제40강 : System Call
Ch 5 System Call
1
Kernel a.out
User a.out
my code
main()
{
add( );
sub( );
printf( );
library
FILE (
local buffer
file descriptor
}
printf(3)
user
(system)
file table
/
u-ofile
fd
inode
table
a
0
1
2
3
4
offset
b
/
data
block data
block
sys call
system call
write(2)
trap( )
a
sys_write()
You have 2 write( ) functions .
One in my a.out as a library function.
another in kernel a.out.
library write( ) is the caller
kernel sys_write() is callee
this issues trap i.e. chmodk (with para)
this implements system call2
Invoking a system call
In Linux, system call interface is provided by the C library.
Kernel a.out
User a.out
my code
main()
{
add( );
sub( );
printf( );
library
printf(3)
{
write(2)
}
trap()
{
…..
}
write()
{ load arg
chmodk
}
sys_write()
{
…..
}
system call wrapper routine
(system call interface)
3
Inside Wrapper Routine
• Only purpose is to issue a system call
• C library function example
– libc.a :
write() {
…..
movl 5, %eax
int $0x80
}
: push arguments
: system call number
: cause trap
4
Compiling
System Call Wrapper Routine
• Before compile
printf()
{
write(2)
}
After compile
<wrapper routine>
arguments into registers (up to five)
system call # into eax register
software interrupt instruction ($0x80)
• interrupt causes system to switch to kernel mode
• kernel executes system call handler system_call()
• which is coded in assembly (entry.S)
5
System Call Handling in Kernel
asmlinkage long sys_write(void)
{
return current->tgid;
}
• Naming convention:
– “sys_ ” followed by system call name write
• asmlinkage modifier on declaration
– required for all system calls
6
Kernel system call function
• Should verify the parameters
– Every argument must be checked
– If pointer is passed, check whether
• points to
• proper
the process’s user address space?
copy_to_user()
access rights?
user
a.out
kernel
a.out
• Accessing user space
– copy_to_user()
– copy_from_user()
– capable()
copy_trom_user()
write to user space -- subyte()
read from user space -- fubyte()
check permission
7
System Call Number
• Unique number that reference system call
– include/asm-i386/unistd.h
• sys_call_table (in entry.S)
– NR_syscalls
• Max number of implementable system calls
sys_call_table
system
call
number
• Architecture dependent
• It cannot be changed
(since all a.out’s use these numbers)
*sys_write()
NR_syscalls
8
Write a New System Call?
• Pros
– Simple to implement
– Good Performance --- binding is fast on Linux
• Cons
– Need new syscall number
• new program is platform dependent.
• This program may not run on other platform
– Cannot change existing system call (only can add)
• Do you really need a new system call?
9
Alternative to New System Call
• Implement a new file fdNEW
• Let this file correspond to a new system call
– read(fdNEW), write(fdNEW), ioctl(fdNEW) to it.
– which pass data & control between user & kernel
• Nobody adds new system call in Linux
• Linux keeps very clean system call layer
10
System Call Implementation
1.
2.
3.
4.
Add an entry at the end of system call table
Define syscall # in include/asm/unistd.h
system
Add syscall function into kernel image
call
number
User-Space wrapper
–
–
–
–
sys_call_table
glibc does not support new system call wrapper!
Linux provides a set of macro
_syscalln() (where n is # of parameters 0 to 6)
Example of using macro:
•
*sys_write()
For new system call open()
– long open(const char *filename, int flags, int mode)
•
Without library support, use this macro
–
_syscall3(long, open, const char*, filename, int, flags, int, mode)
11