linux-kernel - Re: [PATCH] crypto: hash - Make HASH_MAX

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Message-ID: <20250811165712.GB1268@sol>
Date: Mon, 11 Aug 2025 09:57:12 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH] crypto: hash - Make HASH_MAX_DESCSIZE a bit more obvious

On Mon, Aug 11, 2025 at 10:10:44AM +0300, Linus Torvalds wrote:
> On Mon, 11 Aug 2025 at 07:44, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> >
> > The patch below should make the constant a bit more obvious.
> 
> Indeed.
> 
> It would be good to maybe minimize the on-stack max-sized allocations,
> but that's a separate issue. Several hundred bytes is a noticeable
> part of the stack, and it's not always clear that it's a shallow stack
> with not a lot else going on..
> 
> (I just randomly picked the btrfs csum hash to look at, which can
> apparently be one of crc32c / xxhash64 / sha256 or blake2b, and which
> is then used at bio submission time, and I wouldn't be surprised if it
> probably has a pretty deep stack at that point already).

HASH_MAX_DESCSIZE has to be enough for *any* algorithm accessible via
the crypto_shash API, which makes HMAC-SHA3-224 be the limiting factor.
By converting users to use the library APIs instead, they will instead
use strongly-typed contexts that are sized correctly for the algorithms
actually being used.  In the btrfs csum case, the applicable sizes are:

    shash_desc + HASH_MAX_DESCSIZE: 377
    blake2b: 232
    sha256: 104
    xxhash64: 76
    crc32c: 4

So the reduction for btrfs will be 377 => 232.  But blake2b is missing a
library API, so I need to add that first.

- Eric