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

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Message-Id: <20220916062953.2972556-1-masahiroy@kernel.org>
Date:   Fri, 16 Sep 2022 15:29:53 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Yann Sionneau <ysionneau@...ray.eu>,
        Nicolas Schier <nicolas@...sle.eu>
Subject: [PATCH] linux/export: use inline assembler to populate symbol CRCs

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>
---

 include/linux/export-internal.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/export-internal.h b/include/linux/export-internal.h
index c2b1d4fd5987..fb90f326b1b5 100644
--- a/include/linux/export-internal.h
+++ b/include/linux/export-internal.h
@@ -12,6 +12,9 @@
 
 /* __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__ */
-- 
2.34.1