提交Linux内核Patch

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上一篇博客使用QEMU和GDB调试Linux内核，最后使用内核提供的GDB扩展功能获取当前运行进程，发现内核已经不再使用thread_info获取当前进程，而是使用Per-CPU变量。而且从内核4.9版本开始，thread_info也不再位于内核栈底部，然而内核提供的辅助调试函数lx_thread_info()仍然通过内核栈底地址获取thread_info，很明显这是个Bug，于是决定将其修复并提交一个内核Patch，提交后很快就得到内核维护人员的回应，将Patch提交到了内核主分支。

Linux内核Patch提交还是采用邮件列表方式，不过提供了自动化工具。

Bug修复

Bug的原因已经很明确了，先看下问题代码scripts/gdb/linux/tasks.py：

def get_thread_info(task):
    thread_info_ptr_type = thread_info_type.get_type().pointer()
    if utils.is_target_arch("ia64"):
        ...
    else:
        thread_info = task['stack'].cast(thread_info_ptr_type)
    return thread_info.dereference()

还是使用的老的流程，通过栈底地址获取thread_info。

从内核4.9版本开始，已将thread_info移到了task_struct(include\linux\sched.h)，而且一定是第一个字段：

struct task_struct {
#ifdef CONFIG_THREAD_INFO_IN_TASK
    /*
     * For reasons of header soup (see current_thread_info()), this
     * must be the first element of task_struct.
     */
    struct thread_info      thread_info;
#endif
    ...
}

所以修复很简单，只需要判断task的第一个字段是否为thread_info，如果是，则直接将其返回；如果不是，还是走原先的流程：

$ git diff ./
diff --git a/scripts/gdb/linux/tasks.py b/scripts/gdb/linux/tasks.py
index 1bf949c43b76..f6ab3ccf698f 100644
--- a/scripts/gdb/linux/tasks.py
+++ b/scripts/gdb/linux/tasks.py
@@ -96,6 +96,8 @@ def get_thread_info(task):
         thread_info_addr = task.address + ia64_task_size
         thread_info = thread_info_addr.cast(thread_info_ptr_type)
     else:
+        if task.type.fields()[0].type == thread_info_type.get_type():
+            return task['thread_info']
         thread_info = task['stack'].cast(thread_info_ptr_type)
     return thread_info.dereference()

Git配置

添加用户和Email配置，用于git send-email发送Patch。

这里使用Gmail邮箱服务，在Linux项目.git/config配置中添加如下内容：

[user]
    name = Xi Kangjie
    email = imxikangjie@gmail.com
[sendemail]
    smtpEncryption = tls
    smtpServer = smtp.gmail.com
    smtpUser = imxikangjie@gmail.com
    smtpServerPort = 587

注意在Google账户配置中允许不够安全的应用登陆，否则后面发送Patch会收到如下警告：

Patch生成

Bug修复后，先检查下代码是否符合规范：

$ ./scripts/checkpatch.pl --file scripts/gdb/linux/tasks.py
total: 0 errors, 0 warnings, 137 lines checked

scripts/gdb/linux/tasks.py has no obvious style problems and is ready for submission.

没问题就可以写提交日志了：

$ git add scripts/gdb/linux/tasks.py
$ git commit -s

-s自动添加签发人Signed-off-by: Xi Kangjie <imxikangjie@gmail.com>，表示该Patch是你创建的，你会对该Patch负责。日志的第一行为简短描述，会成为邮件标题（Subject），之后空一行，添加详细描述，会成为邮件内容，再空一行，添加签发人。

将最近一次提交生成Patch：

$ git format-patch HEAD~
0001-scripts-gdb-fix-get_thread_info.patch

再次检查Patch是否符合规范：

$ ./scripts/checkpatch.pl 0001-scripts-gdb-fix-get_thread_info.patch
ERROR: Please use git commit description style 'commit <12+ chars of sha1> ("<title line>")' - ie: 'commit c65eacbe290b ("sched/core: Allow putting thread_info into task_struct")'
#10:
- c65eacbe290b (sched/core: Allow putting thread_info into task_struct)

ERROR: Please use git commit description style 'commit <12+ chars of sha1> ("<title line>")' - ie: 'commit 15f4eae70d36 ("x86: Move thread_info into task_struct")'
#11:
- 15f4eae70d36 (x86: Move thread_info into task_struct)

total: 2 errors, 0 warnings, 8 lines checked

NOTE: For some of the reported defects, checkpatch may be able to
      mechanically convert to the typical style using --fix or --fix-inplace.

0001-scripts-gdb-fix-get_thread_info.patch has style problems, please review.

NOTE: If any of the errors are false positives, please report
      them to the maintainer, see CHECKPATCH in MAINTAINERS.

看来格式有错误，引用的提交描述不符合规范，直接修改Patch文件，再次检查：

$ ./scripts/checkpatch.pl 0001-scripts-gdb-fix-get_thread_info.patch
total: 0 errors, 0 warnings, 8 lines checked

0001-scripts-gdb-fix-get_thread_info.patch has no obvious style problems and is ready for submission.

Patch提交

获取Patch相关维护人员：

$ ./scripts/get_maintainer.pl 0001-scripts-gdb-fix-get_thread_info.patch
Jan Kiszka <jan.kiszka@siemens.com> (supporter:GDB KERNEL DEBUGGING HELPER SCRIPTS)
Kieran Bingham <kieran@bingham.xyz> (supporter:GDB KERNEL DEBUGGING HELPER SCRIPTS)
Xi Kangjie <imxikangjie@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:2/2=100%)
linux-kernel@vger.kernel.org (open list)

发送Patch:

$ git send-email --to jan.kiszka@siemens.com --to kieran@bingham.xyz --cc linux-kernel@vger.kernel.org 0001-scripts-gdb-fix-get_thread_info.patch
0001-scripts-gdb-fix-get_thread_info.patch
(mbox) Adding cc: Xi Kangjie <imxikangjie@gmail.com> from line 'From: Xi Kangjie <imxikangjie@gmail.com>'
(body) Adding cc: Xi Kangjie <imxikangjie@gmail.com> from line 'Signed-off-by: Xi Kangjie <imxikangjie@gmail.com>'

From: Xi Kangjie <imxikangjie@gmail.com>
To: jan.kiszka@siemens.com,
        kieran@bingham.xyz
Cc: linux-kernel@vger.kernel.org,
        Xi Kangjie <imxikangjie@gmail.com>
Subject: [PATCH] scripts/gdb: fix get_thread_info
Date: Thu, 18 Jan 2018 21:01:59 +0000
Message-Id: <20180118210159.17223-1-imxikangjie@gmail.com>
X-Mailer: git-send-email 2.13.2

    The Cc list above has been expanded by additional
    addresses found in the patch commit message. By default
    send-email prompts before sending whenever this occurs.
    This behavior is controlled by the sendemail.confirm
    configuration setting.

    For additional information, run 'git send-email --help'.
    To retain the current behavior, but squelch this message,
    run 'git config --global sendemail.confirm auto'.

Send this email? ([y]es|[n]o|[q]uit|[a]ll): y
Password for 'smtp://imxikangjie@gmail.com@smtp.gmail.com:587':
OK. Log says:
Server: smtp.gmail.com
MAIL FROM:<imxikangjie@gmail.com>
RCPT TO:<jan.kiszka@siemens.com>
RCPT TO:<kieran@bingham.xyz>
RCPT TO:<linux-kernel@vger.kernel.org>
RCPT TO:<imxikangjie@gmail.com>
From: Xi Kangjie <imxikangjie@gmail.com>
To: jan.kiszka@siemens.com,
        kieran@bingham.xyz
Cc: linux-kernel@vger.kernel.org,
        Xi Kangjie <imxikangjie@gmail.com>
Subject: [PATCH] scripts/gdb: fix get_thread_info
Date: Thu, 18 Jan 2018 21:01:59 +0000
Message-Id: <20180118210159.17223-1-imxikangjie@gmail.com>
X-Mailer: git-send-email 2.13.2

Result: 250 2.0.0 OK 1516281059 v9sm14814354pfj.88 - gsmtp

提交成功后，就能在内核邮件列表中看到自己的邮件[PATCH] scripts/gdb: fix get_thread_info，以及维护人员的回复Re: [PATCH] scripts/gdb: fix get_thread_info。

Linux内核被划分成不同的子系统，如网络、内存管理等，不同的子系统有相应的维护人员，一个Patch会首先提交到子系统分支，再被维护人员提交到主分支。我的Patch被提交到了mm-tree（维护人员是Andrew Morton），见mm-commits邮件列表scripts-gdb-fix-get_thread_info.patch added to -mm tree，Andrew Morton确认没问题后，会将Patch发送给Linus Torvalds，见mm-commits[patch 4/6] scripts/gdb/linux/tasks.py: fix get_thread_info，我的Patch还被发送给了stable分支，见stable邮件列表[patch 4/6] scripts/gdb/linux/tasks.py: fix get_thread_info。

最终由Linus Torvalds将Patch合并到主分支，scripts/gdb/linux/tasks.py: fix get_thread_info。

看到自己的代码在世界的某个角落运转，推动世界向前发展，才是真正的享受。

参考：