From: Nathan Chancellor <natechancellor@gmail.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
clang-built-linux@googlegroups.com,
Nick Desaulniers <ndesaulniers@google.com>,
Nathan Chancellor <natechancellor@gmail.com>
Subject: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp
Date: Sun, 13 Oct 2019 19:51:01 -0700 [thread overview]
Message-ID: <20191014025101.18567-4-natechancellor@gmail.com> (raw)
In-Reply-To: <20191014025101.18567-1-natechancellor@gmail.com>
r374662 gives LLVM the ability to convert certain loops into a reference
to bcmp as an optimization; this breaks prom_init_check.sh:
CALL arch/powerpc/kernel/prom_init_check.sh
Error: External symbol 'bcmp' referenced from prom_init.c
make[2]: *** [arch/powerpc/kernel/Makefile:196: prom_init_check] Error 1
bcmp is defined in lib/string.c as a wrapper for memcmp so this could be
added to the whitelist. However, commit 450e7dd4001f ("powerpc/prom_init:
don't use string functions from lib/") copied memcmp as prom_memcmp to
avoid KASAN instrumentation so having bcmp be resolved to regular memcmp
would break that assumption. Furthermore, because the compiler is the
one that inserted bcmp, we cannot provide something like prom_bcmp.
To prevent LLVM from being clever with optimizations like this, use
-ffreestanding to tell LLVM we are not hosted so it is not free to make
transformations like this.
Link: https://github.com/ClangBuiltLinux/linux/issues/647
Link: https://github.com/llvm/llvm-project/commit/76cdcf25b883751d83402baea6316772aa73865c
Reviewed-by: Nick Desaulneris <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
v1 -> v3:
* New patch in the series
v3 -> v4:
* Rebase on v5.4-rc3.
* Add Nick's reviewed-by tag.
* Update the LLVM commit reference to the latest applied version (r374662)
as it was originally committed as r370454, reverted in r370788, and
reapplied as r374662.
arch/powerpc/kernel/Makefile | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index f1f362146135..7f0ee465dfb6 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -21,7 +21,7 @@ CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
-CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
+CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector) -ffreestanding
ifdef CONFIG_FUNCTION_TRACER
# Do not trace early boot code
--
2.23.0
next prev parent reply other threads:[~2019-10-14 2:51 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 18:20 [PATCH v3 0/3] LLVM/Clang fixes for pseries_defconfig Nathan Chancellor
2019-09-11 18:20 ` [PATCH v3 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-09-11 18:20 ` [PATCH v3 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-09-11 20:55 ` Nick Desaulniers
2019-09-11 18:20 ` [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Nathan Chancellor
2019-09-11 21:01 ` Nick Desaulniers
2019-09-12 5:43 ` Nathan Chancellor
2019-09-12 17:30 ` Nick Desaulniers
2019-10-14 2:50 ` [PATCH v4 0/3] LLVM/Clang fixes for pseries_defconfig Nathan Chancellor
2019-10-14 2:50 ` [PATCH v4 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-10-14 2:51 ` [PATCH v4 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-10-14 2:51 ` Nathan Chancellor [this message]
2019-10-14 9:35 ` [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Segher Boessenkool
2019-10-14 15:56 ` Nick Desaulniers
2019-10-14 19:11 ` Segher Boessenkool
2019-10-18 19:00 ` Nathan Chancellor
2019-10-18 20:02 ` Segher Boessenkool
2019-10-22 5:15 ` Nathan Chancellor
2019-10-22 8:57 ` Segher Boessenkool
2019-10-30 4:12 ` Nathan Chancellor
2019-10-14 16:03 ` [PATCH v4 0/3] LLVM/Clang fixes for pseries_defconfig Nick Desaulniers
2019-11-19 4:57 ` [PATCH v5 0/3] LLVM/Clang fixes for a few defconfigs Nathan Chancellor
2019-11-19 4:57 ` [PATCH v5 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-11-26 1:13 ` Michael Ellerman
2019-11-19 4:57 ` [PATCH v5 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-11-19 4:57 ` [PATCH v5 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Nathan Chancellor
2019-11-25 20:15 ` [PATCH v5 0/3] LLVM/Clang fixes for a few defconfigs Nick Desaulniers
2019-11-28 4:59 ` Michael Ellerman
2019-11-28 7:45 ` Nathan Chancellor
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