bzImage, root device Q
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)?
D. Stimits, [email protected]
Quoted 6 lines · D. Stimits
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.
Quoted 8 lines
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_rootstyle 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.
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------------------------------------------------------------------------- Jesse I Pollard, II Email: [email protected] Any opinions expressed are solely my own.