linux-kernel - Re: lib/crypto/chacha.c:65:1: warning: the frame size of 1604 bytes is larger than 1024 bytes

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Date:   Thu, 27 Aug 2020 18:25:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        kbuild-all@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: lib/crypto/chacha.c:65:1: warning: the frame size of 1604 bytes
 is larger than 1024 bytes

On Thu, Aug 27, 2020 at 1:51 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Thu, Aug 27, 2020 at 12:41:53PM +0200, Ard Biesheuvel wrote:
> >
> > That does not help, unfortunately.
> >
> > What does seem to work is
> >
> > struct chacha_state { u32 x[16]; };
> >
> > struct chacha_state chacha_permute(struct chacha_state st, int nrounds)
>
> Passing 64 bytes by value is not good.
>
> Passing struct chacha_state as a pointer doesn't work either.

Marking the function as __always_inline avoids the problem, as it
lets the compiler see the issue, but seems to produce somewhat
worse object code.

I also tested with clang-11, which supports both -fsanitize-bounds
and -fprofile-arcs but only needs 8 bytes of stack for this function.

One more data point, I looked at the actual object code and found
that neither -fprofile-arcs  nor -fsanitize-bounds has a noticeable
impact on the object code output by themselves (aside of not
leading to the warning as you already mentioned). I would
conclude that there is an actual problem with gcc here.

      Arnd