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compiler hardening updates for v5.16-rc1 

This collects various compiler hardening feature related updates:
 
 - gcc-plugins:
   - remove support for GCC 4.9 and older (Ard Biesheuvel)
   - remove duplicate include in gcc-common.h (Ye Guojin)
   - Explicitly document purpose and deprecation schedule (Kees Cook)
   - Remove cyc_complexity (Kees Cook)
 
 - instrumentation:
   - Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO (Kees Cook)
 
 - Clang LTO:
   - kallsyms: strip LTO suffixes from static functions (Nick Desaulniers)
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Merge tag 'hardening-v5.16-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull compiler hardening updates from Kees Cook:
 "These are various compiler-related hardening feature updates. Notable
  is the addition of an explicit limited rationale for, and deprecation
  schedule of, gcc-plugins.

  gcc-plugins:
   - remove support for GCC 4.9 and older (Ard Biesheuvel)
   - remove duplicate include in gcc-common.h (Ye Guojin)
   - Explicitly document purpose and deprecation schedule (Kees Cook)
   - Remove cyc_complexity (Kees Cook)

  instrumentation:
   - Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO (Kees Cook)

  Clang LTO:
   - kallsyms: strip LTO suffixes from static functions (Nick Desaulniers)"

* tag 'hardening-v5.16-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  gcc-plugins: remove duplicate include in gcc-common.h
  gcc-plugins: Remove cyc_complexity
  gcc-plugins: Explicitly document purpose and deprecation schedule
  kallsyms: strip LTO suffixes from static functions
  gcc-plugins: remove support for GCC 4.9 and older
  hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO
This commit is contained in:
Linus Torvalds 2021-11-01 17:09:03 -07:00
parent 01463374c5 6425392acf
commit f594e28d80
13 changed files with 77 additions and 322 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
Documentation/kbuild/gcc-plugins.rst
View File

@ -32,6 +32,32 @@ This infrastructure was ported from grsecurity [6]_ and PaX [7]_.
.. [7] https://pax.grsecurity.net/
Purpose
=======
GCC plugins are designed to provide a place to experiment with potential
compiler features that are neither in GCC nor Clang upstream. Once
their utility is proven, the goal is to upstream the feature into GCC
(and Clang), and then to finally remove them from the kernel once the
feature is available in all supported versions of GCC.
Specifically, new plugins should implement only features that have no
upstream compiler support (in either GCC or Clang).
When a feature exists in Clang but not GCC, effort should be made to
bring the feature to upstream GCC (rather than just as a kernel-specific
GCC plugin), so the entire ecosystem can benefit from it.
Similarly, even if a feature provided by a GCC plugin does *not* exist
in Clang, but the feature is proven to be useful, effort should be spent
to upstream the feature to GCC (and Clang).
After a feature is available in upstream GCC, the plugin will be made
unbuildable for the corresponding GCC version (and later). Once all
kernel-supported versions of GCC provide the feature, the plugin will
be removed from the kernel.
Files
=====
@ -70,7 +96,6 @@ Enable the GCC plugin infrastructure and some plugin(s) you want to use
in the kernel config::
CONFIG_GCC_PLUGINS=y
CONFIG_GCC_PLUGIN_CYC_COMPLEXITY=y
CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
...
@ -89,4 +114,3 @@ The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files
right under scripts/gcc-plugins/. Creating subdirectories is not supported.
It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins
and a relevant Kconfig file.
See the cyc_complexity_plugin.c (CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) GCC plugin.

6
Makefile
View File

@ -831,12 +831,12 @@ endif
# Initialize all stack variables with a zero value.
ifdef CONFIG_INIT_STACK_ALL_ZERO
# Future support for zero initialization is still being debated, see
# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
# renamed or dropped.
KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
ifdef CONFIG_CC_IS_CLANG
# https://bugs.llvm.org/show_bug.cgi?id=45497
KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
endif
endif
# While VLAs have been removed, GCC produces unreachable stack probes
# for the randomize_kstack_offset feature. Disable it for all compilers.

50
kernel/kallsyms.c
View File

@ -164,26 +164,46 @@ static unsigned long kallsyms_sym_address(int idx)
return kallsyms_relative_base - 1 - kallsyms_offsets[idx];
}
#if defined(CONFIG_CFI_CLANG) && defined(CONFIG_LTO_CLANG_THIN)
/*
* LLVM appends a hash to static function names when ThinLTO and CFI are
* both enabled, i.e. foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
* This causes confusion and potentially breaks user space tools, so we
* strip the suffix from expanded symbol names.
*/
static inline bool cleanup_symbol_name(char *s)
static bool cleanup_symbol_name(char *s)
{
char *res;
res = strrchr(s, '$');
if (res)
*res = '\0';
if (!IS_ENABLED(CONFIG_LTO_CLANG))
return false;
return res != NULL;
/*
* LLVM appends various suffixes for local functions and variables that
* must be promoted to global scope as part of LTO. This can break
* hooking of static functions with kprobes. '.' is not a valid
* character in an identifier in C. Suffixes observed:
* - foo.llvm.[0-9a-f]+
* - foo.[0-9a-f]+
* - foo.[0-9a-f]+.cfi_jt
*/
res = strchr(s, '.');
if (res) {
*res = '\0';
return true;
}
if (!IS_ENABLED(CONFIG_CFI_CLANG) ||
!IS_ENABLED(CONFIG_LTO_CLANG_THIN) ||
CONFIG_CLANG_VERSION >= 130000)
return false;
/*
* Prior to LLVM 13, the following suffixes were observed when thinLTO
* and CFI are both enabled:
* - foo$[0-9]+
*/
res = strrchr(s, '$');
if (res) {
*res = '\0';
return true;
}
return false;
}
#else
static inline bool cleanup_symbol_name(char *s) { return false; }
#endif
/* Lookup the address for this symbol. Returns 0 if not found. */
unsigned long kallsyms_lookup_name(const char *name)

2
scripts/Makefile.gcc-plugins
View File

@ -1,7 +1,5 @@
# SPDX-License-Identifier: GPL-2.0
gcc-plugin-$(CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) += cyc_complexity_plugin.so
gcc-plugin-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY) += latent_entropy_plugin.so
gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY) \
+= -DLATENT_ENTROPY_PLUGIN

20
scripts/gcc-plugins/Kconfig
View File

@ -19,24 +19,10 @@ menuconfig GCC_PLUGINS
if GCC_PLUGINS
config GCC_PLUGIN_CYC_COMPLEXITY
bool "Compute the cyclomatic complexity of a function" if EXPERT
depends on !COMPILE_TEST # too noisy
help
The complexity M of a function's control flow graph is defined as:
M = E - N + 2P
where
E = the number of edges
N = the number of nodes
P = the number of connected components (exit nodes).
Enabling this plugin reports the complexity to stderr during the
build. It mainly serves as a simple example of how to create a
gcc plugin for the kernel.
config GCC_PLUGIN_SANCOV
bool
# Plugin can be removed once the kernel only supports GCC 6+
depends on !CC_HAS_SANCOV_TRACE_PC
help
This plugin inserts a __sanitizer_cov_trace_pc() call at the start of
basic blocks. It supports all gcc versions with plugin support (from
@ -83,8 +69,6 @@ config GCC_PLUGIN_RANDSTRUCT
the existing seed and will be removed by a make mrproper or
make distclean.
Note that the implementation requires gcc 4.7 or newer.
This plugin was ported from grsecurity/PaX. More information at:
* https://grsecurity.net/
* https://pax.grsecurity.net/

69
scripts/gcc-plugins/cyc_complexity_plugin.c
View File

@ -1,69 +0,0 @@
/*
* Copyright 2011-2016 by Emese Revfy <re.emese@gmail.com>
* Licensed under the GPL v2, or (at your option) v3
*
* Homepage:
* https://github.com/ephox-gcc-plugins/cyclomatic_complexity
*
* https://en.wikipedia.org/wiki/Cyclomatic_complexity
* The complexity M is then defined as:
* M = E - N + 2P
* where
*
* E = the number of edges of the graph
* N = the number of nodes of the graph
* P = the number of connected components (exit nodes).
*
* Usage (4.5 - 5):
* $ make clean; make run
*/
#include "gcc-common.h"
__visible int plugin_is_GPL_compatible;
static struct plugin_info cyc_complexity_plugin_info = {
.version = "20160225",
.help = "Cyclomatic Complexity\n",
};
static unsigned int cyc_complexity_execute(void)
{
int complexity;
expanded_location xloc;
/* M = E - N + 2P */
complexity = n_edges_for_fn(cfun) - n_basic_blocks_for_fn(cfun) + 2;
xloc = expand_location(DECL_SOURCE_LOCATION(current_function_decl));
fprintf(stderr, "Cyclomatic Complexity %d %s:%s\n", complexity,
xloc.file, DECL_NAME_POINTER(current_function_decl));
return 0;
}
#define PASS_NAME cyc_complexity
#define NO_GATE
#define TODO_FLAGS_FINISH TODO_dump_func
#include "gcc-generate-gimple-pass.h"
__visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
{
const char * const plugin_name = plugin_info->base_name;
PASS_INFO(cyc_complexity, "ssa", 1, PASS_POS_INSERT_AFTER);
if (!plugin_default_version_check(version, &gcc_version)) {
error(G_("incompatible gcc/plugin versions"));
return 1;
}
register_callback(plugin_name, PLUGIN_INFO, NULL,
&cyc_complexity_plugin_info);
register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL,
&cyc_complexity_pass_info);
return 0;
}

132
scripts/gcc-plugins/gcc-common.h
View File

@ -27,9 +27,7 @@
#include "except.h"
#include "function.h"
#include "toplev.h"
#if BUILDING_GCC_VERSION >= 5000
#include "expr.h"
#endif
#include "basic-block.h"
#include "intl.h"
#include "ggc.h"
@ -39,11 +37,7 @@
#include "params.h"
#endif
#if BUILDING_GCC_VERSION <= 4009
#include "pointer-set.h"
#else
#include "hash-map.h"
#endif
#if BUILDING_GCC_VERSION >= 7000
#include "memmodel.h"
@ -92,16 +86,13 @@
#include "stmt.h"
#include "gimplify.h"
#include "gimple.h"
#include "tree-ssa-operands.h"
#include "tree-phinodes.h"
#include "tree-cfg.h"
#include "gimple-iterator.h"
#include "gimple-ssa.h"
#include "ssa-iterators.h"
#if BUILDING_GCC_VERSION >= 5000
#include "builtins.h"
#endif
/* missing from basic_block.h... */
void debug_dominance_info(enum cdi_direction dir);
@ -152,125 +143,6 @@ struct register_pass_info NAME##_pass_info = { \
#define TODO_dump_func 0
#define TODO_dump_cgraph 0
#if BUILDING_GCC_VERSION <= 4009
#define TODO_verify_il 0
#define AVAIL_INTERPOSABLE AVAIL_OVERWRITABLE
#define section_name_prefix LTO_SECTION_NAME_PREFIX
#define fatal_error(loc, gmsgid, ...) fatal_error((gmsgid), __VA_ARGS__)
rtx emit_move_insn(rtx x, rtx y);
typedef struct rtx_def rtx_insn;
static inline const char *get_decl_section_name(const_tree decl)
{
if (DECL_SECTION_NAME(decl) == NULL_TREE)
return NULL;
return TREE_STRING_POINTER(DECL_SECTION_NAME(decl));
}
static inline void set_decl_section_name(tree node, const char *value)
{
if (value)
DECL_SECTION_NAME(node) = build_string(strlen(value) + 1, value);
else
DECL_SECTION_NAME(node) = NULL;
}
#endif
#if BUILDING_GCC_VERSION == 4009
typedef struct gimple_statement_asm gasm;
typedef struct gimple_statement_base gassign;
typedef struct gimple_statement_call gcall;
typedef struct gimple_statement_base gcond;
typedef struct gimple_statement_base gdebug;
typedef struct gimple_statement_base ggoto;
typedef struct gimple_statement_phi gphi;
typedef struct gimple_statement_base greturn;
static inline gasm *as_a_gasm(gimple stmt)
{
return as_a<gasm>(stmt);
}
static inline const gasm *as_a_const_gasm(const_gimple stmt)
{
return as_a<const gasm>(stmt);
}
static inline gassign *as_a_gassign(gimple stmt)
{
return stmt;
}
static inline const gassign *as_a_const_gassign(const_gimple stmt)
{
return stmt;
}
static inline gcall *as_a_gcall(gimple stmt)
{
return as_a<gcall>(stmt);
}
static inline const gcall *as_a_const_gcall(const_gimple stmt)
{
return as_a<const gcall>(stmt);
}
static inline gcond *as_a_gcond(gimple stmt)
{
return stmt;
}
static inline const gcond *as_a_const_gcond(const_gimple stmt)
{
return stmt;
}
static inline gdebug *as_a_gdebug(gimple stmt)
{
return stmt;
}
static inline const gdebug *as_a_const_gdebug(const_gimple stmt)
{
return stmt;
}
static inline ggoto *as_a_ggoto(gimple stmt)
{
return stmt;
}
static inline const ggoto *as_a_const_ggoto(const_gimple stmt)
{
return stmt;
}
static inline gphi *as_a_gphi(gimple stmt)
{
return as_a<gphi>(stmt);
}
static inline const gphi *as_a_const_gphi(const_gimple stmt)
{
return as_a<const gphi>(stmt);
}
static inline greturn *as_a_greturn(gimple stmt)
{
return stmt;
}
static inline const greturn *as_a_const_greturn(const_gimple stmt)
{
return stmt;
}
#endif
#define TODO_ggc_collect 0
#define NODE_SYMBOL(node) (node)
#define NODE_DECL(node) (node)->decl
@ -282,7 +154,7 @@ static inline opt_pass *get_pass_for_id(int id)
return g->get_passes()->get_pass_for_id(id);
}
#if BUILDING_GCC_VERSION >= 5000 && BUILDING_GCC_VERSION < 6000
#if BUILDING_GCC_VERSION < 6000
/* gimple related */
template <>
template <>
@ -292,7 +164,6 @@ inline bool is_a_helper<const gassign *>::test(const_gimple gs)
}
#endif
#if BUILDING_GCC_VERSION >= 5000
#define TODO_verify_ssa TODO_verify_il
#define TODO_verify_flow TODO_verify_il
#define TODO_verify_stmts TODO_verify_il
@ -533,7 +404,6 @@ static inline void ipa_remove_stmt_references(symtab_node *referring_node, gimpl
{
referring_node->remove_stmt_references(stmt);
}
#endif
#if BUILDING_GCC_VERSION < 6000
#define get_inner_reference(exp, pbitsize, pbitpos, poffset, pmode, punsignedp, preversep, pvolatilep, keep_aligning) \

19
scripts/gcc-plugins/gcc-generate-gimple-pass.h
View File

@ -78,17 +78,6 @@ static const pass_data _PASS_NAME_PASS_DATA = {
.type = GIMPLE_PASS,
.name = _PASS_NAME_NAME,
.optinfo_flags = OPTGROUP_NONE,
#if BUILDING_GCC_VERSION >= 5000
#elif BUILDING_GCC_VERSION == 4009
.has_gate = _HAS_GATE,
.has_execute = _HAS_EXECUTE,
#else
.gate = _GATE,
.execute = _EXECUTE,
.sub = NULL,
.next = NULL,
.static_pass_number = 0,
#endif
.tv_id = TV_NONE,
.properties_required = PROPERTIES_REQUIRED,
.properties_provided = PROPERTIES_PROVIDED,
@ -102,21 +91,13 @@ public:
_PASS_NAME_PASS() : gimple_opt_pass(_PASS_NAME_PASS_DATA, g) {}
#ifndef NO_GATE
#if BUILDING_GCC_VERSION >= 5000
virtual bool gate(function *) { return _GATE(); }
#else
virtual bool gate(void) { return _GATE(); }
#endif
#endif
virtual opt_pass * clone () { return new _PASS_NAME_PASS(); }
#ifndef NO_EXECUTE
#if BUILDING_GCC_VERSION >= 5000
virtual unsigned int execute(function *) { return _EXECUTE(); }
#else
virtual unsigned int execute(void) { return _EXECUTE(); }
#endif
};
}

19
scripts/gcc-plugins/gcc-generate-ipa-pass.h
View File

@ -146,17 +146,6 @@ static const pass_data _PASS_NAME_PASS_DATA = {
.type = IPA_PASS,
.name = _PASS_NAME_NAME,
.optinfo_flags = OPTGROUP_NONE,
#if BUILDING_GCC_VERSION >= 5000
#elif BUILDING_GCC_VERSION == 4009
.has_gate = _HAS_GATE,
.has_execute = _HAS_EXECUTE,
#else
.gate = _GATE,
.execute = _EXECUTE,
.sub = NULL,
.next = NULL,
.static_pass_number = 0,
#endif
.tv_id = TV_NONE,
.properties_required = PROPERTIES_REQUIRED,
.properties_provided = PROPERTIES_PROVIDED,
@ -180,20 +169,12 @@ public:
_VARIABLE_TRANSFORM) {}
#ifndef NO_GATE
#if BUILDING_GCC_VERSION >= 5000
virtual bool gate(function *) { return _GATE(); }
#else
virtual bool gate(void) { return _GATE(); }
#endif
virtual opt_pass *clone() { return new _PASS_NAME_PASS(); }
#ifndef NO_EXECUTE
#if BUILDING_GCC_VERSION >= 5000
virtual unsigned int execute(function *) { return _EXECUTE(); }
#else
virtual unsigned int execute(void) { return _EXECUTE(); }
#endif
#endif
};
}

19
scripts/gcc-plugins/gcc-generate-rtl-pass.h
View File

@ -78,17 +78,6 @@ static const pass_data _PASS_NAME_PASS_DATA = {
.type = RTL_PASS,
.name = _PASS_NAME_NAME,
.optinfo_flags = OPTGROUP_NONE,
#if BUILDING_GCC_VERSION >= 5000
#elif BUILDING_GCC_VERSION == 4009
.has_gate = _HAS_GATE,
.has_execute = _HAS_EXECUTE,
#else
.gate = _GATE,
.execute = _EXECUTE,
.sub = NULL,
.next = NULL,
.static_pass_number = 0,
#endif
.tv_id = TV_NONE,
.properties_required = PROPERTIES_REQUIRED,
.properties_provided = PROPERTIES_PROVIDED,
@ -102,21 +91,13 @@ public:
_PASS_NAME_PASS() : rtl_opt_pass(_PASS_NAME_PASS_DATA, g) {}
#ifndef NO_GATE
#if BUILDING_GCC_VERSION >= 5000
virtual bool gate(function *) { return _GATE(); }
#else
virtual bool gate(void) { return _GATE(); }
#endif
#endif
virtual opt_pass *clone() { return new _PASS_NAME_PASS(); }
#ifndef NO_EXECUTE
#if BUILDING_GCC_VERSION >= 5000
virtual unsigned int execute(function *) { return _EXECUTE(); }
#else
virtual unsigned int execute(void) { return _EXECUTE(); }
#endif
#endif
};
}

19
scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h
View File

@ -78,17 +78,6 @@ static const pass_data _PASS_NAME_PASS_DATA = {
.type = SIMPLE_IPA_PASS,
.name = _PASS_NAME_NAME,
.optinfo_flags = OPTGROUP_NONE,
#if BUILDING_GCC_VERSION >= 5000
#elif BUILDING_GCC_VERSION == 4009
.has_gate = _HAS_GATE,
.has_execute = _HAS_EXECUTE,
#else
.gate = _GATE,
.execute = _EXECUTE,
.sub = NULL,
.next = NULL,
.static_pass_number = 0,
#endif
.tv_id = TV_NONE,
.properties_required = PROPERTIES_REQUIRED,
.properties_provided = PROPERTIES_PROVIDED,
@ -102,21 +91,13 @@ public:
_PASS_NAME_PASS() : simple_ipa_opt_pass(_PASS_NAME_PASS_DATA, g) {}
#ifndef NO_GATE
#if BUILDING_GCC_VERSION >= 5000
virtual bool gate(function *) { return _GATE(); }
#else
virtual bool gate(void) { return _GATE(); }
#endif
#endif
virtual opt_pass *clone() { return new _PASS_NAME_PASS(); }
#ifndef NO_EXECUTE
#if BUILDING_GCC_VERSION >= 5000
virtual unsigned int execute(function *) { return _EXECUTE(); }
#else
virtual unsigned int execute(void) { return _EXECUTE(); }
#endif
#endif
};
}

2
scripts/gcc-plugins/structleak_plugin.c
View File

@ -103,10 +103,8 @@ static void finish_type(void *event_data, void *data)
if (type == NULL_TREE || type == error_mark_node)
return;
#if BUILDING_GCC_VERSION >= 5000
if (TREE_CODE(type) == ENUMERAL_TYPE)
return;
#endif
if (TYPE_USERSPACE(type))
return;

14
security/Kconfig.hardening
View File

@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
config CC_HAS_AUTO_VAR_INIT_ZERO
# GCC ignores the -enable flag, so we can test for the feature with
# a single invocation using the flag, but drop it as appropriate in
# the Makefile, depending on the presence of Clang.
def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
@ -53,7 +56,8 @@ choice
config GCC_PLUGIN_STRUCTLEAK_USER
bool "zero-init structs marked for userspace (weak)"
depends on GCC_PLUGINS
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
select GCC_PLUGIN_STRUCTLEAK
help
Zero-initialize any structures on the stack containing
@ -64,7 +68,8 @@ choice
config GCC_PLUGIN_STRUCTLEAK_BYREF
bool "zero-init structs passed by reference (strong)"
depends on GCC_PLUGINS
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
depends on !(KASAN && KASAN_STACK)
select GCC_PLUGIN_STRUCTLEAK
help
@ -82,7 +87,8 @@ choice
config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
bool "zero-init everything passed by reference (very strong)"
depends on GCC_PLUGINS
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
depends on !(KASAN && KASAN_STACK)
select GCC_PLUGIN_STRUCTLEAK
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