Human interfaces

Welcome to the first article of the book. The article is named after USB class and Linux facility Human Interface Devices (HID). HID facility in Linux supports keyboard, mouse and other input devices. Console, multimedia (or just media), sound (audio), video, graphics also are in the focus of this article. Security and debugging topics are very related to user and human interactions so are described here too.

human interfaces
text interfaces
security
debugging
multimedia subsystems
human interface devices, input devices
HI drivers

Text interfaces[edit | edit source]

Char devices[edit | edit source]

A character device file is an sequential access file like interface. This kind of devices is used in many facilities from keyboard input to default interface for a sophisticated new driver. You can find a list of registered char devices on the beginning the listing of /proc/devices. Input devices keyboard and mouse are examples of char devices.

Common key function cdev_add_id adds a char device to the system.

Tip: Browse the cross-referencing site to explore nearby API and use cases

💾 Historical: It is one of the most simple, fundamental and oldest concepts derived from UNIX.

⚲ API:

dev_t_id - device id consists of MAJOR_id and MINOR_id numbers
cdev_init_id
cdev_device_add_id
- cdev_add_id
register_chrdev_id, unregister_chrdev_id
alloc_chrdev_region_id / register_chrdev_region_id,
unregister_chrdev_region_id
uapi/linux/major.h_inc
linux/cdev.h_inc

⚙️ Internals:

fs/char_dev.c_src

📚 References

💾 Historical

Text terminals and console[edit | edit source]

🗝️ Acronyms:

tty - 💾 historically TeleTYpewriter, means just terminal
pty - pseudoterminal
pts - pseudoterminal slave
ptmx - pseudoterminal master

⚲ API:

To find out current terminal:
- readlink /proc/self/fd/0
- man 1 tty
- man 1 who -m
linux/tty.h_inc
register_console_id obviously registers console_id
- example virtio_console_id
include/linux/console.h_src
man 2 ioctl_console

⚙️ Internals:

📚 References

💾 Historical

LDD3:TTY Drivers

Security[edit | edit source]

The purpose of security is to restrict access across interfaces. One of the most frequently executed functions is may_open_id. It rejects access of unauthorized users to open a file. It uses common function inode_permission_id which checks for access rights to a given inode.

Authorization[edit | edit source]

🔧 TODO

⚲ API:

Basic classic UNIX authorization is based on ownership and access mode.

Ownership is encoded by owning user id uid_t_id and owning group id gid_t_id.

umode_t_id - just typedef used for encoding access mode. S_IRUSR_id - minimal "read only by user" access mode. S_IALLUGO_id - full access mode. Please read the source for details for other modes.


modes	bits	Read	Write	Execute
bit offset		2	1	0
Other	0-2	or	ow	ox
Group	3-5	gr	gw	gx
User	6-8	ur	uw	ux

man 2 chown ↪ do_fchownat_id changes ownership for file or directory
man 2 chmod ↪ do_fchmodat_id changes access mode for file or directory
man 2 access, man 2 faccessat ↪ do_faccessat_id checks access rights

Common authorization errors:

EPERM_id - "Operation not permitted"
EACCES_id - "Permission denied"

⚙️ Internals:

may_open_id rejects unauthorized file opening
- inode_permission_id checks for access rights to a given inode

📚 References:

File-system permissions

Credentials[edit | edit source]

🔧 TODO

⚲ API:

man 1 id, man 1 test - shell utilities
man 2 getuid ↪ current_uid_id
man 2 getgid
man 2 geteuid is used by utility man 1 whoami
Real, effective, and saved user/group IDs:
- man 2 getresuid, getresgid
- man 2 setreuid, setregid
man 2 setfsuid - set user identity used for filesystem checks
man 2 umask
man 1 stat, man 2 stat ↪ vfs_fstat_id, vfs_fstatat_id
man 2 statx ↪ do_statx_id
struct cred_id - the security context of a task

⚙️ Internals:

📚 References:

Cryptography[edit | edit source]

🔧 TODO

🗝️ Acronyms:

AES - Advanced Encryption Standard

⚲ API:

AF_ALG_id - User Space Interface_doc
linux/crypto.h_inc - Scatterlist Cryptographic API.
crypto_inc

⚙️ Internals:

crypto_src
drivers/crypto_src
lib/crypto_src
arch/x86/crypto_src
fs/crypto_src - per-file encryption
fs/ecryptfs_src eCrypt FS - Encrypted filesystem that operates on the VFS layer.
dm-crypt, drivers/md/dm-crypt.c_src

📚 References

Namespaces[edit | edit source]

Linux namespaces provide the way to make tasks work with different objects using the same id. For example same IPC id may refer to different objects or same pid may refer to different tasks when used in different namespaces.

🔧 TODO

⚲ API:

⚙️ Internals:

📚 References:

Control Groups[edit | edit source]

🔧 TODO: cgroups

⚲ API:

⚙️ Internals:

📚 References:

Audit[edit | edit source]

🔧 TODO:

📚 References:

Debugging[edit | edit source]

User space debug interfaces[edit | edit source]

⚲ Interfaces

man 1 dmesg prints or control the kernel ring buffer
man 1 strace traces system calls and signals
man 2 ptrace process trace
man 2 syslog
man 3 klogctl
man 5 core
/sys/kernel/debug/ - debugfs
dmesg --console-level <level>
gdb /usr/src/linux/vmlinux /proc/kcore

dynamic_doc debug
- ⌨ Example:
- echo "module atkbd +pfl" | sudo tee /sys/kernel/debug/dynamic_debug/control

⚙️ Internals

handle_sysrq_id

📚 References

Printing to log[edit | edit source]

Most common functions

pr_devel_id- conditional debug-level message
pr_debug_id- conditional debug-level or dynamic_doc message
- ⌨ Example:
- echo "module atkbd +pfl" | sudo tee /sys/kernel/debug/dynamic_debug/control
Message logging with printk_doc
Log messages with other levels:
- pr_info_id, pr_notice_id, pr_warn_id, pr_crit_id, pr_alert_id, pr_emerg_id

🔧 TODO. 🚀 advanced features

pr_cont_id- continues a previous log message in the same line
print_hex_dump_bytes_id
print_hex_dump_debug_id
linux/printk.h_inc
dump_stack_id

⚙️ Internals

kgdb and kdb[edit | edit source]

⚲ Interfaces

⚙️ Internals

kernel/debug_src

📚 References

Using kgdb, kdb and the kernel debugger internals_doc

📚 References for debugging

Multimedia subsystems[edit | edit source]

Graphics[edit | edit source]

Old graphics (not to be confused with v4l):

⚲ API:

video_inc

⚙️ Internals:

drivers/video_src

Direct Rendering Manager (DRM)[edit | edit source]

DRM is responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations such as configuring the mode setting of the display. User-space programs can use the DRM API to command the GPU to do hardware-accelerated 3D rendering and video decoding, as well as GPGPU computing.

⚲ API:

/sys/class/drm/
drm_inc
uapi/drm_inc
drm_dev_register_id registers drm_device_id

Advanced Linux Sound Architecture (ALSA)[edit | edit source]

ALSA is a software framework and part of the Linux kernel that provides an API for sound card device drivers. Some of the goals of the ALSA project at its inception were automatic configuration of sound-card hardware and graceful handling of multiple sound devices in a system.

The sound servers PulseAudio, JACK (low-latency professional-grade audio editing and mixing) and PipeWire, the higher-level abstraction APIs OpenAL, SDL audio, etc. work on top of ALSA and implemented sound card device drivers. On Linux systems, ALSA succeeded the older Open Sound System (OSS).

⚲ API:

/proc/asound/cards, /sys/class/sound/
snd_card_id - central struct
snd_card_new_id
snd_card_register_id
snd_device_ops_id
snd_device_new_id creates an ALSA device component
uapi/sound/asound.h_inc
sound/core.h_inc

⚙️ Internals:

📚 References:

Video4Linux (V4L2)[edit | edit source]

V4L is a collection of device drivers and an API for supporting realtime video capture on Linux systems. It supports many USB webcams, TV tuners, and related devices, standardizing their output, so programmers can easily add video support to their applications. MythTV, tvtime and Tvheadend are typical applications that use the V4L framework.

⚲ API: