[PATCH] hardened conditionals
Alexandre Oliva
oliva@adacore.com
Sat Oct 9 03:30:04 GMT 2021
This patch introduces optional passes to harden conditionals used in
branches, and in computing boolean expressions, by adding redundant
tests of the reversed conditions, and trapping in case of unexpected
results. Though in abstract machines the redundant tests should never
fail, CPUs may be led to misbehave under certain kinds of attacks,
such as of power deprivation, and these tests reduce the likelihood of
going too far down an unexpected execution path.
This patch was regstrapped on x86_64-linux-gnu. It was also
bootstrapped along with an extra common.opt that enables both passes
unconditionally. Ok to install?
for gcc/ChangeLog
* common.opt (fharden-compares): New.
(fharden-conditional-branches): New.
* doc/invoke.texi: Document new options.
* gimple-harden-conditionals.cc: New.
* passes.def: Add new passes.
* tree-pass.h (make_pass_harden_compares): Declare.
(make_pass_harden_conditional_branches): Declare.
for gcc/ada/ChangeLog
* doc/gnat_rm/security_hardening_features.rst
(Hardened Conditionals): New.
for gcc/testsuite/ChangeLog
* c-c++-common/torture/harden-comp.c: New.
* c-c++-common/torture/harden-cond.c: New.
---
gcc/Makefile.in | 1
.../doc/gnat_rm/security_hardening_features.rst | 40 ++
gcc/common.opt | 8
gcc/doc/invoke.texi | 19 +
gcc/gimple-harden-conditionals.cc | 379 ++++++++++++++++++++
gcc/passes.def | 2
gcc/testsuite/c-c++-common/torture/harden-comp.c | 14 +
gcc/testsuite/c-c++-common/torture/harden-cond.c | 18 +
gcc/tree-pass.h | 3
9 files changed, 484 insertions(+)
create mode 100644 gcc/gimple-harden-conditionals.cc
create mode 100644 gcc/testsuite/c-c++-common/torture/harden-comp.c
create mode 100644 gcc/testsuite/c-c++-common/torture/harden-cond.c
diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 64252697573a7..7209ed117d09d 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1389,6 +1389,7 @@ OBJS = \
gimple-if-to-switch.o \
gimple-iterator.o \
gimple-fold.o \
+ gimple-harden-conditionals.o \
gimple-laddress.o \
gimple-loop-interchange.o \
gimple-loop-jam.o \
diff --git a/gcc/ada/doc/gnat_rm/security_hardening_features.rst b/gcc/ada/doc/gnat_rm/security_hardening_features.rst
index 1c46e3a4c7b88..52240d7e3dd54 100644
--- a/gcc/ada/doc/gnat_rm/security_hardening_features.rst
+++ b/gcc/ada/doc/gnat_rm/security_hardening_features.rst
@@ -87,3 +87,43 @@ types and subtypes, may be silently ignored. Specifically, it is not
currently recommended to rely on any effects this pragma might be
expected to have when calling subprograms through access-to-subprogram
variables.
+
+
+.. Hardened Conditionals:
+
+Hardened Conditionals
+=====================
+
+GNAT can harden conditionals to protect against control flow attacks.
+
+This is accomplished by two complementary transformations, each
+activated by a separate command-line option.
+
+The option *-fharden-compares* enables hardening of compares that
+compute results stored in variables, adding verification that the
+reversed compare yields the opposite result.
+
+The option *-fharden-conditional-branches* enables hardening of
+compares that guard conditional branches, adding verification of the
+reversed compare to both execution paths.
+
+These transformations are introduced late in the compilation pipeline,
+long after boolean expressions are decomposed into separate compares,
+each one turned into either a conditional branch or a compare whose
+result is stored in a boolean variable or temporary. Compiler
+optimizations, if enabled, may also turn conditional branches into
+stored compares, and vice-versa. Conditionals may also be optimized
+out entirely, if their value can be determined at compile time, and
+occasionally multiple compares can be combined into one.
+
+It is thus difficult to predict which of these two options will affect
+a specific compare operation expressed in source code. Using both
+options ensures that every compare that is not optimized out will be
+hardened.
+
+The addition of reversed compares can be observed by enabling the dump
+files of the corresponding passes, through command line options
+*-fdump-tree-hardcmp* and *-fdump-tree-hardcbr*, respectively.
+
+They are separate options, however, because of the significantly
+different performance impact of the hardening transformations.
diff --git a/gcc/common.opt b/gcc/common.opt
index e867055fc000d..89f2e6da6e56e 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1719,6 +1719,14 @@ fguess-branch-probability
Common Var(flag_guess_branch_prob) Optimization
Enable guessing of branch probabilities.
+fharden-compares
+Common Var(flag_harden_compares) Optimization
+Harden conditionals not used in branches, checking reversed conditions.
+
+fharden-conditional-branches
+Common Var(flag_harden_conditional_branches) Optimization
+Harden conditional branches by checking reversed conditions.
+
; Nonzero means ignore `#ident' directives. 0 means handle them.
; Generate position-independent code for executables if possible
; On SVR4 targets, it also controls whether or not to emit a
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 281dd10fff96c..5ed6a55f729e1 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -595,6 +595,7 @@ Objective-C and Objective-C++ Dialects}.
-fasan-shadow-offset=@var{number} -fsanitize-sections=@var{s1},@var{s2},... @gol
-fsanitize-undefined-trap-on-error -fbounds-check @gol
-fcf-protection=@r{[}full@r{|}branch@r{|}return@r{|}none@r{|}check@r{]} @gol
+-fharden-compares -fharden-conditional-branches @gol
-fstack-protector -fstack-protector-all -fstack-protector-strong @gol
-fstack-protector-explicit -fstack-check @gol
-fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
@@ -15491,6 +15492,24 @@ which functions and calls should be skipped from instrumentation
Currently the x86 GNU/Linux target provides an implementation based
on Intel Control-flow Enforcement Technology (CET).
+@item -fharden-compares
+@opindex fharden-compares
+For every logical test that survives gimple optimizations and is
+@emph{not} the condition in a conditional branch (for example,
+conditions tested for conditional moves, or to store in boolean
+variables), emit extra code to compute and verify the reversed
+condition, and to call @code{__builtin_trap} if the results do not
+match. Use with @samp{-fharden-conditional-branches} to cover all
+conditionals.
+
+@item -fharden-conditional-branches
+@opindex fharden-conditional-branches
+For every non-vectorized conditional branch that survives gimple
+optimizations, emit extra code to compute and verify the reversed
+condition, and to call @code{__builtin_trap} if the result is
+unexpected. Use with @samp{-fharden-compares} to cover all
+conditionals.
+
@item -fstack-protector
@opindex fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing
diff --git a/gcc/gimple-harden-conditionals.cc b/gcc/gimple-harden-conditionals.cc
new file mode 100644
index 0000000000000..08187b21ebddf
--- /dev/null
+++ b/gcc/gimple-harden-conditionals.cc
@@ -0,0 +1,379 @@
+/* Harden conditionals.
+ Copyright (C) 2021 Free Software Foundation, Inc.
+ Contributed by Alexandre Oliva <oliva@adacore.com>.
+
+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
+<http://www.gnu.org/licenses/>. */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "tree.h"
+#include "fold-const.h"
+#include "gimple.h"
+#include "gimplify.h"
+#include "tree-pass.h"
+#include "ssa.h"
+#include "gimple-iterator.h"
+#include "tree-cfg.h"
+#include "basic-block.h"
+#include "cfghooks.h"
+#include "cfgloop.h"
+#include "diagnostic.h"
+#include "intl.h"
+
+namespace {
+
+/* This pass introduces redundant, but reversed conditionals at every
+ compare, be it used for conditional branches, other conditional
+ operations, or for boolean computation. This doesn't make sense
+ for abstract CPUs, but this kind of hardening may avoid undesirable
+ execution paths on CPUs under such attacks as of power
+ deprivation. */
+
+/* Define a pass to harden conditionals other than branches. */
+const pass_data pass_data_harden_compares = {
+ GIMPLE_PASS,
+ "hardcmp",
+ OPTGROUP_NONE,
+ TV_NONE,
+ PROP_cfg | PROP_ssa, // properties_required
+ 0, // properties_provided
+ 0, // properties_destroyed
+ 0, // properties_start
+ TODO_update_ssa
+ | TODO_cleanup_cfg
+ | TODO_verify_il, // properties_finish
+};
+
+class pass_harden_compares : public gimple_opt_pass
+{
+public:
+ pass_harden_compares (gcc::context *ctxt)
+ : gimple_opt_pass (pass_data_harden_compares, ctxt)
+ {}
+ opt_pass *clone () { return new pass_harden_compares (m_ctxt); }
+ virtual bool gate (function *) {
+ return flag_harden_compares;
+ }
+ virtual unsigned int execute (function *);
+};
+
+/* This pass introduces redundant, but reversed conditionals at every
+ conditional branch. This doesn't make sense for abstract CPUs, but
+ this kind of hardening may avoid undesirable execution paths on
+ CPUs under such attacks as of power deprivation. This pass must
+ run after the above, otherwise it will re-harden the checks
+ introduced by the above. */
+
+/* Define a pass to harden conditionals in branches. */
+const pass_data pass_data_harden_conditional_branches = {
+ GIMPLE_PASS,
+ "hardcbr",
+ OPTGROUP_NONE,
+ TV_NONE,
+ PROP_cfg | PROP_ssa, // properties_required
+ 0, // properties_provided
+ 0, // properties_destroyed
+ 0, // properties_start
+ TODO_update_ssa
+ | TODO_cleanup_cfg
+ | TODO_verify_il, // properties_finish
+};
+
+class pass_harden_conditional_branches : public gimple_opt_pass
+{
+public:
+ pass_harden_conditional_branches (gcc::context *ctxt)
+ : gimple_opt_pass (pass_data_harden_conditional_branches, ctxt)
+ {}
+ opt_pass *clone () { return new pass_harden_conditional_branches (m_ctxt); }
+ virtual bool gate (function *) {
+ return flag_harden_conditional_branches;
+ }
+ virtual unsigned int execute (function *);
+};
+
+}
+
+static inline tree
+detach_value (gimple_stmt_iterator *gsip, tree val)
+{
+ if (TREE_CONSTANT (val) || TREE_CODE (val) != SSA_NAME)
+ {
+ gcc_checking_assert (is_gimple_min_invariant (val));
+ return val;
+ }
+
+ tree ret = copy_ssa_name (val);
+
+ vec<tree, va_gc> *inputs = NULL;
+ vec<tree, va_gc> *outputs = NULL;
+ vec_safe_push (outputs,
+ build_tree_list
+ (build_tree_list
+ (NULL_TREE, build_string (2, "=g")),
+ ret));
+ vec_safe_push (inputs,
+ build_tree_list
+ (build_tree_list
+ (NULL_TREE, build_string (1, "0")),
+ val));
+ gasm *detach = gimple_build_asm_vec ("", inputs, outputs,
+ NULL, NULL);
+ gsi_insert_before (gsip, detach, GSI_SAME_STMT);
+
+ SSA_NAME_DEF_STMT (ret) = detach;
+
+ return ret;
+}
+
+static inline void
+insert_check_and_trap (gimple_stmt_iterator *gsip, int flags,
+ enum tree_code cop, tree lhs, tree rhs)
+{
+ basic_block chk = gsi_bb (*gsip);
+
+ gcond *cond = gimple_build_cond (cop, lhs, rhs, NULL, NULL);
+ gsi_insert_before (gsip, cond, GSI_SAME_STMT);
+
+ basic_block trp = create_empty_bb (chk);
+
+ gimple_stmt_iterator gsit = gsi_after_labels (trp);
+ gcall *trap = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0);
+ gsi_insert_before (&gsit, trap, GSI_SAME_STMT);
+
+ if (dump_file)
+ fprintf (dump_file,
+ "Adding reversed compare to block %i, and trap to block %i\n",
+ chk->index, trp->index);
+
+ if (BB_PARTITION (chk))
+ BB_SET_PARTITION (trp, BB_COLD_PARTITION);
+
+ int true_false_flag = flags & (EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+ gcc_assert (true_false_flag);
+ int neg_true_false_flag = (~flags) & (EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+
+ single_succ_edge (chk)->flags &= ~EDGE_FALLTHRU;
+ single_succ_edge (chk)->flags |= neg_true_false_flag;
+ make_edge (chk, trp, true_false_flag);
+
+ if (dom_info_available_p (CDI_DOMINATORS))
+ set_immediate_dominator (CDI_DOMINATORS, trp, chk);
+ if (current_loops)
+ add_bb_to_loop (trp, current_loops->tree_root);
+}
+
+static inline void
+insert_edge_check_and_trap (edge e, enum tree_code cop, tree lhs, tree rhs)
+{
+ int flags = e->flags;
+ basic_block src = e->src;
+ basic_block dest = e->dest;
+
+ basic_block chk = split_edge (e);
+ e = NULL;
+
+ if (dump_file)
+ fprintf (dump_file,
+ "Splitting edge %i->%i into block %i\n",
+ src->index, dest->index, chk->index);
+
+ gimple_stmt_iterator gsik = gsi_after_labels (chk);
+
+ bool same_p = (lhs == rhs);
+ lhs = detach_value (&gsik, lhs);
+ rhs = same_p ? lhs : detach_value (&gsik, rhs);
+
+ insert_check_and_trap (&gsik, flags, cop, lhs, rhs);
+}
+
+unsigned int
+pass_harden_conditional_branches::execute (function *fun)
+{
+ basic_block bb;
+ FOR_EACH_BB_REVERSE_FN (bb, fun)
+ {
+ gimple_stmt_iterator gsi = gsi_last_bb (bb);
+
+ if (gsi_end_p (gsi))
+ continue;
+
+ gcond *cond = dyn_cast <gcond *> (gsi_stmt (gsi));
+ if (!cond)
+ continue;
+
+ /* Turn:
+
+ if (x op y) goto l1; else goto l2;
+
+ into:
+
+ if (x op y) goto l1'; else goto l2';
+ l1': if (x' cop y') goto l1'trap; else goto l1;
+ l1'trap: __builtin_trap ();
+ l2': if (x' cop y') goto l2; else goto l2'trap;
+ l2'trap: __builtin_trap ();
+
+ where cop is a complementary boolean operation to op; l1', l1'trap,
+ l2' and l2'trap are newly-created labels; and x' and y' hold the same
+ value as x and y, but in a way that does not enable the compiler to
+ optimize the redundant compare away.
+ */
+
+ enum tree_code op = gimple_cond_code (cond);
+ tree lhs = gimple_cond_lhs (cond);
+ tree rhs = gimple_cond_rhs (cond);
+
+ enum tree_code cop = invert_tree_comparison (op, HONOR_NANS (lhs));
+
+ if (cop == ERROR_MARK)
+ /* ??? Can we do better? */
+ continue;
+
+ insert_edge_check_and_trap (EDGE_SUCC (bb, 0), cop, lhs, rhs);
+ insert_edge_check_and_trap (EDGE_SUCC (bb, 1), cop, lhs, rhs);
+ }
+
+ return 0;
+}
+
+gimple_opt_pass *
+make_pass_harden_conditional_branches (gcc::context *ctxt)
+{
+ return new pass_harden_conditional_branches (ctxt);
+}
+
+unsigned int
+pass_harden_compares::execute (function *fun)
+{
+ basic_block bb;
+ FOR_EACH_BB_REVERSE_FN (bb, fun)
+ for (gimple_stmt_iterator gsi = gsi_last_bb (bb);
+ !gsi_end_p (gsi); gsi_prev (&gsi))
+ {
+ gassign *asgn = dyn_cast <gassign *> (gsi_stmt (gsi));
+ if (!asgn)
+ continue;
+
+ /* Turn:
+
+ z = x op y;
+
+ into:
+
+ z = x op y;
+ z' = x' cop y';
+ if (z == z') __builtin_trap ();
+
+ where cop is a complementary boolean operation to op; and x'
+ and y' hold the same value as x and y, but in a way that does
+ not enable the compiler to optimize the redundant compare
+ away.
+ */
+
+ enum tree_code op = gimple_assign_rhs_code (asgn);
+
+ enum tree_code cop;
+
+ switch (op)
+ {
+ case EQ_EXPR:
+ case NE_EXPR:
+ case GT_EXPR:
+ case GE_EXPR:
+ case LT_EXPR:
+ case LE_EXPR:
+ case LTGT_EXPR:
+ case UNEQ_EXPR:
+ case UNGT_EXPR:
+ case UNGE_EXPR:
+ case UNLT_EXPR:
+ case UNLE_EXPR:
+ case ORDERED_EXPR:
+ case UNORDERED_EXPR:
+ cop = invert_tree_comparison (op,
+ HONOR_NANS
+ (gimple_assign_rhs1 (asgn)));
+
+ if (cop == ERROR_MARK)
+ /* ??? Can we do better? */
+ continue;
+
+ break;
+
+ /* ??? Maybe handle these too? */
+ case TRUTH_NOT_EXPR: /* Unary! */
+ case TRUTH_ANDIF_EXPR:
+ case TRUTH_ORIF_EXPR:
+ case TRUTH_AND_EXPR:
+ case TRUTH_OR_EXPR:
+ case TRUTH_XOR_EXPR:
+ default:
+ continue;
+ }
+
+ /* These are the operands for the verification. */
+ tree lhs = gimple_assign_lhs (asgn);
+ tree op1 = gimple_assign_rhs1 (asgn);
+ tree op2 = gimple_assign_rhs2 (asgn);
+
+ /* Vector booleans can't be used in conditional branches.
+ ??? Can we do better? */
+ if (VECTOR_TYPE_P (TREE_TYPE (op1)))
+ continue;
+
+ gcc_checking_assert (TREE_CODE (TREE_TYPE (lhs)) == BOOLEAN_TYPE);
+
+ tree rhs = copy_ssa_name (lhs);
+
+ gimple_stmt_iterator gsi_split = gsi;
+ gsi_next (&gsi_split);
+
+ bool same_p = (op1 == op2);
+ op1 = detach_value (&gsi_split, op1);
+ op2 = same_p ? op1 : detach_value (&gsi_split, op2);
+
+ gassign *asgnck = gimple_build_assign (rhs, cop, op1, op2);
+ gsi_insert_before (&gsi_split, asgnck, GSI_SAME_STMT);
+
+ if (!gsi_end_p (gsi_split))
+ {
+ gsi_prev (&gsi_split);
+ split_block (bb, gsi_stmt (gsi_split));
+ gsi_next (&gsi_split);
+ gcc_checking_assert (gsi_end_p (gsi_split));
+
+ if (dump_file)
+ fprintf (dump_file, "Splitting block %i\n", bb->index);
+ }
+
+ gcc_checking_assert (single_succ_p (bb));
+
+ insert_check_and_trap (&gsi_split, EDGE_TRUE_VALUE,
+ EQ_EXPR, lhs, rhs);
+ }
+
+ return 0;
+}
+
+gimple_opt_pass *
+make_pass_harden_compares (gcc::context *ctxt)
+{
+ return new pass_harden_compares (ctxt);
+}
diff --git a/gcc/passes.def b/gcc/passes.def
index 8e4638d20edac..2d81086df1d39 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -421,6 +421,8 @@ along with GCC; see the file COPYING3. If not see
NEXT_PASS (pass_lower_resx);
NEXT_PASS (pass_nrv);
NEXT_PASS (pass_gimple_isel);
+ NEXT_PASS (pass_harden_conditional_branches);
+ NEXT_PASS (pass_harden_compares);
NEXT_PASS (pass_cleanup_cfg_post_optimizing);
NEXT_PASS (pass_warn_function_noreturn);
NEXT_PASS (pass_warn_access);
diff --git a/gcc/testsuite/c-c++-common/torture/harden-comp.c b/gcc/testsuite/c-c++-common/torture/harden-comp.c
new file mode 100644
index 0000000000000..1ee0b3663443d
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/torture/harden-comp.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-fharden-compares -fdump-tree-hardcmp -ffat-lto-objects" } */
+
+int
+f (int i, int j)
+{
+ return i < j;
+}
+
+/* { dg-final { scan-tree-dump-times "Splitting block" 1 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "Adding reversed compare" 1 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 1 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "_\[0-9\]* = i_\[0-9\]*\[(\]D\[)\] < j_\[0-9\]*\[(\]D\[)\];" 1 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "_\[0-9\]* = i_\[0-9\]* >= j_\[0-9\]*;" 1 "hardcmp" } } */
diff --git a/gcc/testsuite/c-c++-common/torture/harden-cond.c b/gcc/testsuite/c-c++-common/torture/harden-cond.c
new file mode 100644
index 0000000000000..86de8e155ed38
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/torture/harden-cond.c
@@ -0,0 +1,18 @@
+/* { dg-do compile } */
+/* { dg-options "-fharden-conditional-branches -fdump-tree-hardcbr -ffat-lto-objects" } */
+
+extern int f1 (void);
+extern int f2 (void);
+
+
+int
+f (int i, int j)
+{
+ return (i < j) ? f1 () : f2 ();
+}
+
+/* { dg-final { scan-tree-dump-times "Splitting edge" 2 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "Adding reversed compare" 2 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 2 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "if \[(\]i_\[0-9\]*\[(\]D\[)\] < j_\[0-9\]*\[(\]D\[)\]\[)\]" 1 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "if \[(\]i_\[0-9\]* >= j_\[0-9\]*\[)\]" 2 "hardcbr" } } */
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index 9d19228c385f7..7eae2f26f5195 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -643,6 +643,9 @@ extern gimple_opt_pass *make_pass_update_address_taken (gcc::context *ctxt);
extern gimple_opt_pass *make_pass_convert_switch (gcc::context *ctxt);
extern gimple_opt_pass *make_pass_lower_vaarg (gcc::context *ctxt);
extern gimple_opt_pass *make_pass_gimple_isel (gcc::context *ctxt);
+extern gimple_opt_pass *make_pass_harden_compares (gcc::context *ctxt);
+extern gimple_opt_pass *make_pass_harden_conditional_branches (gcc::context
+ *ctxt);
/* Current optimization pass. */
extern opt_pass *current_pass;
--
Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
Free Software Activist GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts. Ask me about <https://stallmansupport.org>
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