Subject: [PATCH] ubsan: remove overflow checks

From: Andrey Ryabinin <[email-masked]>
Date: Wed, 10 Feb 2021 02:23:48 +0300

Since GCC 8.0 -fsanitize=signed-integer-overflow doesn't work with -fwrapv.
-fwrapv makes signed overflows defines and GCC essentially disables
ubsan checks. On GCC < 8.0 -fwrapv doesn't have influence on
-fsanitize=signed-integer-overflow setting, so it kinda works
but generates false-positves and violates uaccess rules:

lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled

Disable signed overflow checks to avoid these problems.
Remove unsigned overflow checks as well.
Unsigned overflow appeared as side effect of the commit
 cdf8a76fda4a ("ubsan: move cc-option tests into Kconfig"),
but it never worked (kernel doesn't boot). And unsigned overflows
are allowed by C standard, so it just pointless.

Signed-off-by: Andrey Ryabinin <[email-masked]>
Cc: Peter Zijlstra <[email-masked]>
Cc: Josh Poimboeuf <[email-masked]>
Cc: Randy Dunlap <[email-masked]>
Cc: Stephen Rothwell <[email-masked]>
Cc: Dmitry Vyukov <[email-masked]>
Cc: Kees Cook <[email-masked]>
Cc: Alexander Viro <[email-masked]>
---
 lib/Kconfig.ubsan      | 17 -----------
 lib/test_ubsan.c       | 49 ------------------------------
 lib/ubsan.c            | 68 ------------------------------------------
 scripts/Makefile.ubsan |  2 --
 4 files changed, 136 deletions(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 3a0b1c930733..e5372a13511d 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -112,23 +112,6 @@ config UBSAN_UNREACHABLE
 	  This option enables -fsanitize=unreachable which checks for control
 	  flow reaching an expected-to-be-unreachable position.
 
-config UBSAN_SIGNED_OVERFLOW
-	bool "Perform checking for signed arithmetic overflow"
-	default UBSAN
-	depends on $(cc-option,-fsanitize=signed-integer-overflow)
-	help
-	  This option enables -fsanitize=signed-integer-overflow which checks
-	  for overflow of any arithmetic operations with signed integers.
-
-config UBSAN_UNSIGNED_OVERFLOW
-	bool "Perform checking for unsigned arithmetic overflow"
-	depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
-	depends on !X86_32 # avoid excessive stack usage on x86-32/clang
-	help
-	  This option enables -fsanitize=unsigned-integer-overflow which checks
-	  for overflow of any arithmetic operations with unsigned integers. This
-	  currently causes x86 to fail to boot.
-
 config UBSAN_OBJECT_SIZE
 	bool "Perform checking for accesses beyond the end of objects"
 	default UBSAN
diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
index 5e5d9355ef49..7e7bbd0f3fd2 100644
--- a/lib/test_ubsan.c
+++ b/lib/test_ubsan.c
@@ -11,51 +11,6 @@ typedef void(*test_ubsan_fp)(void);
 			#config, IS_ENABLED(config) ? "y" : "n");	\
 	} while (0)
 
-static void test_ubsan_add_overflow(void)
-{
-	volatile int val = INT_MAX;
-	volatile unsigned int uval = UINT_MAX;
-
-	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
-	val += 2;
-
-	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
-	uval += 2;
-}
-
-static void test_ubsan_sub_overflow(void)
-{
-	volatile int val = INT_MIN;
-	volatile unsigned int uval = 0;
-	volatile int val2 = 2;
-
-	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
-	val -= val2;
-
-	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
-	uval -= val2;
-}
-
-static void test_ubsan_mul_overflow(void)
-{
-	volatile int val = INT_MAX / 2;
-	volatile unsigned int uval = UINT_MAX / 2;
-
-	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
-	val *= 3;
-
-	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
-	uval *= 3;
-}
-
-static void test_ubsan_negate_overflow(void)
-{
-	volatile int val = INT_MIN;
-
-	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
-	val = -val;
-}
-
 static void test_ubsan_divrem_overflow(void)
 {
 	volatile int val = 16;
@@ -155,10 +110,6 @@ static void test_ubsan_object_size_mismatch(void)
 }
 
 static const test_ubsan_fp test_ubsan_array[] = {
-	test_ubsan_add_overflow,
-	test_ubsan_sub_overflow,
-	test_ubsan_mul_overflow,
-	test_ubsan_negate_overflow,
 	test_ubsan_shift_out_of_bounds,
 	test_ubsan_out_of_bounds,
 	test_ubsan_load_invalid_value,
diff --git a/lib/ubsan.c b/lib/ubsan.c
index bec38c64d6a6..26229973049d 100644
--- a/lib/ubsan.c
+++ b/lib/ubsan.c
@@ -163,74 +163,6 @@ static void ubsan_epilogue(void)
 	}
 }
 
-static void handle_overflow(struct overflow_data *data, void *lhs,
-			void *rhs, char op)
-{
-
-	struct type_descriptor *type = data->type;
-	char lhs_val_str[VALUE_LENGTH];
-	char rhs_val_str[VALUE_LENGTH];
-
-	if (suppress_report(&data->location))
-		return;
-
-	ubsan_prologue(&data->location, type_is_signed(type) ?
-			"signed-integer-overflow" :
-			"unsigned-integer-overflow");
-
-	val_to_string(lhs_val_str, sizeof(lhs_val_str), type, lhs);
-	val_to_string(rhs_val_str, sizeof(rhs_val_str), type, rhs);
-	pr_err("%s %c %s cannot be represented in type %s\n",
-		lhs_val_str,
-		op,
-		rhs_val_str,
-		type->type_name);
-
-	ubsan_epilogue();
-}
-
-void __ubsan_handle_add_overflow(void *data,
-				void *lhs, void *rhs)
-{
-
-	handle_overflow(data, lhs, rhs, '+');
-}
-EXPORT_SYMBOL(__ubsan_handle_add_overflow);
-
-void __ubsan_handle_sub_overflow(void *data,
-				void *lhs, void *rhs)
-{
-	handle_overflow(data, lhs, rhs, '-');
-}
-EXPORT_SYMBOL(__ubsan_handle_sub_overflow);
-
-void __ubsan_handle_mul_overflow(void *data,
-				void *lhs, void *rhs)
-{
-	handle_overflow(data, lhs, rhs, '*');
-}
-EXPORT_SYMBOL(__ubsan_handle_mul_overflow);
-
-void __ubsan_handle_negate_overflow(void *_data, void *old_val)
-{
-	struct overflow_data *data = _data;
-	char old_val_str[VALUE_LENGTH];
-
-	if (suppress_report(&data->location))
-		return;
-
-	ubsan_prologue(&data->location, "negation-overflow");
-
-	val_to_string(old_val_str, sizeof(old_val_str), data->type, old_val);
-
-	pr_err("negation of %s cannot be represented in type %s:\n",
-		old_val_str, data->type->type_name);
-
-	ubsan_epilogue();
-}
-EXPORT_SYMBOL(__ubsan_handle_negate_overflow);
-
-
 void __ubsan_handle_divrem_overflow(void *_data, void *lhs, void *rhs)
 {
 	struct overflow_data *data = _data;
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 0e53a93e8f15..9e2092fd5206 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -8,8 +8,6 @@ ubsan-cflags-$(CONFIG_UBSAN_LOCAL_BOUNDS)	+= -fsanitize=local-bounds
 ubsan-cflags-$(CONFIG_UBSAN_SHIFT)		+= -fsanitize=shift
 ubsan-cflags-$(CONFIG_UBSAN_DIV_ZERO)		+= -fsanitize=integer-divide-by-zero
 ubsan-cflags-$(CONFIG_UBSAN_UNREACHABLE)	+= -fsanitize=unreachable
-ubsan-cflags-$(CONFIG_UBSAN_SIGNED_OVERFLOW)	+= -fsanitize=signed-integer-overflow
-ubsan-cflags-$(CONFIG_UBSAN_UNSIGNED_OVERFLOW)	+= -fsanitize=unsigned-integer-overflow
 ubsan-cflags-$(CONFIG_UBSAN_OBJECT_SIZE)	+= -fsanitize=object-size
 ubsan-cflags-$(CONFIG_UBSAN_BOOL)		+= -fsanitize=bool
 ubsan-cflags-$(CONFIG_UBSAN_ENUM)		+= -fsanitize=enum
-- 
2.26.2