# Linux Kernel Module + GCOV

###### tags: `blog`

## 簡介

gcov 是一個測量程式執行 coverage 的工具，原理是編譯程式時在所有branch上插入紀錄函式
便可得知執行期間的執行狀況

通常 gcov 會用在一般 C/C++ 程式上，但是 Linux Kernel 其實也支援 gcov profiling，因此可以用 gcov 來紀錄 Linux Kernel 的 Coverage，當然 Kernel module 也不例外

這篇會分享用 gcov profiling Kernel module 的步驟

## 1. Kerenl 支援

先確定 Kernel 有支援 gcov profiling，可以在察看現在的 kernel 的 config 是否有以下參數，沒有的話要自己重新編譯一個

CONFIG_DEBUG_FS=y
CONFIG_GCOV_KERNEL=y

另外，若也想 profile 整個 Kenel， 則需要 `CONFIG_GCOV_PROFILE_ALL=y` 參數，注意該 kernel 會因此變慢不少，且並非所有硬體架構都支援

若 Kernel 支援，再確認是否已開啟 debugfs

ls /sys/kernel/debug/gcov

如果沒有就開啟

mount -t debugfs none /sys/kernel/debug

## 2. 修改 Makefile

`Use cp -P (capital P) to never traverse any symbolic link and copy the symbolic link instead.`

PWD=`pwd`

$ sudo ls -l sys/kernel/debug/gcov/$PWD
sample.gcda  sample.gcno

sample.gcno 會是 symbolic link

因為在 gcov 資料夾下權限很麻煩 搬回來處理
(cp -P 會直接複製 soft link)

$ sudo cp -P /sys/kernel/debug/gcov/$PWD/* .

## 用 gcov 產生可讀的檔案

$ sudo sudo gcov sample.gcda
File 'include/linux/kasan-checks.h'
Lines executed:0.00% of 2
Creating 'kasan-checks.h.gcov'
Cannot open source file include/linux/kasan-checks.h
...

$ vim sample.c.gcov

        1:   72:static int __init sample_init(void)
        -:   73:{
        1:   74:        printk("Loaded sample module\n");
        -:   75:
        1:   76:        ret_reg_dev = register_chrdev(42, "sample", &sample_ops);
        1:   77:        if (ret_reg_dev) {
    #####:   78:                printk("Error on register_chrdev\n");
    #####:   79:                return ret_reg_dev;
        -:   80:        }
        1:   81:        sample_class = class_create(THIS_MODULE, "sample");
        -:   82:
        2:   83:        if (IS_ERR(sample_class)) {
    #####:   84:                printk("Error on class_create\n");
    #####:   85:                return PTR_ERR(sample_class);
        -:   86:        }
        2:   87:        device =
        1:   88:                device_create(sample_class, NULL, MKDEV(42, 0), NULL, "sample");
        -:   89:
        2:   90:        if (IS_ERR(device)) {
    #####:   91:                printk("Error on device_create\n");
    #####:   92:                return PTR_ERR(device);
        -:   93:        }
        -:   94:
        -:   95:        return 0;
        -:   96:}
        -:   97:

# lcov

lcov 會

$ sudo geninfo  . -o ./coverage.info
Found gcov version: 8.3.0
Scanning . for .gcda files ...
Found 1 data files in .
Processing sample.gcda
Cannot open source file include/linux/kasan-checks.h
Cannot open source file include/linux/err.h
Cannot open source file include/linux/fs.h
...
geninfo: WARNING: skipping empty file arch#x86#include#asm#atomic.h.gcov
geninfo: WARNING: skipping empty file arch#x86#include#asm#jump_label.h.gcov
...
Finished .info-file creation

$ genhtml coverage.info -o html
Reading data file coverage.info
Found 1 entries.
Found common filename prefix "/home/pschen/sslab/kernel_module_example/gup"
Writing .css and .png files.
Generating output.
Processing file module/sample.c
Writing directory view page.
Overall coverage rate:
  lines......: 71.2% (37 of 52 lines)
  functions..: 85.7% (6 of 7 functions)

## 重置 profiling 結果
`sudo echo 1 > /sys/kernel/debug/gcov/resest`

## Reference

* http://ltp.sourceforge.net/coverage/lcov.php
* https://01.org/linuxgraphics/gfx-docs/drm/dev-tools/gcov.html
* http://techvolve.blogspot.com/2014/03/how-to-gcovlcov-for-linux-kernel-modules.html