt (gsi); if (!last || gimple_code (last) != GIMPLE_COND) return alloca_type_and_limit (ALLOCA_UNBOUNDED); enum tree_code cond_code = gimple_cond_code (last); if (e->flags & EDGE_TRUE_VALUE) ; else if (e->flags & EDGE_FALSE_VALUE) cond_code = invert_tree_comparison (cond_code, false); else return alloca_type_and_limit (ALLOCA_UNBOUNDED); // Check for: // if (ARG .COND. N) // goto ; // else // goto ; // : // alloca(ARG); if ((cond_code == LE_EXPR || cond_code == LT_EXPR || cond_code == GT_EXPR || cond_code == GE_EXPR) && (gimple_cond_lhs (last) == arg || gimple_cond_lhs (last) == arg_casted)) { if (TREE_CODE (gimple_cond_rhs (last)) == INTEGER_CST) { tree rhs = gimple_cond_rhs (last); int tst = wi::cmpu (wi::to_widest (rhs), max_size); if ((cond_code == LT_EXPR && tst == -1) || (cond_code == LE_EXPR && (tst == -1 || tst == 0))) return alloca_type_and_limit (ALLOCA_OK); else { // Let's not get too specific as to how large the limit // may be. Someone's clearly an idiot when things // degrade into "if (N > Y) alloca(N)". if (cond_code == GT_EXPR || cond_code == GE_EXPR) rhs = integer_zero_node; return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, rhs); } } else return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN); } // Similarly, but check for a comparison with an unknown LIMIT. // if (LIMIT .COND. ARG) // alloca(arg); // // Where LIMIT has a bound of unknown range. // // Note: All conditions of the form (ARG .COND. XXXX) where covered // by the previous check above, so we only need to look for (LIMIT // .COND. ARG) here. tree limit = gimple_cond_lhs (last); if ((gimple_cond_rhs (last) == arg || gimple_cond_rhs (last) == arg_casted) && TREE_CODE (limit) == SSA_NAME) { wide_int min, max; value_range_type range_type = get_range_info (limit, &min, &max); if (range_type == VR_UNDEFINED || range_type == VR_VARYING) return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN); // ?? It looks like the above `if' is unnecessary, as we never // get any VR_RANGE or VR_ANTI_RANGE here. If we had a range // for LIMIT, I suppose we would have taken care of it in // alloca_call_type(), or handled above where we handle (ARG .COND. N). // // If this ever triggers, we should probably figure out why and // handle it, though it is likely to be just an ALLOCA_UNBOUNDED. return alloca_type_and_limit (ALLOCA_UNBOUNDED); } return alloca_type_and_limit (ALLOCA_UNBOUNDED); } // Return TRUE if SSA's definition is a cast from a signed type. // If so, set *INVALID_CASTED_TYPE to the signed type. static bool cast_from_signed_p (tree ssa, tree *invalid_casted_type) { gimple *def = SSA_NAME_DEF_STMT (ssa); if (def && !gimple_nop_p (def) && gimple_assign_cast_p (def) && !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def)))) { *invalid_casted_type = TREE_TYPE (gimple_assign_rhs1 (def)); return true; } return false; } // Return TRUE if X has a maximum range of MAX, basically covering the // entire domain, in which case it's no range at all. static bool is_max (tree x, wide_int max) { return wi::max_value (TREE_TYPE (x)) == max; } // Analyze the alloca call in STMT and return the alloca type with its // corresponding limit (if applicable). IS_VLA is set if the alloca // call is really a BUILT_IN_ALLOCA_WITH_ALIGN, signifying a VLA. // // If the alloca call may be too large because of a cast from a signed // type to an unsigned type, set *INVALID_CASTED_TYPE to the // problematic signed type. static struct alloca_type_and_limit alloca_call_type (gimple *stmt, bool is_vla, tree *invalid_casted_type) { gcc_assert (gimple_alloca_call_p (stmt)); bool tentative_cast_from_signed = false; tree len = gimple_call_arg (stmt, 0); tree len_casted = NULL; wide_int min, max; struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_UNBOUNDED); gcc_assert (!is_vla || (unsigned HOST_WIDE_INT) warn_vla_limit > 0); gcc_assert (is_vla || (unsigned HOST_WIDE_INT) warn_alloca_limit > 0); // Adjust warn_alloca_max_size for VLAs, by taking the underlying // type into account. unsigned HOST_WIDE_INT max_size; if (is_vla) max_size = (unsigned HOST_WIDE_INT) warn_vla_limit; else max_size = (unsigned HOST_WIDE_INT) warn_alloca_limit; // Check for the obviously bounded case. if (TREE_CODE (len) == INTEGER_CST) { if (tree_to_uhwi (len) > max_size) return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE, len); if (integer_zerop (len)) return alloca_type_and_limit (ALLOCA_ARG_IS_ZERO); ret = alloca_type_and_limit (ALLOCA_OK); } // Check the range info if available. else if (TREE_CODE (len) == SSA_NAME) { value_range_type range_type = get_range_info (len, &min, &max); if (range_type == VR_RANGE) { if (wi::leu_p (max, max_size)) ret = alloca_type_and_limit (ALLOCA_OK); else { // A cast may have created a range we don't care // about. For instance, a cast from 16-bit to // 32-bit creates a range of 0..65535, even if there // is not really a determinable range in the // underlying code. In this case, look through the // cast at the original argument, and fall through // to look at other alternatives. // // We only look at through the cast when its from // unsigned to unsigned, otherwise we may risk // looking at SIGNED_INT < N, which is clearly not // what we want. In this case, we'd be interested // in a VR_RANGE of [0..N]. // // Note: None of this is perfect, and should all go // away with better range information. But it gets // most of the cases. gimple *def = SSA_NAME_DEF_STMT (len); if (gimple_assign_cast_p (def)) { tree rhs1 = gimple_assign_rhs1 (def); tree rhs1type = TREE_TYPE (rhs1); // Bail if the argument type is not valid. if (!INTEGRAL_TYPE_P (rhs1type)) return alloca_type_and_limit (ALLOCA_OK); if (TYPE_UNSIGNED (rhs1type)) { len_casted = rhs1; range_type = get_range_info (len_casted, &min, &max); } } // An unknown range or a range of the entire domain is // really no range at all. if (range_type == VR_VARYING || (!len_casted && is_max (len, max)) || (len_casted && is_max (len_casted, max))) { // Fall through. } else if (range_type == VR_ANTI_RANGE) return alloca_type_and_limit (ALLOCA_UNBOUNDED); else if (range_type != VR_VARYING) return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, max); } } else if (range_type == VR_ANTI_RANGE) { // There may be some wrapping around going on. Catch it // with this heuristic. Hopefully, this VR_ANTI_RANGE // nonsense will go away, and we won't have to catch the // sign conversion problems with this crap. // // This is here to catch things like: // void foo(signed int n) { // if (n < 100) // alloca(n); // ... // } if (cast_from_signed_p (len, invalid_casted_type)) { // Unfortunately this also triggers: // // __SIZE_TYPE__ n = (__SIZE_TYPE__)blah; // if (n < 100) // alloca(n); // // ...which is clearly bounded. So, double check that // the paths leading up to the size definitely don't // have a bound. tentative_cast_from_signed = true; } } // No easily determined range and try other things. } // If we couldn't find anything, try a few heuristics for things we // can easily determine. Check these misc cases but only accept // them if all predecessors have a known bound. basic_block bb = gimple_bb (stmt); if (ret.type == ALLOCA_UNBOUNDED) { ret.type = ALLOCA_OK; for (unsigned ix = 0; ix < EDGE_COUNT (bb->preds); ix++) { gcc_assert (!len_casted || TYPE_UNSIGNED (TREE_TYPE (len_casted))); ret = alloca_call_type_by_arg (len, len_casted, EDGE_PRED (bb, ix), max_size); if (ret.type != ALLOCA_OK) break; } } if (tentative_cast_from_signed && ret.type != ALLOCA_OK) return alloca_type_and_limit (ALLOCA_CAST_FROM_SIGNED); return ret; } // Return TRUE if the alloca call in STMT is in a loop, otherwise // return FALSE. As an exception, ignore alloca calls for VLAs that // occur in a loop since those will be cleaned up when they go out of // scope. static bool in_loop_p (bool is_vla, gimple *stmt) { basic_block bb = gimple_bb (stmt); if (bb->loop_father && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun)) { // Do not warn on VLAs occurring in a loop, since VLAs are // guaranteed to be cleaned up when they go out of scope. // That is, there is a corresponding __builtin_stack_restore // at the end of the scope in which the VLA occurs. tree fndecl = gimple_call_fn (stmt); while (TREE_CODE (fndecl) == ADDR_EXPR) fndecl = TREE_OPERAND (fndecl, 0); if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL && is_vla && DECL_FUNCTION_CODE (fndecl) == BUILT_IN_ALLOCA_WITH_ALIGN) return false; return true; } return false; } unsigned int pass_walloca::execute (function *fun) { basic_block bb; FOR_EACH_BB_FN (bb, fun) { for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si)) { gimple *stmt = gsi_stmt (si); location_t loc = gimple_location (stmt); if (!gimple_alloca_call_p (stmt)) continue; gcc_assert (gimple_call_num_args (stmt) >= 1); bool is_vla = gimple_alloca_call_p (stmt) && gimple_call_alloca_for_var_p (as_a (stmt)); // Strict mode whining for VLAs is handled by the front-end, // so we can safely ignore this case. Also, ignore VLAs if // the user doesn't care about them. if (is_vla && (warn_vla > 0 || !warn_vla_limit)) continue; if (!is_vla && (warn_alloca || !warn_alloca_limit)) { if (warn_alloca) warning_at (loc, OPT_Walloca, G_("use of %")); continue; } tree invalid_casted_type = NULL; struct alloca_type_and_limit t = alloca_call_type (stmt, is_vla, &invalid_casted_type); // Even if we think the alloca call is OK, make sure it's // not in a loop. if (t.type == ALLOCA_OK && in_loop_p (is_vla, stmt)) t = alloca_type_and_limit (ALLOCA_IN_LOOP); enum opt_code wcode = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_; char buff[WIDE_INT_MAX_PRECISION / 4 + 4]; switch (t.type) { case ALLOCA_OK: break; case ALLOCA_BOUND_MAYBE_LARGE: if (warning_at (loc, wcode, is_vla ? G_("argument to variable-length array " "may be too large") : G_("argument to % may be too large")) && t.limit != integer_zero_node) { print_decu (t.limit, buff); inform (loc, G_("limit is %u bytes, but argument " "may be as large as %s"), is_vla ? warn_vla_limit : warn_alloca_limit, buff); } break; case ALLOCA_BOUND_DEFINITELY_LARGE: if (warning_at (loc, wcode, is_vla ? G_("argument to variable-length array " "is too large") : G_("argument to % is too large")) && t.limit != integer_zero_node) { print_decu (t.limit, buff); inform (loc, G_("limit is %u bytes, but argument is %s"), is_vla ? warn_vla_limit : warn_alloca_limit, buff); } break; case ALLOCA_BOUND_UNKNOWN: warning_at (loc, wcode, is_vla ? G_("variable-length array bound is unknown") : G_("% bound is unknown")); break; case ALLOCA_UNBOUNDED: warning_at (loc, wcode, is_vla ? G_("unbounded use of variable-length array") : G_("unbounded use of %")); break; case ALLOCA_IN_LOOP: gcc_assert (!is_vla); warning_at (loc, wcode, G_("use of % within a loop")); break; case ALLOCA_CAST_FROM_SIGNED: gcc_assert (invalid_casted_type != NULL_TREE); warning_at (loc, wcode, is_vla ? G_("argument to variable-length array " "may be too large due to " "conversion from %qT to %qT") : G_("argument to % may be too large " "due to conversion from %qT to %qT"), invalid_casted_type, size_type_node); break; case ALLOCA_ARG_IS_ZERO: warning_at (loc, wcode, is_vla ? G_("argument to variable-length array " "is zero") : G_("argument to % is zero")); break; default: gcc_unreachable (); } } } return 0; } gimple_opt_pass * make_pass_walloca (gcc::context *ctxt) { return new pass_walloca (ctxt); }