linux-kernel - Re: [PATCH] crypto: xts_crypt() return if walk.nbytes is 0

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Date:   Fri, 20 Aug 2021 20:53:15 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Shreyansh Chouhan <chouhan.shreyansh630@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzbot+20191dc583eff8602d2d@...kaller.appspotmail.com
Subject: Re: [PATCH] crypto: xts_crypt() return if walk.nbytes is 0

On Fri, Aug 20, 2021 at 01:14:52PM +0200, Ard Biesheuvel wrote:
>
> In spite of that, I have a slight preference for this version, given
> that it makes it obvious that we bail on two separate conditions:
> - an error has occurred
> - no error has occurred but the resulting walk is empty
> 
> Testing walk.nbytes only needlessly obfuscates the code, as we need to
> return 'err' in the end anyway.

I disagree, this is how most skcipher walkers are structured, they
never explicitly test on err and only terminate the loop when
walk->nbytes hits zero, in which case err is returned as is.

I don't see why this particular skcipher walker should deviate
from that paradigm.  In fact it is exactly that deviation that
caused the bug in the first instance.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt