Re: [PATCH v2] overflow.h: use new generic division helpers to avoid / operator

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Sep 13, 2021 at 05:23:18PM -0700, Nick Desaulniers wrote:
> 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: Kees Cook <keescook@chromium.org>
> Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Suggested-by: Pavel Machek <pavel@ucw.cz>
> Link: https://lore.kernel.org/all/20210909182525.372ee687@canb.auug.org.au/
> Link: https://github.com/ClangBuiltLinux/linux/issues/1438
> Fixes: f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
> add fallback code")
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes v1 -> v2:
> * Change the BUILD_BUG_ON_MSG's to check the divisor! Not the dividend!
> * Change __builtin_choose_expr/__builtin_constant_p soup to _Generic as
>   per Linus.
> * Add Linus' Suggested-by.
> * use __ prefixes on new macros.
> * add parens around use of macro parameters.
> * realign trailing \.
> 
> Note to Rasmus: I did not include comments on the usage. I don't think
> we really intend for folks to be using these, much less so in -stable.
> 
>  include/linux/math64.h   | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/overflow.h |  8 ++++----
>  2 files changed, 41 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/math64.h b/include/linux/math64.h
> index 66deb1fdc2ef..a1a6ad98b5ea 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(divisor) < 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 +153,31 @@ static inline s64 div_s64(s64 dividend, s32 divisor)
>  }
>  #endif
>  
> +#define __div_x64(dividend, divisor) ({			\
> +	BUILD_BUG_ON_MSG(sizeof(divisor) > 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)			\
> +	_Generic((divisor),				\
> +	s64: __div64_x64((dividend), (divisor)),	\
> +	u64: __div64_x64((dividend), (divisor)),	\
> +	default: __div_x64((dividend), (divisor)))
> +
> +#define div_64(dividend, divisor) ({				\
> +	BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u64) ||	\
> +	                 sizeof(divisor) > sizeof(u64),		\
> +	                 "128b div unsupported");		\
> +	_Generic((dividend),					\
> +	s64: __div_64((dividend), (divisor)),			\
> +	u64: __div_64((dividend), (divisor)),			\
> +	default: (dividend) / (divisor));			\
> +})
> +
>  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);			\
>  })
>  
> 
> base-commit: cb83afdc0b865d7c8a74d2b2a1f7dd393e1d196d
> -- 
> 2.33.0.309.g3052b89438-goog
> 

Why is this needed in the 5.10.y tree?  I see that commit fad7cd3310db
("nbd: add the check to prevent overflow in __nbd_ioctl()") is planned
to go into 5.14.y and 5.13.y, but no further back at the moment.

confused,

greg k-h