Buildroot error when building with Ubuntu 21.10 - linux-kernel

Question

I am trying to compile linux for RISCV Arch using buildroot(busybox). I was using 18.04 and 20.04 previously and had no issues compiling it. Right now, I have upgraded it to 21.10 for building some other stuffs. I have moved my toolchain and I can find it using the which command. When I try to compile linux I get some error which I havn't faced in the earlier versions.
>>> host-m4 1.4.18 Building
In file included from /usr/include/signal.h:328,
from ./signal.h:52,
from c-stack.c:49:
c-stack.c:55:26: error: missing binary operator before token "("
55 | #elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
| ^~~~~~~~
CC closein.o
c-stack.c:134:8: error: variably modified 'buffer' at file scope
134 | char buffer[SIGSTKSZ];
| ^~~~~~
CC closeout.o
I am confused on how different versions can cause this error.
Thanks in advance.

Answer 1

It looks like you have hit a change in GNU C Library version 2.34 that can make SIGSTKSZ non-constant.
From the GNU C Library 2.34 release announcement:
Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ. When _DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ are no longer constant on Linux. MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ) and SIGSTKSZ is redefined to sysconf (_SC_SIGSTKSZ). This supports dynamic sized register sets for modern architectural features like Arm SVE.
A possible workaround is to configure buildroot to build host-m4 version 1.4.19 instead of 1.4.18, because it no longer uses SIGSTKSZ.

Answer 2

I saw that error when building on Ubuntu 22.04 for an embedded Linux board using Buildroot. It got stuck when building the host-m4 package. #Ian Abbott is right:
A possible workaround is to configure buildroot to build host-m4 version 1.4.19 instead of 1.4.18, because it no longer uses SIGSTKSZ.
In Buildroot, to update from m4 v 1.4.18 to 1.4.19, simply grab the latest files here: https://github.com/buildroot/buildroot/tree/master/package/m4
As of right now (9 Jan. 2023), the upstream Buildroot m4 version there is
1.4.19, as shown here: https://github.com/buildroot/buildroot/blob/master/package/m4/m4.mk#L7:
################################################################################
#
# m4
#
################################################################################
M4_VERSION = 1.4.19
M4_SOURCE = m4-$(M4_VERSION).tar.xz
M4_SITE = $(BR2_GNU_MIRROR)/m4
M4_LICENSE = GPL-3.0+
M4_LICENSE_FILES = COPYING
$(eval $(host-autotools-package))
How to upgrade/update any buildroot package to the latest upstream version, from the command-line
Here is an example of how to upgrade any buildroot package from the command-line. In these commands, I am upgrading the m4 package, to solve the problem in the OP's question. Change the word m4 in all 3 places to the name of the package you'd like to update:
# initially:
cd path/to/buildroot
git remote add upstream https://github.com/buildroot/buildroot.git
git fetch upstream master
# then (from within the "buildroot" repo or subrepo)
rm -r package/m4
git checkout upstream/master -- package/m4
git add -A
git status
git commit -m "Update m4 library"