Errors and Exception Handling

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Errors and Exception Handling

• Tim Bisson

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Outline for Today

• Kernel Development
• Debugging, error handling, and virtual machines
• Userspace C
• Exception handling?
• C and Exception handling

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Kernel Development

• Implement abstract functionality to leverage
  physical devices
• Optimize physical resources (disk, memory, power)
• File Systems can optimize I/O access for
  mechanical nature of disks
• Debugging/developing OS functionality
  traditionally involved pressing the power-switch
  100s of times per day
• Very time-consuming

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Kernel Debugging

• Printk - kernel-level printing
• Message classification
• KERN_DEBUG, KERN_CRIT, KERN_INFO,
• printk(KERN_CRIT Bug On\n")
• Current process usually faults
• Drivers are usually the culprit, so process using
  that driver dies
• If your lucky, the system wont panic

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Kernel Debugging

• KDB
• Run on live system
• Serial Debugging
• Two machines test and development
• Communicate over gdb through serial port
• Many systems companies have some form of this
  infrastructure
• Core dumps
• Analyze stack trace off-line

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Kernel Development with Virtual Machines

• Virtual machines as test systems - when the
  break, they dont panic the host system
• Just reboot the virtual machines (like restarting
  an app)
• Speeds up kernel development
• Booting is akin to application start-up
• Debugging is easier (run debugger on virtual
  machine)
• Useful for networking, fs, etc development
• Parallels, Xen
• UML
• Run Linux Kernel as a process in Linux

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Example using UML

• UML uses SIGSEGV to fault pages into the UML
  kernel
• handle SIGSEGV pass nostop noprint
• To ignore such signals when debugging a UML
  kernel

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Why get involved with open-source kernel
development

• Contribute to an open source kernel project
• Linux, FreeBSD, NetBSD, DragonFlyBSD
• Microsoft isn't evil, they just make really
  crappy operating systems. - Linus Torvalds
• Tons of cool projects to work on
• http//www.netbsd.org/contrib/projects.html
• http//wiki.dragonflybsd.org/index.cgi/ProjectsPag
  e
• Looks really good on your resume
• Apply to SoC 07 and get paid to work on an
  open-source kernel project for the summer

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Kernel Error Handling

• A difference between application and kernel
  programming is error handling
• Application segmentation faults are harmless
• Debugger can trace the error to source (gdb)
• Kernel faults can often be fatal for the whole
  system
• Drivers are typically responsible for OS failures
• They run in kernel address space
• Their quality is questionable
• Core OS subsystems have error handling too

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File System Error Handling

• Ext4 - new file system for Linux with many new
  features
• Extents allocation, preallocation,
  defragmentation, etc
• Sets error code numbers EIO, ENOMEM, ENOSPC
• Some error handlers
• Ext4_warning()
• Ext4-_error()
• Report failure conditions such as inconsistencies
  or read I/O failures
• Ext4_abort
• unrecoverable failures such as journal I/O or
  ENOMEM
• Unconditionally force file system into read-only
  mode or panic
• Ext4_decode_error() - errno values and return
  string

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File System Error Handling (2)

• ext4_warning()
• ext4_orphan_get() - error handling for bad orphan
  inodes
• ext4_handle_error()
• ext4_get_inode_block() - ensure selected block
  group lt total block groups
• ext4_abort()
• ext4_journal_start_sb() - journalling aborted

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Goto

• Goto is also useful for aggregating error
  handling
• Free() is only called from one place

int function() int ret_val - char
data (char ) malloc (100) / do some
work / if (error) ret_val
error1 goto end / do some
more work / if (error) ret_val
error goto end end /
clean up/ free (data) return ret_val
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Userland C and exception handling

• C doesnt support exception handling
• It supports supports other functionality
• Assert
• Goto
• Signals
• Return/reason codes

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System Call/Library Errors

• When system call or library errors occur, the
  errno variable gets set
• Take a look at errno.h (man errno) for a list
  of possible numbers
• EPERM Operation not permitted (POSIX.1)
• EIO Input/output error (POSIX.1)

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Perror()

• Prints a message of describing last error that
  occurred
• Translates errors into human readable format

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Example

• define SIZE 10
• int main()
• char bufSIZE
• int ret, fd
• ret read(fd,buf, SIZE)
• if (ret ! SIZE)
• perror("Read Error")
• exit(1)
• return 0
• bisson root gcc this.c a.out
• Read Error Bad file descriptor

• Why does the program fail?
• Read(2) sets errno value to EBADF
• Perror(3) describes EBADF

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Signal Handling

• The OS delivers an exception in the form of a
  software interrupt to an executing process
• process must handle event immediately
• Signals are defined by a number
• Processes may define signal handlers for a
  particular signals
• Function called when process receives that signal
• Asynchronous execution

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What good are signals for

• Report errors - invalid memory address reference
• Report asynchronous events
• Ctrl-c, ctrl-z, fg
• Alternative is event polling

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Example - srtgen

• A synthetic soft real-time application generator
• Use SIGINT (ctrl-c) to process current frame,
  dump stats, then exit
• atexit(3) - register a function to be called at
  normal process termination

Int bool 0 void sig_int (int s)
fprintf(stderr, CTRL-C detected, aborting
after this frame\n) quit 1 void
dumpstats() /prints application
statistics/ . int main (int argc, char
argv) signal (SIGINT, sig_int) atexit(dumps
tats) do while(numsamples lt
NUMSAMPLES !quit)
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C and Exception Handling

• Try-Catch-Throw model
• Deals with synchronous and asynchronous errors
• Synchronous error example - divide by zero
• Put code that may generate an exception in a try
  block
• try
• //code that might throw an exception

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Throwing an Exception

• Indicates an exception occurred
• Specify one operand
• Exception object, if operand thrown is an object
• Exception caught by closest handler from try
  block in which exception thrown
• Control transferred to handler
• if (denominator 0)
• throw DivideByZeroException()
• Exception handler need not terminate program, but
  block where exception occurred is terminated

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Catching an Exception

• Exception handlers are the catch block
• Caught if argument type matches throw type
• If not, terminate (abort) called
• catch (DivideByZeroException ex)
• cout ltlt Exception occurred ltlt ex.what() ltlt
  endl
• Use catch() to catch all exceptions

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Simple Example

• int main ()
• char myarray10
• try
• for (int n0 nlt10 n)
• if (ngt9) throw "Out of range"
• myarrayn'z'
• catch (char str)
• cout ltlt "Exception " ltlt str ltlt endl
• return 0

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Re-throwing an Exception

• Exception handler can handle some of the
• exception, then throw it to the calling function
• Uses throw
• void throwException()
• try // Throw an exception and
  immediately catch it.
• cout ltlt "Function throwException\n"
• throw exception()
• catch( exception e )
• cout ltlt "Exception handled in function
  throwException\n"
• throw // re-throw exception for further
  processing
• cout ltlt This should not be print\n //control
  never gets here
• void main( )
• try
• throwException()
• cout ltlt This should not be print\n
  //exception will be thrown

Output Function throwException Exception handled
in function throwException Exception handled in
main Program control continues after catch in
main