From 317686c6733fbec79ccdceaa3b9a4f45fa9f77ae Mon Sep 17 00:00:00 2001 From: jianbosun Date: Sat, 11 Apr 2015 20:38:48 +0800 Subject: [PATCH] Add more explanation for nsenter Now the README in nsenter is a little confused and hard for new commings to understand. Signed-off-by: Sun Jianbo --- nsenter/README.md | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-) diff --git a/nsenter/README.md b/nsenter/README.md index ac94cba05..d1a60ef98 100644 --- a/nsenter/README.md +++ b/nsenter/README.md @@ -1,6 +1,25 @@ ## nsenter -The `nsenter` package registers a special init constructor that is called before the Go runtime has -a chance to boot. This provides us the ability to `setns` on existing namespaces and avoid the issues -that the Go runtime has with multiple threads. This constructor is only called if this package is -registered, imported, in your go application and the argv 0 is `nsenter`. +The `nsenter` package registers a special init constructor that is called before +the Go runtime has a chance to boot. This provides us the ability to `setns` on +existing namespaces and avoid the issues that the Go runtime has with multiple +threads. This constructor will be called if this package is registered, +imported, in your go application. + +The `nsenter` package will `import "C"` and it uses [cgo](https://golang.org/cmd/cgo/) +package. In cgo, if the import of "C" is immediately preceded by a comment, that comment, +called the preamble, is used as a header when compiling the C parts of the package. +So every time we import package `nsenter`, the C code function `nsexec()` would be +called. And package `nsenter` is now only imported in Docker execdriver, so every time +before we call `execdriver.Exec()`, that C code would run. + +`nsexec()` will first check the environment variable `_LIBCONTAINER_INITPID` +which will give the process of the container that should be joined. Namespaces fd will +be found from `/proc/[pid]/ns` and set by `setns` syscall. + +And then get the pipe number from `_LIBCONTAINER_INITPIPE`, error message could +be transfered through it. If tty is added, `_LIBCONTAINER_CONSOLE_PATH` will +have value and start a console for output. + +Finally, `nsexec()` will clone a child process , exit the parent process and let +the Go runtime take over.