linux-kernel - Re: [PATCH] lib/crypto: blake2b: Limit frame size workaround to GCC

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Message-ID: <20251123202629.GA49083@sol>
Date: Sun, 23 Nov 2025 12:26:29 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: david laight <david.laight@...box.com>
Cc: Thorsten Blum <thorsten.blum@...ux.dev>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	Ard Biesheuvel <ardb@...nel.org>, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/crypto: blake2b: Limit frame size workaround to GCC
 < 12.2 on i386

On Sun, Nov 23, 2025 at 06:58:18PM +0000, david laight wrote:
> On Sun, 23 Nov 2025 18:00:01 +0100
> Thorsten Blum <thorsten.blum@...ux.dev> wrote:
> 
> > On 23. Nov 2025, at 10:28, david laight wrote:
> > > On Sat, 22 Nov 2025 11:55:31 +0100
> > > Thorsten Blum <thorsten.blum@...ux.dev> wrote:
> > >   
> > >> The GCC bug only occurred on i386 and has been resolved since GCC 12.2.
> > >> Limit the frame size workaround to GCC < 12.2 on i386.
> > >> 
> > >> Signed-off-by: Thorsten Blum <thorsten.blum@...ux.dev>
> > >> ---
> > >> lib/crypto/Makefile | 4 ++++
> > >> 1 file changed, 4 insertions(+)
> > >> 
> > >> diff --git a/lib/crypto/Makefile b/lib/crypto/Makefile
> > >> index b5346cebbb55..5ee36a231484 100644
> > >> --- a/lib/crypto/Makefile
> > >> +++ b/lib/crypto/Makefile
> > >> @@ -33,7 +33,11 @@ obj-$(CONFIG_CRYPTO_LIB_GF128MUL) += gf128mul.o
> > >> 
> > >> obj-$(CONFIG_CRYPTO_LIB_BLAKE2B) += libblake2b.o
> > >> libblake2b-y := blake2b.o
> > >> +ifeq ($(CONFIG_X86_32),y)
> > >> +ifeq ($(CONFIG_CC_IS_GCC)_$(call gcc-min-version, 120200),y_)
> > >> CFLAGS_blake2b.o := -Wframe-larger-than=4096 #  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105930
> > >> +endif # CONFIG_CC_IS_GCC
> > >> +endif # CONFIG_X86_32  
> > > 
> > > Isn't that just going to cause a run-time stack overflow?  
> > 
> > My change doesn't cause a runtime stack overflow, it's just a compiler
> > warning. There's more information in commit 1d3551ced64e ("crypto:
> > blake2b: effectively disable frame size warning").
> > 
> > Given the kernel test robot results with GCC 15.1.0 on m68k, we should
> > probably make this conditional on GCC (any version). Clang produces much
> > smaller stack frames and should be fine with the default warning
> > threshold.
> 
> But if anyone tries to run the kernel they'll need space for the '3k monster stack'.
> So changing the limit is 'fine' for a test build, but not for a proper build.
> (Yes this has been wrong since Linus did the original patch in 2022.)
> 
> Does allmodconfig set COMPILE_TEST ?
> If so that could be included in the conditional.
> 
> A more interesting question is whether the change can just be removed.
> I'd guess no one is actively using gcc 12.1 any more.

How about we roll up the BLAKE2b rounds loop if !CONFIG_64BIT?

- Eric