Advanced%20Operating%20Systems

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Advanced Operating Systems

• Implementing System Calls

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System Call Implementation

• Under Linux system calls have 2 separate function
  calls
• The do_xxx call which does the work
• The sys_xxx macro which deals with arguments a
  call numbers

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System Call Implementation

• System calls involve switching from user to
  supervisor mode through an interrrupt (0x80 on
  PCs)
• _syscall macro generates the actual system call
  number (unistd.h)
• Interrupt routine handles the call
  (arch/kernel/entry.s)
• If legitimate, the routine looked up in
  sys_call_table

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System Call Implementation

• Some call may be traced - ie debugged calls
  using the syscall_trace function
• On return from system call a number of
  administrative tasks must be done
• Send parent signals
• Any pending interrupt service routines called
• Signals handled

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Implementing fork() clone()

• Under Linux fork() and clone() are essentially
  the same call with some (important) differences
• Fork() create a completely new process
• Clone() creates a new thread within a process

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Linux fork() versus traditional fork()

• UNIX fork() creates
• New process environment
• Copies data
• Copies text
• Copies files, locks and signals
• Creates new PID

• Linux fork() creates
• New process environment
• Makes data write protected (copies on write)
• Copies files, locks and signals
• Creates new PID

Text may be shared
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Implementing fork() clone()

• They both call do_fork() however clone() sets up
  some extra parameters beforehand
• Set up a new set of register and stack pointers
• do_fork() is called as follows
• do_fork(SIGCHILD, regs, esp, regs)
• Or
• do_fork(clone_flags, newsp, regs)

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Implementing fork() clone()

• do_fork()
• Creates a new stack using kmalloc()
• Gets a new process table entry
• The child or thread inherits all of the parents
  task structure but changes some entries
• Exec flag, time, signal flags

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Implementing fork() clone()

• The difference between fork and clone is found at
  the final section of the call when the task
  substructures are copied (or not!)
• This copies files, file systems, signal handlers,
  memory management
• The parent gets the child or thread pid

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Fork and Clone
Process 1
Process 1
Process 1
Process 1
User Space
Thread
Thread
Kernel
Kernel Space
Kernel
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Executable formats

• Before looking at exec a word on file formats
  there are a number of different formats
• a.out the original UNIX format
• COFF Common Object File Format
• ELF Executable and Linkable Format
• There are others lots of them!

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Linux and file formats

• Linux supports a large number of file formats
  each ones is tried until one works
• Done by do_exec
• Use linux_binfmt
• Loads binary
• Loads libs
• Deals with the core file
• They can be load as modules

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ELF
E L F 0x8048090 52 52 2
e_indent e_entry e_phoff e_phentsize e_phnum
PT_LOAD 0 0x804800 68532 68532 PF_R, PF_X
p_type p_offset p_vaddr p_filesz p_memsz p_flags
Physical Header
p_type p_offset p_vaddr p_filesz p_memsz p_flags
PT_LOAD 68536 0x8059BB8 2200 4248 PF_R, PF_W
Physical Header
CODE
DATA