Filesystems do not necessarily need to be backed by a physical block device, but their contents can also be generated dynamically. This allows for passing information from the kernel to userland (and vice versa), which can be easily obtained by regular file I/O operations. The /proc filesystem was one of the first virtual filesystems used by Linux, and a more recent addition is sysfs, which presents a hierarchically structured representation of (nearly) all objects known to the kernel.
This chapter also discussed some generic routines to implement virtual filesystems and additionally considered how pseudo-filesystems that are not visible to userland carry information important for the kernel itself.
Continue reading here: Extended Attributes and Access Control Lists
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