8.5.2 Path Traversal

Figure fig-vnode-fs-xyz also shows the new structure of file system X, after Y had been mounted, as file system Z.

The sequence of in-kernel operations to, say, read the file /usr/local/bin/tex would be as follows:

1.

The system call read() is executed. It begins by looking up the file.

2.

The generic lookup function performs a VOP_LOOKUP(rootvp, "usr"). It tries to look for the next component in the path, starting from the current lookup directory (root vnode).

3.

The lookup function is translated into ufs_lookup. The vnode X2 is found. Note that X2 is not the same vnode as Z2! X2 is hidden, while Z2 overshadows it.

4.

The lookup function now notices that X2's v_vfsmountedhere field is non-null, so it knows that X2 is a mount point. It calls the VOP_ROOT function on the vfs that is ``mounted here,'' that translates to nfs_lookup. This function returns the root vnode of the Y file system as it is mounted on X. This root vnode is X2. The ``magic'' part that happens at this point is that the lookup routine now resumes its path traversal but on the mounted file system.

5.

An nfs_lookup is performed on the Z2 vnode for the component "local", that will return the vnode Z5.

6.

An NFS lookup is performed on vnode Z5 for the component "bin", that will return the vnode Z8.

7.

An NFS lookup is performed on vnode Z8 for the component "tex", that will return the vnode for the file.

8.

The lookup is complete and returns the newly found vnode for component "tex" to the read() system call.

9.

The generic read function performs a VOP_READ on the newly found vnode. Since that vnode is an NFS one, the read is translated into nfs_read.

10.

Actual reading of the file /usr/local/bin/tex begins in earnest.