KERNEL_MOUNT(9)

💅 KERNEL_MOUNT(9) 💅

okay so this is the KERNEL_MOUNT(9) page from the Kernel Developer's Manual, and it's lowkey iconic: it's the kernel-side API for building mount arguments. every single detail is staying, no cap. 🫶

🌸 NAME 🌸

free_mntarg, kernel_mount, mount_arg, mount_argb, mount_argf, mount_argsu — functions provided as part of the kernel mount interface

✨ SYNOPSIS ✨

void
free_mntarg(struct mntarg *ma);

int
kernel_mount(struct mntarg *ma, int flags);

struct mntarg *
mount_arg(struct mntarg *ma, const char *name, const void *val,
    int len);

struct mntarg *
mount_argb(struct mntarg *ma, int flag, const char *name);

struct mntarg *
mount_argf(struct mntarg *ma, const char *name, const char *fmt, ...);

struct mntarg *
mount_argsu(struct mntarg *ma, const char *name, const void *val,
    int len);

🦋 DESCRIPTION 🦋

Okay so the kernel_mount() family of functions are provided as an API for building a list of mount arguments which will be used to mount file systems from inside the kernel. By accumulating a list of arguments, the API takes shape and provides the information necessary for the kernel to control the mount(8) utility. When an error occurs, the process will stop. This will not cause a panic(9). 💖

The header of the structure is stored in src/sys/kern/vfs_mount.c which permits automatic structure creation to ease the mount process. Memory allocation must always be freed when the entire process is complete, it is an error otherwise. 🧠

• free_mntarg() — used to free or clear the mntarg structure. 🧹
• kernel_mount() — pulls information from the structure to perform the mount request on a given file system. Additionally, the kernel_mount() function always calls the free_mntarg() function. If ma contains any error code generated during the construction, that code will be called and the file system mount will not be attempted. 🚫
• mount_arg() — takes a plain argument and crafts parts of the structure with regards to various mount options. If the length is a value less than 0, strlen(3) is used. This argument will be referenced until either free_mntarg() or kernel_mount() is called. 🔗
• mount_argb() — used to add boolean arguments to the structure. The flag is the boolean value and name must start with "no", otherwise a panic will occur. 😱
• mount_argf() — adds printf(9) style arguments to the current structure. 🖨️
• mount_argsu() — will add arguments to the structure from a userland string. 📝

⚠️ fr fr: mount_argb() is not playing around — if name does not start with "no", a panic will occur. that's not a vibe, it's a rule. 💀

🎀 EXAMPLES 🎀

An example of the *_cmount() function:

static int
msdosfs_cmount(struct mntarg *ma, void *data, int flags, struct thread *td)
{
        struct msdosfs_args args;
        int error;

        if (data == NULL)
                return (EINVAL);
        error = copyin(data, &args, sizeof(args));
        if (error)
                return (error);

        ma = mount_argsu(ma, "from", args.fspec, MAXPATHLEN);
        ma = mount_arg(ma, "export", &args.export, sizeof(args.export));
        ma = mount_argf(ma, "uid", "%d", args.uid);
        ma = mount_argf(ma, "gid", "%d", args.gid);
        ma = mount_argf(ma, "mask", "%d", args.mask);
        ma = mount_argf(ma, "dirmask", "%d", args.dirmask);

        ma = mount_argb(ma, args.flags & MSDOSFSMNT_SHORTNAME, "noshortname");
        ma = mount_argb(ma, args.flags & MSDOSFSMNT_LONGNAME, "nolongname");
        ma = mount_argb(ma, !(args.flags & MSDOSFSMNT_NOWIN95), "nowin95");
        ma = mount_argb(ma, args.flags & MSDOSFSMNT_KICONV, "nokiconv");

        ma = mount_argsu(ma, "cs_win", args.cs_win, MAXCSLEN);
        ma = mount_argsu(ma, "cs_dos", args.cs_dos, MAXCSLEN);
        ma = mount_argsu(ma, "cs_local", args.cs_local, MAXCSLEN);

        error = kernel_mount(ma, flags);

        return (error);
}

🌈 SEE ALSO 🌈

• VFS(9)
• VFS_MOUNT(9)

⭐ HISTORY ⭐

The kernel_mount() family of functions and this manual page first appeared in FreeBSD 6.0.

👑 AUTHORS 👑

The kernel_mount() family of functions and API was developed by Poul-Henning Kamp <phk@FreeBSD.org>. This manual page was written by Tom Rhodes <trhodes@FreeBSD.org>.

xoxo, your Debian girly 💖 — this KERNEL_MOUNT(9) page is dated November 20, 2021, and we are signing off. 🎀✨