Re: bzImage, root device Q

From: Jesse Pollard (
Date: Fri Jul 20 2001 - 05:46:52 EST

On Fri, 20 Jul 2001, D. Stimits wrote:
>When booting to a bzImage kernel, bytes 508 and 509 can be used to name
>the minor and major number of the intended root device (although it can
>be overridden with a command line parameter). Other characteristics are
>also available this way, through bytes in the kernel. rdev makes a
>convenient way to hex edit those bytes.
>What I'm more curious about is how does the kernel know what filesystem
>_type_ the root is? Are there similar bytes in the bzImage, and can rdev
>change this? And is there a command line syntax to allow specifying
>filesystem type (e.g., something like "vmlinuz root=/dev/scd0,iso9660"
>or "vmlinuz root=/dev/scd0,xfs")? Or is this limited in some way,
>requiring mount on one or a few known filesystem types ("linux native"
>subset comes to mind), followed by a chroot or pivot_root style command
>(which in turn means no direct root mount of some filesystem types)?

Take a look at fs/super.c - function mount_root().

It reads the file system superblock (from the major/minor specified root
device) and determines the filesystem from that. There is a loop that
cycles through all known (ie built in) file systems until one works.

If none do, then it panics.

Jesse I Pollard, II

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