[PATCH 3/7] kcov: x86: introduce CONFIG_KCOV_ENABLE_GUARDS

[Alexander Potapenko <glider@google.com>]

The new config switches coverage instrumentation to using
  __sanitizer_cov_trace_pc_guard(u32 *guard)
instead of
  __sanitizer_cov_trace_pc(void)

Each callback receives a unique 32-bit guard variable residing in the
__sancov_guards section. Those guards can be used by kcov to deduplicate
the coverage on the fly.

As a first step, we make the new instrumentation mode 1:1 compatible with
the old one.

Cc: x86@kernel.org
Signed-off-by: Alexander Potapenko <glider@google.com>
---
 arch/x86/kernel/vmlinux.lds.S     |  1 +
 include/asm-generic/vmlinux.lds.h | 14 ++++++-
 include/linux/kcov.h              |  2 +
 kernel/kcov.c                     | 61 +++++++++++++++++++++----------
 lib/Kconfig.debug                 | 16 ++++++++
 scripts/Makefile.kcov             |  4 ++
 scripts/module.lds.S              | 23 ++++++++++++
 tools/objtool/check.c             |  1 +
 8 files changed, 101 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 0deb4887d6e96..2acfbbde33820 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -390,6 +390,7 @@ SECTIONS
 		. = ALIGN(PAGE_SIZE);
 		__bss_stop = .;
 	}
+	SANCOV_GUARDS_BSS
 
 	/*
 	 * The memory occupied from _text to here, __end_of_kernel_reserve, is
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 0d5b186abee86..3ff150f152737 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -102,7 +102,8 @@
  * sections to be brought in with rodata.
  */
 #if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
-defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
+	defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG) || \
+	defined(CONFIG_KCOV_ENABLE_GUARDS)
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
 #else
 #define TEXT_MAIN .text
@@ -121,6 +122,17 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 #define SBSS_MAIN .sbss
 #endif
 
+#if defined(CONFIG_KCOV_ENABLE_GUARDS)
+#define SANCOV_GUARDS_BSS			\
+	__sancov_guards(NOLOAD) : {		\
+		__start___sancov_guards = .;	\
+		*(__sancov_guards);		\
+		__stop___sancov_guards = .;	\
+	}
+#else
+#define SANCOV_GUARDS_BSS
+#endif
+
 /*
  * GCC 4.5 and later have a 32 bytes section alignment for structures.
  * Except GCC 4.9, that feels the need to align on 64 bytes.
diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index e1f7d793c1cb3..7ec2669362fd1 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -107,6 +107,8 @@ typedef unsigned long long kcov_u64;
 #endif
 
 void __sanitizer_cov_trace_pc(void);
+void __sanitizer_cov_trace_pc_guard(u32 *guard);
+void __sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);
 void __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2);
 void __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2);
 void __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2);
diff --git a/kernel/kcov.c b/kernel/kcov.c
index 8fcbca236bec5..b97f429d17436 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -193,27 +193,15 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
 	return ip;
 }
 
-/*
- * Entry point from instrumented code.
- * This is called once per basic-block/edge.
- */
-void notrace __sanitizer_cov_trace_pc(void)
+static void sanitizer_cov_write_subsequent(unsigned long *area, int size,
+					   unsigned long ip)
 {
-	struct task_struct *t;
-	unsigned long *area;
-	unsigned long ip = canonicalize_ip(_RET_IP_);
-	unsigned long pos;
-
-	t = current;
-	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
-		return;
-
-	area = t->kcov_state.s.area;
 	/* The first 64-bit word is the number of subsequent PCs. */
-	pos = READ_ONCE(area[0]) + 1;
-	if (likely(pos < t->kcov_state.s.size)) {
-		/* Previously we write pc before updating pos. However, some
-		 * early interrupt code could bypass check_kcov_mode() check
+	unsigned long pos = READ_ONCE(area[0]) + 1;
+
+	if (likely(pos < size)) {
+		/*
+		 * Some early interrupt code could bypass check_kcov_mode() check
 		 * and invoke __sanitizer_cov_trace_pc(). If such interrupt is
 		 * raised between writing pc and updating pos, the pc could be
 		 * overitten by the recursive __sanitizer_cov_trace_pc().
@@ -224,7 +212,40 @@ void notrace __sanitizer_cov_trace_pc(void)
 		area[pos] = ip;
 	}
 }
+
+/*
+ * Entry point from instrumented code.
+ * This is called once per basic-block/edge.
+ */
+#ifndef CONFIG_KCOV_ENABLE_GUARDS
+void notrace __sanitizer_cov_trace_pc(void)
+{
+	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
+		return;
+
+	sanitizer_cov_write_subsequent(current->kcov_state.s.area,
+				       current->kcov_state.s.size,
+				       canonicalize_ip(_RET_IP_));
+}
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
+#else
+void notrace __sanitizer_cov_trace_pc_guard(u32 *guard)
+{
+	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
+		return;
+
+	sanitizer_cov_write_subsequent(current->kcov_state.s.area,
+				       current->kcov_state.s.size,
+				       canonicalize_ip(_RET_IP_));
+}
+EXPORT_SYMBOL(__sanitizer_cov_trace_pc_guard);
+
+void notrace __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
+						 uint32_t *stop)
+{
+}
+EXPORT_SYMBOL(__sanitizer_cov_trace_pc_guard_init);
+#endif
 
 #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
 static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
@@ -252,7 +273,7 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 	start_index = 1 + count * KCOV_WORDS_PER_CMP;
 	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
 	if (likely(end_pos <= max_pos)) {
-		/* See comment in __sanitizer_cov_trace_pc(). */
+		/* See comment in sanitizer_cov_write_subsequent(). */
 		WRITE_ONCE(area[0], count + 1);
 		barrier();
 		area[start_index] = type;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 35796c290ca35..a81d086b8e1ff 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2135,6 +2135,8 @@ config ARCH_HAS_KCOV
 config CC_HAS_SANCOV_TRACE_PC
 	def_bool $(cc-option,-fsanitize-coverage=trace-pc)
 
+config CC_HAS_SANCOV_TRACE_PC_GUARD
+	def_bool $(cc-option,-fsanitize-coverage=trace-pc-guard)
 
 config KCOV
 	bool "Code coverage for fuzzing"
@@ -2151,6 +2153,20 @@ config KCOV
 
 	  For more details, see Documentation/dev-tools/kcov.rst.
 
+config KCOV_ENABLE_GUARDS
+	depends on KCOV
+	depends on CC_HAS_SANCOV_TRACE_PC_GUARD
+	bool "Use fsanitize-coverage=trace-pc-guard for kcov"
+	help
+	  Use coverage guards instrumentation for kcov, passing
+	  -fsanitize-coverage=trace-pc-guard to the compiler.
+
+	  Every coverage callback is associated with a global variable that
+	  allows to efficiently deduplicate coverage at collection time.
+
+	  This comes at a cost of increased binary size (4 bytes of .bss
+	  per basic block, plus 1-2 instructions to pass an extra parameter).
+
 config KCOV_ENABLE_COMPARISONS
 	bool "Enable comparison operands collection by KCOV"
 	depends on KCOV
diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 67e8cfe3474b7..ec63d471d5773 100644
--- a/scripts/Makefile.kcov
+++ b/scripts/Makefile.kcov
@@ -1,5 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
+ifeq ($(CONFIG_KCOV_ENABLE_GUARDS),y)
+kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC_GUARD) += -fsanitize-coverage=trace-pc-guard
+else
 kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC)	+= -fsanitize-coverage=trace-pc
+endif
 kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)	+= -fsanitize-coverage=trace-cmp
 kcov-flags-$(CONFIG_GCC_PLUGIN_SANCOV)		+= -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
 
diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 450f1088d5fd3..ec7e9247f8de6 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -64,6 +64,29 @@ SECTIONS {
 		MOD_CODETAG_SECTIONS()
 	}
 #endif
+
+#ifdef CONFIG_KCOV_ENABLE_GUARDS
+	__sancov_guards(NOLOAD) : {
+		__start___sancov_guards = .;
+		*(__sancov_guards);
+		__stop___sancov_guards = .;
+	}
+
+	.text : {
+		*(.text .text.[0-9a-zA-Z_]*)
+		*(.text..L*)
+	}
+
+	.init.text : {
+		*(.init.text .init.text.[0-9a-zA-Z_]*)
+		*(.init.text..L*)
+	}
+	.exit.text : {
+		*(.exit.text .exit.text.[0-9a-zA-Z_]*)
+		*(.exit.text..L*)
+	}
+#endif
+
 	MOD_SEPARATE_CODETAG_SECTIONS()
 }
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index ce973d9d8e6d8..a5db690dd2def 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1149,6 +1149,7 @@ static const char *uaccess_safe_builtin[] = {
 	"write_comp_data",
 	"check_kcov_mode",
 	"__sanitizer_cov_trace_pc",
+	"__sanitizer_cov_trace_pc_guard",
 	"__sanitizer_cov_trace_const_cmp1",
 	"__sanitizer_cov_trace_const_cmp2",
 	"__sanitizer_cov_trace_const_cmp4",
-- 
2.49.0.604.gff1f9ca942-goog