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Date: Tue, 14 Sep 2021 11:47:58 -0700
From: Kees Cook <keescook@chromium.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josef Bacik <josef@toxicpanda.com>, Jens Axboe <axboe@kernel.dk>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sasha Levin <sashal@kernel.org>, llvm@lists.linux.dev,
	stable <stable@vger.kernel.org>, Arnd Bergmann <arnd@kernel.org>,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Pavel Machek <pavel@ucw.cz>
Subject: Re: [PATCH 5.10] overflow.h: use new generic division helpers to
 avoid / operator
Message-ID: <202109141144.1AE2DDB@keescook>
References: <20210913203201.1844253-1-ndesaulniers@google.com>
 <195b2f47-b92e-a00b-a2bc-d91bfdbd9d12@rasmusvillemoes.dk>
 <202109141031.AEFD06F03F@keescook>
 <CAHk-=wjac_3K+NQNO6tjQZU1KLgba==BOvHmHE2sgNSVj3j85g@mail.gmail.com>
 <CAHk-=whiQBofgis_rkniz8GBP9wZtSZdcDEffgSLO62BUGV3gg@mail.gmail.com>
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In-Reply-To: <CAHk-=whiQBofgis_rkniz8GBP9wZtSZdcDEffgSLO62BUGV3gg@mail.gmail.com>

On Tue, Sep 14, 2021 at 11:30:03AM -0700, Linus Torvalds wrote:
>  	case NBD_SET_SIZE_BLOCKS:
> -		if (check_mul_overflow((loff_t)arg, config->blksize, &bytesize))
> +		bytesize = arg << config->blksize_bits;
> +		if (bytesize >> config->blksize_bits != arg)
>  			return -EINVAL;

FWIW, it's probably better to avoid open-coding the check. There are
helpers for shift-left too. :)

+		if (check_shl_overflow(arg, config->blksize_bits, &bytesize))

-- 
Kees Cook