linux-kernel - [PATCH AUTOSEL 5.10 2/7] powerpc/64s: Disable stack variable initialisation for prom

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Date:   Mon,  1 Aug 2022 15:02:56 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Sasha Levin <sashal@...nel.org>, christophe.leroy@...roup.eu,
        paulus@...abs.org, peterz@...radead.org, ast@...nel.org,
        npiggin@...il.com, dja@...ens.net, masahiroy@...nel.org,
        aneesh.kumar@...ux.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 5.10 2/7] powerpc/64s: Disable stack variable initialisation for prom_init

From: Michael Ellerman <mpe@...erman.id.au>

[ Upstream commit be640317a1d0b9cf42fedb2debc2887a7cfa38de ]

With GCC 12 allmodconfig prom_init fails to build:

  Error: External symbol 'memset' referenced from prom_init.c
  make[2]: *** [arch/powerpc/kernel/Makefile:204: arch/powerpc/kernel/prom_init_check] Error 1

The allmodconfig build enables KASAN, so all calls to memset in
prom_init should be converted to __memset by the #ifdefs in
asm/string.h, because prom_init must use the non-KASAN instrumented
versions.

The build failure happens because there's a call to memset that hasn't
been caught by the pre-processor and converted to __memset. Typically
that's because it's a memset generated by the compiler itself, and that
is the case here.

With GCC 12, allmodconfig enables CONFIG_INIT_STACK_ALL_PATTERN, which
causes the compiler to emit memset calls to initialise on-stack
variables with a pattern.

Because prom_init is non-user-facing boot-time only code, as a
workaround just disable stack variable initialisation to unbreak the
build.

Reported-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Link: https://lore.kernel.org/r/20220718134418.354114-1-mpe@ellerman.id.au
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/kernel/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 376104c166fc..db2bdc4cec64 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -20,6 +20,7 @@ CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_prom_init.o += -fno-stack-protector
 CFLAGS_prom_init.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_prom_init.o += -ffreestanding
+CFLAGS_prom_init.o += $(call cc-option, -ftrivial-auto-var-init=uninitialized)

 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code
-- 
2.35.1