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added stuff for ext4

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Om Raheja 2024-08-18 14:50:11 -04:00
parent b11bfe8ed6
commit 09425c76cf
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writeup/3.md
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@ -14,3 +14,28 @@ Below that, we have a struct called `ext2fs_vfsops` which just contains values t
At this point, we should see where in the mounting process ext4 fails. Create an ext4 filesystem using qemu and mount it; make sure that the virtual machine manager marks it as a readonly filesystem before mounting it to ensure that it doesn't get corrupted (at least, not yet). At this point, we should see where in the mounting process ext4 fails. Create an ext4 filesystem using qemu and mount it; make sure that the virtual machine manager marks it as a readonly filesystem before mounting it to ensure that it doesn't get corrupted (at least, not yet).
Finally, on line 106 we have the `ext2fs_mountroot` function. This is where the bulk of our analysis will start.
```c
struct m_ext2fs *fs;
struct mount *mp;
struct proc *p = curproc; /* XXX */
struct ufsmount *ump;
int error;
/*
* Get vnodes for swapdev and rootdev.
*/
if (bdevvp(swapdev, &swapdev_vp) || bdevvp(rootdev, &rootvp))
panic("ext2fs_mountroot: can't setup bdevvp's");
```
So here we have to figure out what exactly the bdevvp function is.
From the OpenBSD man page:
> bdevvp() will create a vnode for a block device, and is used for the root device and swap areas, among other things.
I guess our question starts at, what exactly is a virtual node? It's nothing more than "an abstract ayer on top of a more concrete filesystem." (Wikipedia).
Meaning nothing more than what we have to do in order for there to be no difference when we run `ls`, `cd`, `rm`, `mkdir`, etc. Abstractions are necessary.