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Subject[PATCH 2/2] export.h, genksyms: do not make genksyms calculate CRC of trimmed symbols
Date
Arnd Bergmann reported false-positive modpost warnings detected by
his randconfig testing of linux-next:

https://lkml.org/lkml/2019/9/6/619

Actually, this happens under the combination of CONFIG_MODVERSIONS=y
and CONFIG_TRIM_UNUSED_KSYMS=y since commit 15bfc2348d54 ("modpost:
check for static EXPORT_SYMBOL* functions").

For example, arch/arm/config/multi_v7_defconfig + CONFIG_MODVERSIONS=y
+ CONFIG_TRIM_UNUSED_KSYMS=y produces the following false-positives:

WARNING: "__lshrdi3" [vmlinux] is a static (unknown)
WARNING: "__ashrdi3" [vmlinux] is a static (unknown)
WARNING: "__aeabi_lasr" [vmlinux] is a static (unknown)
WARNING: "__aeabi_llsr" [vmlinux] is a static (unknown)
WARNING: "ftrace_set_clr_event" [vmlinux] is a static (unknown)
WARNING: "__muldi3" [vmlinux] is a static (unknown)
WARNING: "__aeabi_ulcmp" [vmlinux] is a static (unknown)
WARNING: "__ucmpdi2" [vmlinux] is a static (unknown)
WARNING: "__aeabi_lmul" [vmlinux] is a static (unknown)
WARNING: "__bswapsi2" [vmlinux] is a static (unknown)
WARNING: "__bswapdi2" [vmlinux] is a static (unknown)
WARNING: "__ashldi3" [vmlinux] is a static (unknown)
WARNING: "__aeabi_llsl" [vmlinux] is a static (unknown)

The root cause of the problem is not in the modpost, but in the
implementation of CONFIG_TRIM_UNUSED_KSYMS.

If there is at least one untrimmed symbol in the file, genksyms is
invoked to calculate CRC of *all* the symbols in that file even if
some of them have been trimmed due to no caller existing.

As a result, .tmp_*.ver files contain CRC of trimmed symbols, thus
unneeded __crc* symbols are added to objects. It has been harmless
until recently.

Since commit 15bfc2348d54 ("modpost: check for static EXPORT_SYMBOL*
functions"), it is harmful because the bogus __crc* symbols make
modpost call sym_update_crc(), and then new_symbol(), but there is
no one that clears the ->is_static member.

I gave Fixes to the first commit that uncovered the issue, but the
potential problem has long existed since commit f235541699bc
("export.h: allow for per-symbol configurable EXPORT_SYMBOL()").

Fixes: 15bfc2348d54 ("modpost: check for static EXPORT_SYMBOL* functions")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---

include/linux/export.h | 42 ++++++++++++++-----------------------
scripts/genksyms/keywords.c | 6 +-----
2 files changed, 17 insertions(+), 31 deletions(-)

diff --git a/include/linux/export.h b/include/linux/export.h
index cdd98a0d918c..7d8c112a8b61 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -18,9 +18,6 @@ extern struct module __this_module;
#define THIS_MODULE ((struct module *)0)
#endif

-#ifdef CONFIG_MODULES
-
-#if !defined(__GENKSYMS__)
#ifdef CONFIG_MODVERSIONS
/* Mark the CRC weak since genksyms apparently decides not to
* generate a checksums for some symbols */
@@ -74,6 +71,12 @@ struct kernel_symbol {
};
#endif

+#ifdef __GENKSYMS__
+
+#define ___EXPORT_SYMBOL(sym, sec) __GENKSYMS_EXPORT_SYMBOL(sym)
+
+#else
+
/* For every exported symbol, place a struct in the __ksymtab section */
#define ___EXPORT_SYMBOL(sym, sec) \
extern typeof(sym) sym; \
@@ -83,7 +86,9 @@ struct kernel_symbol {
= #sym; \
__KSYMTAB_ENTRY(sym, sec)

-#if defined(__DISABLE_EXPORTS)
+#endif
+
+#if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)

/*
* Allow symbol exports to be disabled completely so that C code may
@@ -117,37 +122,22 @@ struct kernel_symbol {
#define __cond_export_sym_0(sym, sec) /* nothing */

#else
-#define __EXPORT_SYMBOL ___EXPORT_SYMBOL
-#endif

-#define EXPORT_SYMBOL(sym) \
- __EXPORT_SYMBOL(sym, "")
+#define __EXPORT_SYMBOL(sym, sec) ___EXPORT_SYMBOL(sym, sec)

-#define EXPORT_SYMBOL_GPL(sym) \
- __EXPORT_SYMBOL(sym, "_gpl")
-
-#define EXPORT_SYMBOL_GPL_FUTURE(sym) \
- __EXPORT_SYMBOL(sym, "_gpl_future")
+#endif /* CONFIG_MODULES */

+#define EXPORT_SYMBOL(sym) __EXPORT_SYMBOL(sym, "")
+#define EXPORT_SYMBOL_GPL(sym) __EXPORT_SYMBOL(sym, "_gpl")
+#define EXPORT_SYMBOL_GPL_FUTURE(sym) __EXPORT_SYMBOL(sym, "_gpl_future")
#ifdef CONFIG_UNUSED_SYMBOLS
-#define EXPORT_UNUSED_SYMBOL(sym) __EXPORT_SYMBOL(sym, "_unused")
-#define EXPORT_UNUSED_SYMBOL_GPL(sym) __EXPORT_SYMBOL(sym, "_unused_gpl")
+#define EXPORT_UNUSED_SYMBOL(sym) __EXPORT_SYMBOL(sym, "_unused")
+#define EXPORT_UNUSED_SYMBOL_GPL(sym) __EXPORT_SYMBOL(sym, "_unused_gpl")
#else
#define EXPORT_UNUSED_SYMBOL(sym)
#define EXPORT_UNUSED_SYMBOL_GPL(sym)
#endif

-#endif /* __GENKSYMS__ */
-
-#else /* !CONFIG_MODULES... */
-
-#define EXPORT_SYMBOL(sym)
-#define EXPORT_SYMBOL_GPL(sym)
-#define EXPORT_SYMBOL_GPL_FUTURE(sym)
-#define EXPORT_UNUSED_SYMBOL(sym)
-#define EXPORT_UNUSED_SYMBOL_GPL(sym)
-
-#endif /* CONFIG_MODULES */
#endif /* !__ASSEMBLY__ */

#endif /* _LINUX_EXPORT_H */
diff --git a/scripts/genksyms/keywords.c b/scripts/genksyms/keywords.c
index c586d32dd2c3..7a85c4e21175 100644
--- a/scripts/genksyms/keywords.c
+++ b/scripts/genksyms/keywords.c
@@ -3,11 +3,7 @@ static struct resword {
const char *name;
int token;
} keywords[] = {
- { "EXPORT_SYMBOL", EXPORT_SYMBOL_KEYW },
- { "EXPORT_SYMBOL_GPL", EXPORT_SYMBOL_KEYW },
- { "EXPORT_SYMBOL_GPL_FUTURE", EXPORT_SYMBOL_KEYW },
- { "EXPORT_UNUSED_SYMBOL", EXPORT_SYMBOL_KEYW },
- { "EXPORT_UNUSED_SYMBOL_GPL", EXPORT_SYMBOL_KEYW },
+ { "__GENKSYMS_EXPORT_SYMBOL", EXPORT_SYMBOL_KEYW },
{ "__asm", ASM_KEYW },
{ "__asm__", ASM_KEYW },
{ "__attribute", ATTRIBUTE_KEYW },
--
2.17.1
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