RETPOLINE

我的任务是移植系统，将CentOS5.8的运用程序和驱动在CentOS6.8中重新编译并需要正常运行，在此过程中遇到一个模块加载的问题，特此记录下来

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问题如下：

我的内核模块名为switcher.ko

进入到内核模块文件夹目录中，开始加载模块

insmod switcher.ko

提示错误

insmod: error inserting 'switcher.ko': -1 Unknown symbol in module

shell窗口下没有提示其他错误信息，因为我加了printk打印，所以这时候我查看系统日志信息，获取模块加载失败原因

tail -f /var/log/messages

获取到如下信息

Jan 21 10:26:12 etc kernel: switcher: Unknown symbol __x86_indirect_thunk_edx
Jan 21 10:26:12 etc kernel: switcher: Unknown symbol __x86_indirect_thunk_ecx
Jan 21 10:26:12 etc kernel: switcher: Unknown symbol __x86_indirect_thunk_esi
Jan 21 10:26:12 etc kernel: switcher: Unknown symbol __x86_indirect_thunk_eax
Jan 21 10:26:12 etc kernel: switcher: Unknown symbol __x86_indirect_thunk_ebx

经过查询获知，这个是通过内核连接获取到模块

arch/x86/include/asm/asm-prototypes.h:15:#define INDIRECT_THUNK(reg) extern asmlinkage void __x86_indirect_thunk_r ## reg(void);

查询INDIRECT_THUNK可以了解该定义域CONFIG_RETPOLINE宏定义有关

retpoline是什么东西？

我百度了一下，好像是在某个内核版本之后添加封装接口，每次调用特定接口时会通过该retpoline进行安全校验，具体的不清楚，目前来说我的内核即使开启了该内核选项还是提示该错误

开启该内核选项

cd /lib/modules/2.6.32-642.el6.i686/build
make menuconfig
Processor type and features  --->
    Avoid speculative indirect branches in kernel 

查看该内核模块信息

modinfo switcher.ko
filename:       switcher.ko
description:    Support to control 2 Marvel 88E6097 switch chips
author:         dongyun,gktel.com.cn
license:        GPL
retpoline:      Y
srcversion:     EDB1348DCA1EA7250B98486
depends:        lpcio
vermagic:       2.6.32-696.30.1.el6.i686 SMP mod_unload modversions 686 

retpoline一直为Y状态，通过在该模块编译环境中了解

gcc -Wp,-MD,/home/whoami/come_drivers/marvell-switch/DSDT_2.8b/src/platform/.gtMiiSmiIf.o.d  -nostdinc -isystem /usr/lib/gcc/i686-redhat-linux/4.4.7/include -Iinclude  -I/usr/src/kernels/2.6.32-696.30.1.el6.i686/include/uapi -I/usr/src/kernels/2.6.32-696.30.1.el6.i686/arch/x86/include -Iarch/include/generated -Iinclude -include /usr/src/kernels/2.6.32-696.30.1.el6.i686/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mpreferred-stack-boundary=2 -march=i686 -mtune=generic -Wa,-mtune=generic32 -ffreestanding -fstack-protector -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_AVX=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mindirect-branch=thunk-extern -mindirect-branch-register -DRETPOLINE -Wframe-larger-than=1024 -Wno-unused-but-set-variable -fomit-frame-pointer -g -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fno-dwarf2-cfi-asm -fconserve-stack  -DMODULE -D"KBUILD_STR(s)=\#s" -D"KBUILD_BASENAME=KBUILD_STR(gtMiiSmiIf)"  -D"KBUILD_MODNAME=KBUILD_STR(switcher)" -D"DEBUG_HASH=14" -D"DEBUG_HASH2=61" -c -o /home/whoami/come_drivers/marvell-switch/DSDT_2.8b/src/platform/.tmp_gtMiiSmiIf.o /home/whoami/come_drivers/marvell-switch/DSDT_2.8b/src/platform/gtMiiSmiIf.c

编译该模块时使用了-DRETPOLINE

我的修改方法

# Avoid indirect branches in kernel to deal with Spectre
ifdef CONFIG_RETPOLINE
    RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
    ifneq ($(RETPOLINE_CFLAGS),)
        # KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
    else
        # Fail brew build, but let other users proceed with warning
        RETPOLINE_MSG := CONFIG_RETPOLINE=y, but not supported by the compiler. Toolchain update recommended.
        KBUILD_USER := $(shell id -un)
        ifeq ($(KBUILD_USER), mockbuild)
            $(error $(RETPOLINE_MSG))
        else
            $(warning $(RETPOLINE_MSG))
        endif
    endif
endif

注释 KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE，使其编译的时候不使用-DRETPOLINE

然后重新编译模块，没有在提示上面的错误，模块能够正常加载

modinfo switcher.ko 
filename:       switcher.ko
description:    Support to control 2 Marvel 88E6097 switch chips
author:         dongyun,gktel.com.cn
license:        GPL
srcversion:     EDB1348DCA1EA7250B98486
depends:        lpcio
vermagic:       2.6.32-696.30.1.el6.i686 SMP mod_unload modversions 686