linux-kernel - Re: [PATCH libcrypto 2/2] crypto: chacha20poly1305: statically check fixed array lengths

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Message-ID: <20251119184500.GA3303394@ax162>
Date: Wed, 19 Nov 2025 11:45:00 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	kernel test robot <lkp@...el.com>,
	Eric Biggers <ebiggers@...nel.org>,
	Ard Biesheuvel <ardb@...nel.org>, Kees Cook <kees@...nel.org>,
	linux-crypto@...r.kernel.org, llvm@...ts.linux.dev,
	oe-kbuild-all@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH libcrypto 2/2] crypto: chacha20poly1305: statically check
 fixed array lengths

On Wed, Nov 19, 2025 at 05:46:44PM +0100, Jason A. Donenfeld wrote:
> Hey Linus,
> 
> On Wed, Nov 19, 2025 at 5:29 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> >  On Wed, 19 Nov 2025 at 04:46, kernel test robot <lkp@...el.com> wrote:
> > >
> > > >> drivers/net/wireguard/cookie.c:193:2: warning: array argument is too small; contains 31 elements, callee requires at least 32 [-Warray-bounds]
> >
> > Hmm. Is this a compiler bug?
> 
> It's not. It's a 0day test bot bug! My original patch had in it some
> commentary about what a bug would look like when it's caught by the
> compiler. In order to provoke that compiler output, I mentioned in the
> commit message that this diff will produce such and such result:
> 
> diff --git a/drivers/net/wireguard/cookie.h b/drivers/net/wireguard/cookie.h
> index c4bd61ca03f2..2839c46029f8 100644
> --- a/drivers/net/wireguard/cookie.h
> +++ b/drivers/net/wireguard/cookie.h
> @@ -13,7 +13,7 @@ struct wg_peer;
> 
>  struct cookie_checker {
>         u8 secret[NOISE_HASH_LEN];
> -       u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN];
> +       u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN - 1];
>         u8 message_mac1_key[NOISE_SYMMETRIC_KEY_LEN];
>         u64 secret_birthdate;
>         struct rw_semaphore secret_lock;
> 
> It looks like the 0day test bot just went through the email and
> applied all the `diff --git ...` hunks, without taking into account
> the context area above where the actual patches start.

I don't think it is a bot issue. Just running 'b4 shazam' (i.e. just
applying the patch with 'git am') on the series results in:

commit 6ddc87109d4bb589d02cc3a8b037c99fdc4cbbf9
Author: Jason A. Donenfeld <Jason@...c4.com>
Date:   Tue Nov 18 18:02:40 2025 +0100

    crypto: chacha20poly1305: statically check fixed array lengths
    
    Several parameters of the chacha20poly1305 functions require arrays of
    an exact length. Use the new min_array_size() macro to instruct gcc and
    clang to statically check that the caller is passing an object of at
    least that length.
    
    Here it is in action, with this faulty patch:

diff --git a/drivers/net/wireguard/cookie.h b/drivers/net/wireguard/cookie.h
index c4bd61ca03f2..2839c46029f8 100644
--- a/drivers/net/wireguard/cookie.h
+++ b/drivers/net/wireguard/cookie.h
@@ -13,7 +13,7 @@ struct wg_peer;
 
 struct cookie_checker {
 	u8 secret[NOISE_HASH_LEN];
-	u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN];
+	u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN - 1];
 	u8 message_mac1_key[NOISE_SYMMETRIC_KEY_LEN];
 	u64 secret_birthdate;
 	struct rw_semaphore secret_lock;

followed by the rest of the patch for me.

Cheers,
Nathan