Re: [PATCH v2] crypto: ecc - Unbreak the build on arm with CONFIG_KASAN_STACK=y

[Andy Shevchenko]

On Wed, May 06, 2026 at 03:27:49PM +0200, Lukas Wunner wrote:
> Andrew reports build breakage of arm allmodconfig, reproducible with gcc
> 14.2.0 and 15.2.0:
> 
>   crypto/ecc.c: In function 'ecc_point_mult':
>   crypto/ecc.c:1380:1: error: the frame size of 1360 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
> 
> gcc aggressively inlines functions called by ecc_point_mult() (without
> there being any explicit inline declarations), which pushes stack usage
> close to the limit imposed by CONFIG_FRAME_WARN.  allmodconfig implies
> CONFIG_KASAN_STACK=y, which increases the stack above that limit.
> 
> In the bugzilla entry linked below, gcc maintainers explain that gcc
> estimates extra stack usage caused by inlining, but ASAN instrumentation
> is added in post-IPA passes and thus the inlining heuristics cannot
> account for it.
> 
> It could be argued that -Werror=frame-larger-than=1280 instructs the
> compiler to avoid inlining beyond that limit lest the build breaks,
> which would imply gcc behaves incorrectly.  But gcc maintainers reject
> this notion and believe that a warning switch should never affect code
> generation, even if it is promoted to an error.
> 
> One way to unbreak the build is to limit inlining via -finline-limit=100
> or by explicitly declaring some functions noinline.  However while it
> does keep stack usage of individual functions below the limit, *total*
> stack usage increases.
> 
> A longterm solution is to refactor ecc.c for reduced stack usage.  It
> currently performs ECC point multiplication with a Montgomery ladder
> which uses co-Z (conjugate) addition to trade off memory for speed.
> The algorithm is susceptible to timing attacks and needs to be replaced
> with a constant time Montgomery ladder, which should consume less memory
> and thus resolve the stack usage issue as a side effect.
> 
> In the interim, raise the limit for ecc.c, as is already done for
> several other files in the source tree.
> 
> Constrain to gcc because clang 19.1.7 does not exhibit the issue.  It
> makes do with a 724 bytes stack frame even though it inlines almost the
> same functions as gcc.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

...

> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124949

Perhaps also mention the algo change as that one sounds to me even more
critical than this issue per se.

> +ifeq ($(CONFIG_ARM)$(CONFIG_KASAN_STACK)$(CONFIG_CC_IS_GCC),yyy)
> +CFLAGS_ecc.o += $(call cc-option,-Wframe-larger-than=1536)
> +endif

-- 
With Best Regards,
Andy Shevchenko