linux-kernel - [PATCH 6.0 648/862] linux/export: use inline assembler to populate symbol CRCs

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Message-Id: <20221019083318.553316034@linuxfoundation.org>
Date:   Wed, 19 Oct 2022 10:32:15 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Yann Sionneau <ysionneau@...ray.eu>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 6.0 648/862] linux/export: use inline assembler to populate symbol CRCs

From: Masahiro Yamada <masahiroy@...nel.org>

[ Upstream commit f3304ecd7f060db1d4197fbdce5a503259f770d3 ]

Since commit 7b4537199a4a ("kbuild: link symbol CRCs at final link,
removing CONFIG_MODULE_REL_CRCS"), the module versioning on the
(non-upstreamed-yet) kvx Linux port is broken due to unexpected padding
for __crc_* symbols. The kvx GCC adds padding so u32 gets 8-byte
alignment instead of 4.

I do not know if this happens for upstream architectures in general,
but any compiler has the freedom to insert padding for faster access.

Use the inline assembler to directly specify the wanted data layout.
This is how we previously did before the breakage.

Link: https://lore.kernel.org/lkml/20220817161438.32039-1-ysionneau@kalray.eu/
Link: https://lore.kernel.org/linux-kbuild/31ce5305-a76b-13d7-ea55-afca82c46cf2@kalray.eu/
Fixes: 7b4537199a4a ("kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS")
Reported-by: Yann Sionneau <ysionneau@...ray.eu>
Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
Tested-by: Yann Sionneau <ysionneau@...ray.eu>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 include/linux/export-internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/export-internal.h b/include/linux/export-internal.h
index c2b1d4fd5987..fe7e6ba918f1 100644
--- a/include/linux/export-internal.h
+++ b/include/linux/export-internal.h
@@ -10,8 +10,10 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 
-/* __used is needed to keep __crc_* for LTO */
 #define SYMBOL_CRC(sym, crc, sec)   \
-	u32 __section("___kcrctab" sec "+" #sym) __used __crc_##sym = crc
+	asm(".section \"___kcrctab" sec "+" #sym "\",\"a\""	"\n" \
+	    "__crc_" #sym ":"					"\n" \
+	    ".long " #crc					"\n" \
+	    ".previous"						"\n")
 
 #endif /* __LINUX_EXPORT_INTERNAL_H__ */
-- 
2.35.1