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+title:Writing linux mount utility
+keywords:c,linux,kernel,mount
+
+# Writing linux mount utility
+
+## Intro
+
+For long time wanted to understand how all stuff works on system level with linux.
+From one side mounting looked like some sort of magic. Decided to write my
+own mount utility. As its looks like only one syscall is needed to do that sys_mount.
+Other part of it is to see how to get all different file systems and find
+options that are possible to pass to mounted partition. All of this also preparations
+to write my own linux userspace and to have non-Linux distro. Main inspiration
+taken form arsv/minibase soure code.
+
+## Syscalls
+
+To implement mount there is enought just with one syscall mount. Its quite simple use case with just 
+passing param to syscall is enought. There is more time spent to figure out with params are supported 
+by each of filesystems.
+
+```c
+int smount(
+    const char *source, 
+    const char *target,
+    const char *filesystemtype,
+    unsigned long mountflags,
+    const void *data)
+{
+    int ret=-1;
+    ret = syscall(SYS_mount,source,target,filesystemtype,mountflags,data);
+    return ret;
+}
+```
+
+Comparison to mount utility is straigforward.
+
+#### mount arguments
+| Param | Descriptio  |
+|---|---|
+| source         | Source directory or file to mount |
+| target         | Target directory where to mount |
+| filesystemtype | filesystem type |
+| mountflags     | mount params |
+| data           | specific to filesystem |
+
+### Supported filesystems
+
+Rund command to find filesystems supported by currently running kernel
+
+```
+cat /proc/filesystems
+``` 
+
+### Mount flags
+
+Mount flags that can be passed when mounting filesystems, those are taken from kernel definition
+
+```c
+#define _MS_RDONLY       (1<<0)
+#define _MS_NOSUID       (1<<1)
+#define _MS_NODEV        (1<<2)
+#define _MS_NOEXEC       (1<<3)
+#define _MS_SYNCHRONOUS  (1<<4)
+#define _MS_REMOUNT      (1<<5)
+#define _MS_MANDLOCK     (1<<6)
+#define _MS_DIRSYNC      (1<<7)
+#define _MS_NOATIME      (1<<10)
+#define _MS_NODIRATIME   (1<<11)
+#define _MS_BIND         (1<<12)
+#define _MS_MOVE         (1<<13)
+#define _MS_REC          (1<<14)
+#define _MS_SILENT       (1<<15)
+#define _MS_POSIXACL     (1<<16)
+#define _MS_UNBINDABLE   (1<<17)
+#define _MS_PRIVATE      (1<<18)
+#define _MS_SLAVE        (1<<19)
+#define _MS_SHARED       (1<<20)
+#define _MS_RELATIME     (1<<21)
+#define _MS_KERNMOUNT    (1<<22)
+#define _MS_I_VERSION    (1<<23)
+#define _MS_STRICTATIME  (1<<24)
+#define _MS_LAZYTIME     (1<<25)
+```
+
+
+
+## Kernel implementation
+
+Lates linux kernel source code where mount syscal is define is  
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/namespace.c  
+
+There is 2 definitions one for syscall and one for mount code
+```c
+SYSCALL_DEFINE5(mount, char user *, dev_name, char user *, dir_name,
+		char user *, type, unsigned long, flags, void user *, data)
+
+long do_mount(
+	const char *dev_name, 
+	const char __user *dir_name,
+	const char *type_page, 
+	unsigned long flags, 
+	void *data_page);
+```
+
+## Filesystems
+
+There is allways a mistery where all parameters that are suppported by particular filesystem comes from.
+I spend some time to find all params that are supported for procfs,devtmps and tempfs.
+
+### Procfs
+
+| Param | Description |
+|---|---|
+| hidepid | __0__ - Everybody may access all proc, __1__ - User can access only their own proc, not other /proc/[pid], __2__ - As for mode 1, extra hides pid directories of others | 
+| gid | usergroup who sees /proc in mode 0 |
+
+### Devtmpfs
+
+
+
+### Tempfs
+
+
+## Implementation
+
+
+## Links
+
+http://git.main.lv/cgit.cgi/nmount.git/  
+https://man7.org/linux/man-pages/man7/signal.7.html  
+https://man7.org/linux/man-pages/man2/mount.2.html  
+https://github.com/arsv/minibase/blob/master/src/rootfs/kmount.c  
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/namespace.c  
+[devtmpfs]  
+https://www.kernel.org/doc/Documentation/filesystems/sysfs.txt  
+https://elixir.bootlin.com/linux/v4.14.184/source/drivers/base/devtmpfs.c  
+https://man7.org/linux/man-pages/man7/file-hierarchy.7.html  
+[procsf]  
+https://www.man7.org/linux/man-pages/man5/proc.5.html  
+https://elixir.bootlin.com/linux/v4.14.184/source/fs/proc/root.c#L33  
+[tmpfs]  
+https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt  
+
+
+
+
+
+