Kernel Services

1
Kernel Services

• CIS 657

2
System Processes

• Three processes created at startup time
• init
• process 1
• user-mode
• administrative tasks (keeps getty running
  shutdown)
• ancestor of all of your processes

• swapper
• process 0
• kernel process
• moves entire processes between main memory and
  secondary storage
• pagedaemon
• process 2
• kernel process
• moves parts of processes in and out

3
Note on Init and Logging In

• init keeps a getty process running on each
  terminal port (tty)
• getty initializes the port and waits for a login
  name (the login prompt)
• getty reads a string and execs login
• login prompts for the password, performs one-way
  encryption, and compares values
• if successful, login sets the user id and execs a
  shell

4
Run-Time Organization

• Top Half
• per-process stack
• library of shared code
• maintains process structure (always resident)
• maintains user structure (can be swapped out)
• division between process/user dependent on memory
• never preempted for another process (but can
  yield the processor)
• can block interrupts by setting processor
  priority level (see discussion on bottom half)

5
Run-Time Organization II

• Bottom Half
• handles hardware interrupts
• asynchronous activities
• special kernel stack (might not be a process
  running)
• bottom half runs at specific priority level
• top half can block specific devices from
  interrupting by setting priority level before it
  modifies work queues
• top half starts I/O requests, waits for bottom
  half to finish

6
Entry Into the Kernel

• Hardware interrupt
• I/O device (disks, network cards, etc.)
• clock (used for scheduling, time of day)
• Hardware trap
• divide by 0, illegal memory reference
• Software-initiated trap
• system call

7
Entry Into the Kernel II

• First, kernel must save machine state
• Why?
• Example sequence
• hardware switches to kernel mode
• hardware pushes onto per-process kernel stack the
  PC, PSW, trap info
• additional asm routine saves all other state that
  the hardware doesnt
• kernel calls a C routine--the handler.

8
Entry Into the Kernel III

• Handlers for each kind of entry
• syscall() for a system call
• trap() for hardware traps
• interrupt handlers for devices
• Each kind of handler takes specific parameters
  (e.g., syscall number, exception frame or the
  unit number for an interrupt).

9
Return From Kernel

• asm routine restores registers and user stack
  pointer (it undoes what the companion asm routine
  did)
• hardware restores the stored PC, PSW, etc.
  (undoes what it did on the way in)
• execution returns at the next instruction in the
  user process

10
Software Interrupts

• Used as low-priority processing mechanism in the
  kernel
• Hardware interrupts have high priority
• Can put work in work queues (cf. network)
• When high-priority work is done, low-priority
  software interrupt does the rest
• might be real interrupt, might be flag checked in
  kernel (architecture-dependent)
• can be preempted by another hardware interrupt

11
What FreeBSD Does for x86

• The cpl variable holds the current priority level
• Various macros are defined to set the cpl to a
  new level (e.g. spl0(), splx(), spltty())
• Interrupts at lower levels are masked (but not
  really--a pending bit is set and the majority of
  the work handling the interrupt is deferred)
• See /usr/src/sys/i386/isa/ipl_funcs.c

12
Clock Interrupts

• The system clock interrupts, or ticks, at regular
  intervals (usually 100 Hz).
• Interrupt handler calls the hardclock() routine,
  which must run quickly
• running for more than one tick will miss the next
  interrupt, causing the time-of-day clock to skew
• lower-priority devices (network devices, disk
  controllers) cannot be serviced while hardclock()
  is running.
• Non-critical clock functions handled by
  softclock()

13
Clock Interrupts II

• What hardclock() does
• increment the time of day
• check for an interval timer on the currently
  running process
• do the job of statclock() if there is no separate
  clock for statistics gathering
• call softclock() directly if the cpl is low
  (saves overhead of a software interrupt that
  would just do that when hardclock() returns)
• See /usr/src/sys/kern/kern_clock.c

14
Statistics

• Hardclock() used to collect resource utilization
  statistics, and forced context switches
• Problems (see McCanne Torek)
• potential for inaccurate measurement of CPU
  utilization
• inaccurate profiling
• Use semi-randomized sampling with a second clock
  (the stat clock, see statclock())
• charge the current process with a tick if it has
  four, recalculate priority
• record what the system was doing at time of tick

15
Softclock()

• Handles events in the callout queue, such as
• timeouts (real-time timer)
• retransmits dropped network packets
• monitors some peripherals that require polling
• process scheduling
• Scheduler would/should be called every second in
  a perfect world uses interval timer to run 1
  second after it last finished
• Discussion why not do scheduling in hardclock()?

16
Callout Queue

• Sorted in time order (soonest first)
• Delta queue
• hardclock() decrements first entry, and
  softclock() only runs when that reaches 0.

queue
1 tick
3 ticks
0 ticks
81 ticks
time
function argument
f(x)
g(y)
f(z)
h(a)
10 ms
40 ms
40 ms
850 ms
when
17
Memory Management

• Two kinds of executable files in BSD Unix
• interpreted
• compiled (directly executed)
• First 16 bits in a file contain a magic number
  telling what kind of file it is.
• ! indicates an interpreted file interpreter
  must be directly executable (!/bin/sh is the
  most common)
• Other magic numbers indicate whether the file can
  be paged and whether the text is sharable.

18
UNIX Process Layout
0xfff00000

• 0-filled bss, stack
• most of process is demand paged into memory (Ch.
  5)

Symbol table
Initialized data
text
0x00000000
a.out header
magic number
19
What Was That Special Stuff?
Per-process kernel stack

• Argv, argc, envp contain arguments and
  environment
• signal code used by kernel to deliver signals
• ps_strings used by ps to located argv of process
• user (u.) area (swappable context)
• red zone is read-only page (not on all arch)

Red zone
User area
Ps_strings struct
Signal code
Env strings
argv strings
Env pointers
argc
argv pointers
20
Timing Services

• Real Time gettimeofday() returns the time since
  1 Jan 1970 in UTC (the Epoch)
• adjtime() allows one to tweak the clock
• keeps multiple machines close enough
• response to normal clock skew
• give a delta argument speed up or slow down the
  counted microseconds per clock tick by 10 until
  delta is reached
• Time is reported in microseconds

21
Interval Timers

• Each process gets three interval timers
• real decrements in real time SIGALRM run from
  timeout queue maintained by softclock()
• profiling decrements only when process runs, but
  tracks both user and kernel-mode execution
  SIGPROF checked by hardclock()
• process virtual decrements only when the process
  is running SIGVTALRM checked by hardclock()

22
User, Group, Other Identifiers

• User ID (uid) 32-bit identifier for all
  processes of each user, set by administrator
• Group ID (gid) 32-bit identifier. Many users in
  one group many groups for each user
• Root uid 0, gid 0.
• These bits are checked on file access

23
Permission Checks

• Checked in order
• If the UIDF UIDP use owner permission bits
• If UIDF ! UIDP, but GIDF GIDP then use group
  permission bits
• If UIDF ! UIDP, and GIDF ! GIDP then use the
  other permission bits
• Recall discussion last time of how uid, gid set
  on login
• Can I own a file that I cant read?

24
Rights Amplification

• Users may need temporary write access on files
  (e.g. passwd)
• setuid() does this
• changes effective user id
• real user id stays the same
• effective uid also saved
• seteuid() changes only the effective user id
• setgid()
• used to work like setuid()
• now just put effective gid into 0th element of
  array

25
Effects of Syscalls on UIDs
Action
Real
Effective
Saved
Exec-normal
R
R
R
Exec-setuid
S
R
S
Seteuid(R)
R
R
S
Seteuid(S)
R
S
S
Seteuid(R)
R
R
S
Exec-normal
R
R
R
R Real UID, S Special-privilege UID
26
Homework Assignment for FreeBSD

• In which file is each of the following defined?
• hardclock()
• softclock()
• statclock()
• In which file or files is the code that checks
  the current interrupt priority level and handles
  soft interrupts?
• Read the McCanne Torek paper, and write an at
  most three paragraph summary of it.
• Due 26 Sep 2001, by e-mail to cis657ta alias