error: macro "__TIME__" might prevent reproducible builds [-Werror=date-time]
1.问题
在进行内核模块编译的时候突然提示如下错误：
/work/kernel/module.c:136:86: error: macro "__TIME__" might prevent reproducible builds [-Werror=date-time]
MODULE_DESCRIPTION("compiled by GCC " __VERSION__ " build on " __TIME__ " " __DATE__);
/work/dte/src/kernel/module.c:136:86: error: macro "__DATE__" might prevent reproducible builds [-Werror=date-time]
1.1开发环境
开发环境如下图所示
2.定位
首先看到错误日志就能确认在内核模块中引用了如下代码“MODULE_DESCRIPTION("compiled by GCC " __VERSION__ " build on " __TIME__ " " __DATE__);”
在4.9(含)的gcc版本中添加了-Wdate-time告警，如果在内核模块代码中使用了__DATE__, __TIME__, or __TIMESTAMP__这几个宏，会产生这几种错误；
点击打开链接
[PATCH 7/7] Makefile: Build with -Werror=date-time if the compilersupports it
From: Josh TriplettDate: Mon Dec 23 2013 - 16:56:19 EST
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GCC 4.9 and newer have a new warning -Wdate-time, which warns on any use
of __DATE__, __TIME__, or __TIMESTAMP__, which would make the build
non-deterministic. Now that the kernel does not use any of those
macros, turn on -Werror=date-time if available, to keep it that way.
The kernel already (optionally) records this information at build time
in a single place; other kernel code should not duplicate that.
Signed-off-by: Josh Triplett
---
Makefile | 3 +++
1 file changed, 3 insertions(+)
diff --git a/Makefile b/Makefile
index 14d592c..188eea7 100644
--- a/Makefile
+++ b/Makefile
@@ -668,6 +668,9 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=implicit-int)
# require functions to have arguments in prototypes, not empty 'int foo()'
KBUILD_CFLAGS += $(call cc-option,-Werror=strict-prototypes)
+# Prohibit date/time macros, which would make the build non-deterministic
+KBUILD_CFLAGS += $(call cc-option,-Werror=date-time)
+
# use the deterministic mode of AR if available
KBUILD_ARFLAGS := $(call ar-option,D)
--
1.8.5.2
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3.解决方法
解决方法有如下几种，可根据需求进行选择
第一种解决方法 在makefile中添加如下选项
EXTRA_CFLAGS += -Wno-error=date-time
EXTRA_CFLAGS +=-Wno-date-time
注意：在第二段定位中已经说到只有在4.9以及之后的gcc版本中才有这两个选项，如果你在gcc版本小于4.9的系统上编译，比如在centos7.2系统中，centos中默认的gcc版本是4.8.5，编译时就会报"cc1: 错误：-Werror=date-time：没有选项 -Wdate-time"错误，所以这种解决方法如果只是使用4.9以上的gcc版本是没有问题的；
第二种解决方法
修改/lib/modules/'uname -r'/build/Makefile文件，我的是/lib/modules/4.4.0-116-generic/build/Makefile
将KBUILD_CFLAGS += $(call cc-option,-Werror=date-time)这一行注释掉
注意:这种方法虽然解决了这个问题，但是修改了系统本身的属性，不太推荐
第三种解决方法
这一种方法也是我比较推荐的方法，修改内核模块的makefile文件，添加一行
ccflags-y+=$(shell if [ $(call cc-version) -ge 0490 ] ; then echo "-Wno-error=date-time -Wno-date-time"; fi ;)
在linux内核的最顶层目录也就是根目录下面的Makefile文件中，有一个变量"KBUILD_CFLAGS"，这个$(KBUILD_CFLAGS)中存放的是传递给gcc编译器的编译选项，如果被编译的文件在被gcc编译时，没有特殊的要求的话就会使用$(KBUILD_CFLAGS)中的编译选项。
但是如果被gcc编译的文件有特殊的编译选项要求的话，使用如下的方式来指定编译选项：
ccflags-y,是指定给$(CC)的编译选项；
asflags-y,是指定给$(AS)的编译选项；
ldflags-y,是指定给$(LD)的编译选项；