buildroot GNU strip分析

使用buildroot构建嵌入式系统带来很多方便。buildroot里使用了很多很好的技术，本文是学习使用buildroot遇到的问题及分析，希望对大家也有帮助。

我们查看一个object文件有not stripped和stripped，而buildroot编译出来的目标文件大多是stripped的。

output/build/test/test2             这个binary文件是not stripped     

output/target/usr/bin/test2      而这个binary文件是stripped，这是为什么呢？

1, GNU strip command

strip - Discard symbols from object files.

http://man7.org/linux/man-pages/man1/strip.1.html

2, buildroot strip选项

buildroot/Config.in

注意 BR2_STRIP_strip 默认是选中的

config BR2_ENABLE_DEBUG

    bool \"build packages with debugging symbols\"

    help

      Build packages with debugging symbols enabled. All libraries

      and binaries in the \'staging\' directory will have debugging

      symbols, which allows remote debugging even if libraries and

      binaries are stripped on the target. Whether libraries and

      binaries are stripped on the target is controlled by the

      BR2_STRIP_* options below.


config BR2_STRIP_strip

    bool \"strip target binaries\"

    depends on !BR2_PACKAGE_HOST_ELF2FLT

    default y

    help

      Binaries and libraries in the target filesystem will be

      stripped using the normal \'strip\' command. This allows to save

      space, mainly by removing debugging symbols. Debugging symbols

      on the target are needed for native debugging, but not when

      remote debugging is used.

makemenuconfig 可以看到选项情况

第3行定义 BR2_STRIP_strip ，你的buildroot使能了strip

第4行定义 BR2_STRIP_EXCLUDE_DIRS , test目录是strip的例外目录

3, buildroot strip的执行的准备工作

定义了执行strip时的命令和参数

1）STRIPCMD 和 STRIP_STRIP_DEBUG

定义在文件 buildroot/package/Makefile.in

ifeq ($(BR2_STRIP_strip),y)
STRIP_STRIP_DEBUG := --strip-debug
TARGET_STRIP = $(TARGET_CROSS)strip
STRIPCMD = $(TARGET_CROSS)strip --remove-section=.comment --remove-section=.note
else
TARGET_STRIP = /bin/true
STRIPCMD = $(TARGET_STRIP)
endif

STRIP_STRIP_DEBUG的意义：Remove debugging symbols only.

而 STRIPCMD 会删掉 .comment, .note 2个section

2） STRIP_FIND_CMD  

定义在文件 buildroot/Makefile

STRIP_FIND_CMD = find $(TARGET_DIR)

ifneq (,$(call qstrip,$(BR2_STRIP_EXCLUDE_DIRS)))

STRIP_FIND_CMD += \\( $(call finddirclauses,$(TARGET_DIR),$(call qstrip,$(BR2_STRIP_EXCLUDE_DIRS))) \\) -prune -o

endif

STRIP_FIND_CMD += -type f \\( -perm /111 -o -name \'*.so*\' \\)

# file exclusions:

# - libpthread.so: a non-stripped libpthread shared library is needed for

#   proper debugging of pthread programs using gdb.

# - ld.so: a non-stripped dynamic linker library is needed for valgrind

# - kernel modules (*.ko): do not function properly when stripped like normal

#   applications and libraries. Normally kernel modules are already excluded

#   by the executable permission check above, so the explicit exclusion is only

#   done for kernel modules with incorrect permissions.

STRIP_FIND_CMD += -not \\( $(call findfileclauses,libpthread*.so* ld-*.so* *.ko $(call qstrip,$(BR2_STRIP_EXCLUDE_FILES))) \\) -print0

大概解读一下STRIP_FIND_CMD：

查找target目录下，除BR2_STRIP_EXCLUDE_DIRS目录和BR2_STRIP_EXCLUDE_FILES文件以外的，所有可执行的regular文件，或者文件名是*.so的文件

另外 libpthread*.so, ld-*.so, *.ko 也被看做是strip例外文件，注释里有解释。

4, buildroot strip的执行

buildroot/Makefile 有调用strip的语句

target-finalize: $(PACKAGES)

    $(STRIP_FIND_CMD) | xargs -0 $(STRIPCMD) 2>/dev/null || true

另外对 libpthread*.so, ld-*.so 执行 Remove debugging symbols only.

# See http://sourceware.org/gdb/wiki/FAQ, \"GDB does not see any threads

# besides the one in which crash occurred; or SIGTRAP kills my program when

# I set a breakpoint\"

ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),y)

    find $(TARGET_DIR)/lib/ -type f -name \'libpthread*.so*\' | \\

        xargs -r $(STRIPCMD) $(STRIP_STRIP_DEBUG)

endif


# Valgrind needs ld.so with enough information, so only strip

# debugging symbols.

    find $(TARGET_DIR)/lib/ -type f -name \'ld-*.so*\' | \\

        xargs -r $(STRIPCMD) $(STRIP_STRIP_DEBUG)

至此，你在output/target下看到的binary文件就是stripped的了