linux-hardening - Re: [PATCH 5/5] overflow: Introduce inc_wrap() and dec

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Date: Mon, 29 Jan 2024 21:16:36 +0100
From: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To: Kees Cook <keescook@...omium.org>
Cc: Mark Rutland <mark.rutland@....com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 linux-hardening@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Miguel Ojeda <ojeda@...nel.org>, Marco Elver <elver@...gle.com>,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 5/5] overflow: Introduce inc_wrap() and dec_wrap()

On 29/01/2024 19.34, Kees Cook wrote:
> This allows replacements of the idioms "var += offset" and "var -= offset"
> with the inc_wrap() and dec_wrap() helpers respectively. They will avoid
> wrap-around sanitizer instrumentation.
> 
> Cc: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 4f945e9e7881..080b18b84498 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -138,6 +138,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>  		__sum;					\
>  	})
>  
> +/**
> + * add_wrap() - Intentionally perform a wrapping increment

inc_wrap

> + * @a: variable to be incremented
> + * @b: amount to add
> + *
> + * Increments @a by @b with wrap-around. Returns the resulting
> + * value of @a. Will not trip any wrap-around sanitizers.
> + */
> +#define inc_wrap(var, offset)					\
> +	({							\
> +		if (check_add_overflow(var, offset, &var)) {	\
> +			/* do nothing */			\
> +		}						\
> +		var;						\

Hm. I wonder if multiple evaluations of var could be a problem.
Obviously never if var is actually some automatic variable, nor if it is
some simple foo->bar expression. But nothing really prevents var from
being, say, foo[gimme_an_index()] or something similarly convoluted.

Does the compiler generate ok code if one does

  typeof(var) *__pvar = &(var);
  if (check_add_overflow(*__pvar, offset, __pvar)) {}
  *__pvar;

[in fact, does it even generate code, i.e. does it compile?]

I dunno, maybe it's overkill to worry about.

Rasmus