Kernel Entry Kernel Exit

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Kernel Entry / Kernel Exit

• Marcus Völp
• Universität Karlsruhe

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System-calls

• What are system-calls used for ?
• Access operating system functionality
• File System
• Network
• Devices
• Perform privileged operations
• Process creation / manipulation
• (Memory Management)
• Privileged instructions

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System-calls
thread

• Kernel executes system-call on behalf of
  user-thread
• Enter supervised mode
• Perform operation
• Leave supervised mode

System-call
user
kernel
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Entering the kernel
syscall(id 42, param) set_syscall_params
(id, param) div 0

• Exception
• undefined opcode
• div 0
• Software interrupt
• int 42
• Special instruction
• sysenter

Exception handler case id 42
read_syscall_params execute systemcall
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Entering the kernel

• Exception
• undefined opcode
• div 0
• Software interrupt
• int 42
• Special instruction
• sysenter

syscall(id 42, param) set_syscall_params
(param) int 42
Int 42 handler read_syscall_params
execute systemcall
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Entering the kernel

• Exception
• undefined opcode
• div 0
• Software interrupt
• int 42
• Special instruction
• sysenter

syscall(id 42, param) set_syscall_params
(id, param) sysenter
Syscall entry point case id 42
read_syscall_params execute systemcall
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from C to C
User-level program in C
User binding
Kernel binding
System-call in C
kernel
user
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User binding

• User-level interface between
• language and system-call
• prepare input parameters
• (Calling convention)
• enter supervisor mode
• prepare output parameters
• (Calling convention)

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Kernel binding

• Kernel level interface between
• kernel entry point and c-syscall function
• (select syscall function)
• extract input parameters
• call c-function of syscall
• prepare return parameters
• return to user land

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Parameter passing

• User Stack / Memory
• Register Set
• UTCB

Read UL Memory

• Stack per Thread
• Memory per Address Space

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Parameter passing

• User Stack / Memory
• Register Set
• UTCB

R1
R2 . . .
Rn

• Unmodified register content on kernel entry
• Convention Kernel overwrites R1, R3
• Private registers for kernel use

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Parameter passing
Kernel Stack

• User Stack / Memory
• Register Set
• UTCB

R1
Rn
R2 . . .
R2
R1
Rn
Limited number of registers available
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Parameter passing

• User Stack / Memory
• Register Set
• UTCB

No page-faults combined with larger memory space.
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Example x86

• C-calling convention

void foo (int p1, int p2, int p3, int p4)
ret _foo p1 2 p2 4 p3 p4 old frame
ptr r1 2 r2 4
sp
main() int r1 2 int r2 4 foo (r1,
r2, r1, r2)
foo params
main locals
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Example MIPS

• C-calling convention

hardwired 0
R0
R1
void foo (int p1, int p2, int p3, int p4)
Return value
R2
R3
p1 2
R4
main() int r1 2 int r2 4 foo (r1,
r2, r1, r2)
p2 4
R5
p3 r1
R6
p4 r2
R7
R8
R30
ret _foo
R31
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Example x86

• Kernel entry with exception
• x86 translates
• exception to software interrupt

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Example x86

• Kernel entry with software interrupt

Register EAX-EBP EFlags
Int_23_handler pusha
Err-No
int 23
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Example x86

• Kernel entry with software interrupt

Register EAX-EBP EFlags
Int_23_handler pusha
Err-No
int 23
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Example x86

• Returning from software interrupt

kernel stack
int_23_handler pusha prepare input
parameters call 23_handler_c_function prepare
return parameters
Register EAX-EBP EFlags
Err-No
popa add esp, 4 // get rid of err-no iret
int 23 nop
U-SS
U-ESP
U-CS
U-EIP
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Example x86

• Kernel entry with sysenter

MSR SYSENTER_CS SYSENTER_EIP SYSENTER_ESP
GDT Kernel CS Kernel SS
Flat 4GB cs rx ss rw

• no stack frame
• no user level registers
• switch to PL 0

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Example x86

• Returning from sysenter with sysexit

GDT Kernel CS Kernel SS User CS User SS
MSR SYSENTER_CS SYSENTER_EIP SYSENTER_ESP
Flat 4GB cs rx ss rw

• EDX gt EIP
• ECX gt ESP
• switch to PL 3

NOTE Sysenter / Sysexit are asymmetric
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Performance int X / iret vs. sysenter / sysexit
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Linux
arch/i386/entry.s
TCB
int 80
ENTRY(sys_call) push eax SAVE_ALL()
GET_CURRENT(ebx) cmp (NR_syscalls), eax jae
badsys testb 0x02, tsk_ptrace(ebx) jne
tracesys call (SYMBOL_NAME(sys_call_table)(,
eax, 4) mov eax, EAX(esp) ENTRY(ret_from_syscall
) cli cmp 0, need_resched(ebx) jne
reschedule cmp 0, sigpending(ebx) jne
signal_return restore_all RESTORE_ALL
USER - EBX USER - ECX USER - EDX USER - ESI USER
- EDI USER - EBP USER - EAX USER - DS USER -
ES EAX SYSCALL ID USER - EIP USER - CS USER
EFLAGS OLD ESP OLD SS
Stack
ENTRY(sys_call_table) .long SYMBOL_NAME(sys_ni_s
yscall) 0 .long SYMBOL_NAME(sys_exit) 1
.long SYMBOL_NAME(sys_fork) 2 .long
SYMBOL_NAME(sys_read) 3 .long
SYMBOL_NAME(sys_fnctl16) 221
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L4 Small Spaces

• High AS switch costs because of TLB flush
• Affinity of certain application sets
• gcc, make,
• quake, X,

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L4 Small Spaces

• no tagged TLB
• gt expensive address space switches