Linux Kernel Architecture

Chapter 10: Filesystems without Persistent Storage

❑ proc_symlinkgenerates a symbolic link.

❑ remove_proc_entrydeletes a dynamically generated entry from theprocdirectory.

The kernel sources include a sample file inDocumentation/DocBook/procfs_example.c.Thisdemon-

strates the options described here and can be used as a template for writingprocroutines. Section 10.1.6

includes some sample kernel source routines that are responsible for interaction between the read/write

routines of theprocfilesystem and the kernel subsystems.

FindingEntries

Userspace applications accessprocfiles as if they were normal files in regular filesystems; in other words,

they follow the same path as the VFS routines described in Chapter 8 when searching for entries. As

discussed there, the lookup process (e.g., of theopensystem call) duly arrives atreal_lookup,which

invokes the function saved in thelookupfunction pointer ofinode_operationsto resolve the filename

by reference to its individual path components. In this section, we take a look at the steps performed by

the kernel to find files in theprocfilesystem.

The search for entries starts at the mount point of theprocfilesystem, usually/proc. In Section 10.1.2

you saw that thelookuppointer of thefile_operationsinstance for the root directory of the process

filesystem points to theproc_root_lookupfunction. Figure 10-3 shows the associated code flow diagram.

proc_root_lookup

proc_lookup

proc_pid_lookup

Figure 10-3: Code flow diagram for

proc_root_lookup.

The kernel uses this routine simply to distinguish between two different types ofprocentries before

delegating the real work to specialized routines. Entries may be files in a process-specific directory, as

with/proc/1/maps. Alternatively, entries may be files registered dynamically by a driver or subsystem

(e.g.,/proc/cpuinfoor/proc/net/dev). It is up to the kernel to distinguish between the two.

The kernel first invokesproc_lookupto find regular entries. If the function finds the file it is looking

for (by sequentially scanning the components of thespecified path), everything is OK, and the lookup

operation is terminated.

Ifproc_lookupfails to find an entry, the kernel invokesproc_pid_lookupto look in the list of process-

specific entries.

These functions are not examined in detail here. All we need to know is that an appropriate inode type

is returned (proc_pid_lookupis discussed again in Section 10.1.7, where the creation and structure of

process-specific inodes are discussed).