linux-kernel - [PATCH 2/4] export.h: make __ksymtab

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Date:   Fri, 26 Feb 2021 01:02:44 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, Nicolas Pitre <nico@...xnic.net>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: [PATCH 2/4] export.h: make __ksymtab_strings per-symbol section

The export symbol tables are placed on own sections (__ksymtab*+<sym>)
and sorted by SORT (an alias of SORT_BY_NAME) because the module
subsystem uses the binary search for symbol resolution.

We did not have a good reason to do so for __ksymtab_strings, but
now I have.

To make CONFIG_TRIM_UNUSED_KSYMS work in one-pass, the linker needs
to trim unused strings of symbols and namespaces. To allow per-symbol
keep/drop choice, __ksymtab_strings must be placed on own sections.
Of course, SORT is unneeded here, though.

This keeps the string unification introduced by commit ce2b617ce8cb
("export.h: reduce __ksymtab_strings string duplication by using "MS"
section flags").

For example, the empty namespaces share the same address.

  $ nm -n
  [ snip ]
  ffffffff8233b53a r __kstrtabns_IO_APIC_get_PCI_irq_vector
  ffffffff8233b53a r __kstrtabns_I_BDEV
  ffffffff8233b53a r __kstrtabns_LZ4_decompress_fast
  ffffffff8233b53a r __kstrtabns_LZ4_decompress_fast_continue
  ffffffff8233b53a r __kstrtabns_LZ4_decompress_fast_usingDict
  ffffffff8233b53a r __kstrtabns_LZ4_decompress_safe
  ffffffff8233b53a r __kstrtabns_LZ4_decompress_safe_continue
    ...

I confirmed no size change in vmlinux.

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 include/asm-generic/export.h      | 2 +-
 include/asm-generic/vmlinux.lds.h | 2 +-
 include/linux/export.h            | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 07a36a874dca..e847f1fde367 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -39,7 +39,7 @@
 __ksymtab_\name:
 	__put \val, __kstrtab_\name
 	.previous
-	.section __ksymtab_strings,"aMS",%progbits,1
+	.section __ksymtab_strings+\name,"aMS",%progbits,1
 __kstrtab_\name:
 	.asciz "\name"
 	.previous
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index c54adce8f6f6..5a2b31890bb8 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -513,7 +513,7 @@
 									\
 	/* Kernel symbol table: strings */				\
         __ksymtab_strings : AT(ADDR(__ksymtab_strings) - LOAD_OFFSET) {	\
-		*(__ksymtab_strings)					\
+		*(__ksymtab_strings+*)					\
 	}								\
 									\
 	/* __*init sections */						\
diff --git a/include/linux/export.h b/include/linux/export.h
index 6271a5d9c988..01e6ab19b226 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -99,7 +99,7 @@ struct kernel_symbol {
 	extern const char __kstrtab_##sym[];					\
 	extern const char __kstrtabns_##sym[];					\
 	__CRC_SYMBOL(sym, sec);							\
-	asm("	.section \"__ksymtab_strings\",\"aMS\",%progbits,1	\n"	\
+	asm("	.section \"__ksymtab_strings+" #sym "\",\"aMS\",%progbits,1\n"	\
 	    "__kstrtab_" #sym ":					\n"	\
 	    "	.asciz 	\"" #sym "\"					\n"	\
 	    "__kstrtabns_" #sym ":					\n"	\
-- 
2.27.0