linux-kernel - [PATCH 07/22] kbuild: lto: merge module sections

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Date:   Wed, 24 Jun 2020 13:31:45 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org, Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 07/22] kbuild: lto: merge module sections

LLD always splits sections with LTO, which increases module sizes. This
change adds a linker script that merges the split sections in the final
module and discards the .eh_frame section that LLD may generate.

Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 Makefile               |  2 ++
 scripts/module-lto.lds | 26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 scripts/module-lto.lds

diff --git a/Makefile b/Makefile
index ee66513a5b66..9ffec5fe1737 100644
--- a/Makefile
+++ b/Makefile
@@ -898,6 +898,8 @@ CC_FLAGS_LTO_CLANG += -fvisibility=default
 # Limit inlining across translation units to reduce binary size
 LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
 KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
+
+KBUILD_LDS_MODULE += $(srctree)/scripts/module-lto.lds
 endif
 
 ifdef CONFIG_LTO
diff --git a/scripts/module-lto.lds b/scripts/module-lto.lds
new file mode 100644
index 000000000000..65884c652bf2
--- /dev/null
+++ b/scripts/module-lto.lds
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
+ * -ffunction-sections, which increases the size of the final module.
+ * Merge the split sections in the final binary.
+ */
+SECTIONS {
+	__patchable_function_entries : { *(__patchable_function_entries) }
+
+	.bss : {
+		*(.bss .bss.[0-9a-zA-Z_]*)
+		*(.bss..L* .bss..compoundliteral*)
+	}
+
+	.data : {
+		*(.data .data.[0-9a-zA-Z_]*)
+		*(.data..L* .data..compoundliteral*)
+	}
+
+	.rodata : {
+		*(.rodata .rodata.[0-9a-zA-Z_]*)
+		*(.rodata..L* .rodata..compoundliteral*)
+	}
+
+	.text : { *(.text .text.[0-9a-zA-Z_]*) }
+}
-- 
2.27.0.212.ge8ba1cc988-goog