commit fad7cd3310db ("nbd: add the check to prevent overflow in
__nbd_ioctl()") raised an issue from the fallback helpers added in
commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
add fallback code")

ERROR: modpost: "__divdi3" [drivers/block/nbd.ko] undefined!

As Stephen Rothwell notes:
  The added check_mul_overflow() call is being passed 64 bit values.
  COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW is not set for this build (see
  include/linux/overflow.h).

Specifically, the helpers for checking whether the results of a
multiplication overflowed (__unsigned_mul_overflow,
__signed_add_overflow) use the division operator when
!COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW.  This is problematic for 64b
operands on 32b hosts.

This was fixed upstream by
commit 76ae847497bc ("Documentation: raise minimum supported version of
GCC to 5.1")
which is not suitable to be backported to stable; I didn't have this
patch ready in time.

Cc: stable@vger.kernel.org
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Kees Cook <redacted>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reported-by: Naresh Kamboju <redacted>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Stephen Rothwell <redacted>
Suggested-by: Pavel Machek <redacted>
Link: https://github.com/ClangBuiltLinux/linux/issues/1438
Link: https://lore.kernel.org/all/20210909182525.372ee687@canb.auug.org.au/ (local)
Link: https://lore.kernel.org/lkml/20210910234047.1019925-1-ndesaulniers@google.com/ (local)
Fixes: f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
add fallback code")
Signed-off-by: Nick Desaulniers <redacted>
---
This kind of generic meta-programming in C and my lack of experience in
doing so makes me very uncomfortable (and slightly ashamed) to send
this. I would appreciate careful review and feedback. I would appreciate
if Naresh can test this with GCC 4.9, which I don't have handy.

Linus also suggested I look into the use of _Generic; I haven't
evaluated that approach yet, but I felt like posting this early for
feedback.

 include/linux/math64.h   | 35 +++++++++++++++++++++++++++++++++++
 include/linux/overflow.h |  8 ++++----
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 66deb1fdc2ef..bc9c12c168d0 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h

@@ -10,6 +10,9 @@
 
 #define div64_long(x, y) div64_s64((x), (y))
 #define div64_ul(x, y)   div64_u64((x), (y))
+#ifndef is_signed_type
+#define is_signed_type(type)       (((type)(-1)) < (type)1)
+#endif
 
 /**
  * div_u64_rem - unsigned 64bit divide with 32bit divisor with remainder

@@ -111,6 +114,15 @@ extern s64 div64_s64(s64 dividend, s64 divisor);
 
 #endif /* BITS_PER_LONG */

+#define div64_x64(dividend, divisor) ({			\
+	BUILD_BUG_ON_MSG(sizeof(dividend) < sizeof(u64),\
+	                 "prefer div_x64");		\
+	__builtin_choose_expr(				\
+		is_signed_type(typeof(dividend)),	\
+		div64_s64(dividend, divisor),		\
+		div64_u64(dividend, divisor));		\
+})
+
 /**
  * div_u64 - unsigned 64bit divide with 32bit divisor
  * @dividend: unsigned 64bit dividend

@@ -141,6 +153,29 @@ static inline s64 div_s64(s64 dividend, s32 divisor)
 }
 #endif
 
+#define div_x64(dividend, divisor) ({			\
+	BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u32),\
+	                 "prefer div64_x64");		\
+	__builtin_choose_expr(				\
+		is_signed_type(typeof(dividend)),	\
+		div_s64(dividend, divisor),		\
+		div_u64(dividend, divisor));		\
+})
+
+#define div_64(dividend, divisor) ({						\
+	BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u64),			\
+	                 "128b div unsupported");				\
+	__builtin_choose_expr(							\
+		__builtin_types_compatible_p(typeof(dividend), s64) ||		\
+		__builtin_types_compatible_p(typeof(dividend), u64),		\

 u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder);
 
 #ifndef mul_u32_u32

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index ef74051d5cfe..2ebdf220c184 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h

@@ -123,8 +123,8 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 	(void) (&__a == __d);				\
 	*__d = __a * __b;				\
 	__builtin_constant_p(__b) ?			\
-	  __b > 0 && __a > type_max(typeof(__a)) / __b : \
-	  __a > 0 && __b > type_max(typeof(__b)) / __a;	 \
+	  __b > 0 && __a > div_64(type_max(typeof(__a)), __b) :	\
+	  __a > 0 && __b > div_64(type_max(typeof(__b)), __a);	\
 })
 
 /*

@@ -195,8 +195,8 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 	(void) (&__a == &__b);						\
 	(void) (&__a == __d);						\
 	*__d = (u64)__a * (u64)__b;					\
-	(__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||	\
-	(__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
+	(__b > 0 && (__a > div_64(__tmax, __b) || __a < div_64(__tmin, __b))) ||		\
+	(__b < (typeof(__b))-1 && (__a > div_64(__tmin, __b) || __a < div_64(__tmax, __b))) ||	\
 	(__b == (typeof(__b))-1 && __a == __tmin);			\
 })