The kernel and some modules make many symbols available for other modules to use via EXPORT_SYMBOL() and variants. Depending on the set of modules being selected in your kernel configuration, many of those exported symbols might never be used. One direct effect of EXPORT_SYMBOL() entries is to force the referenced code and the code it references into the kernel binary with no possibilityes for dead code elimination, eeven though the referenced code is never executed. This patch series provides the option to omit those exported symbols from the kernel and modules that are never referenced by any of the selected modules in the current kernel configuration. In turns this allows for optimizing the compiled code and reducing final binaryes' size. When using LTO the binari size reduction is even more effective. Also, by removing some entry points, it could be argued that this has some security advantages by reducing the attack surface of the kernel. But this is not my main motivation for doing this so I'll let security experts judge the merit of this claim. Here's some numbers showing the effect of this series. Kernel v4.5-rc2 x86 defconfig: $ size vmlinux text data bss dec hex filename 12362563 1856456 1101824 15320843 e9c70b vmlinux $ wc -l Module.symvers 8806 Module.symvers Kernel v4.5-rc2 x86 defconfig + CONFIG_TRIM_UNUSED_EXPSYMS=y: $ size vmlinux text data bss dec hex filename 12059848 1856456 1101824 15018128 e52890 vmlinux $ wc -l Module.symvers 225 Module.symvers Because the x86 defconfig only contains 18 modules, the number of needed exported symbols is only 225 out of a possible 8806 for this configuration. The kernel text size shrank by about 2.4%. More numbers, on ARM this time. Kernel v4.5-rc2 arm defconfig (same as multi_v7_defconfig): $ size vmlinux text data bss dec hex filename 13664222 1554068 351368 15569658 ed92fa vmlinux $ wc -l Module.symvers 10044 Module.symvers Kernel v4.5-rc2 arm defconfig + CONFIG_TRIM_UNUSED_EXPSYMS=y: $ size vmlinux text data bss dec hex filename 13255051 1554132 351240 15160423 e75467 vmlinux $ wc -l Module.symvers 2703 Module.symvers This time many more modules (279 of them) are part of the build configuration. Still, only 2703 out of 10044 exported symbols are required. And despite a smaller number of omitted exports, the kernel shrank by 3%. Now let's add LTO into the mix. Kernel v4.5-rc2 arm defconfig + LTO: $ size vmlinux text data bss dec hex filename 12813766 1538324 344356 14696446 e03ffe vmlinux $ wc -l Module.symvers 8415 Module.symvers Kernel v4.5-rc2 arm defconfig + LTO + CONFIG_TRIM_UNUSED_EXPSYMS=y: $ size vmlinux text data bss dec hex filename 12197437 1536052 338660 14072149 d6b955 vmlinux $ wc -l Module.symvers 1742 Module.symvers This time the kernel shrank by 5%. Finally, let's have a look at a configuration that is potentially more representative of an embedded target. Kernel v4.5-rc2 arm realview_defconfig + LTO: $ size vmlinux text data bss dec hex filename 4422942 209640 126880 4759462 489fa6 vmlinux $ wc -l Module.symvers 5597 Module.symvers Kernel v4.5-rc2 arm realview_defconfig + LTO + CONFIG_TRIM_UNUSED_EXPSYMS=y: $ size vmlinux text data bss dec hex filename 3823485 205416 125800 4154701 3f654d vmlinux $ wc -l Module.symvers 52 Module.symvers Here we reduced the kernel text by about 13.6%. Disabling module support altogether would reduce it by 13.9% i.e. only 0.3% difference. This means the overhead of using modules on embedded targets is greatly reduced. I know that Viro posted a series allowing for assembly code to declare exported symbols but this doesn't conflict (we don't touch the same files) and making this work with his series is very trivial. diffstat: Makefile | 14 +++++++ include/linux/export.h | 22 ++++++++++- init/Kconfig | 16 ++++++++ scripts/Kbuild.include | 3 +- scripts/Makefile.build | 16 ++++++-- scripts/adjust_expsyms.sh | 97 +++++++++++++++++++++++++++++++++++++++++++++++ scripts/basic/fixdep.c | 80 +++++++++++++++++++++++++++++--------- 7 files changed, 224 insertions(+), 24 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
