Linux System Calls
Some system calls are more complex then others because of variable-length argument lists. Examples of a complex system call include open and ioctl. However, even complex system calls must use the same entry point; they just have more overhead for parameter setup. Each syscall macro expands to an assembly routine which sets up the calling stack frame and calls _system_call through an interrupt, via the instruction int $0x80. For example, the setuid system call is coded as
which expands to the assembly code shown in Listing 2.
The user-space call code library can be found in /usr/src/libc/syscall. The hard-coding of the parameter layout and actual system call numbers is not a problem, because system calls are never really changed; they are only “introduced” and “obsoleted”. An obsoleted system call is marked with the old_ prefix in the system call table for entry.S, and reference to it is removed from the next glibc. Once no application uses that system call anymore, its slot is marked “unused” and is potentially reusable for a newly introduced system call.
If a user wishes to trace a program, it is equally important to know what happens during system calls. Thus, the trace of a program usually includes a trace through the system calls as well. This is done through SIGSTOP and SIGCHLD ping-ponging between parent (tracing process) and child (traced process). When a traced process is executed, every system call is preceded by a sys_ptrace call. This makes the traced process send a SIGCHILD to the tracing process each time a system call is made. The traced process immediately enters the TASK_STOPPED state (a flag is set in the task_struct structure). The tracing process can then examine the entire address space of the traced process through the use of _ptrace, which is a multi-purpose system call. The tracing process sends a SIGSTOP to allow execution again.
Adding your own system calls is actually quite easy. Follow this list of steps to do so. Remember, if you do not make these system calls available on all the machines you want your program to run on, the result will be non-portable code.
Create a directory under the /usr/src/linux/ directory to hold your code.
Put any include files in /usr/include/sys/ and /usr/include/linux/.
Add the relocatable module produced by the link of your new kernel code to the ARCHIVES and the subdirectory to the SUBDIRS lines of the top-level Makefile. See fs/Makefile, target fs.o for an example.
Add a #define __NR_xx to unistd.h to assign a call number for your system call, where xx, the index, is something descriptive relating to your system call. It will be used to set up the vector through sys_call_table to invoke your code.
Add an entry point for your system call to the sys_call_table in sys.h. It should match the index (xx) you assigned in the previous step.
The NR_syscalls variable will be recalculated automatically.
Modify any kernel code in kernel/fs/mm/, etc. to take into account the environment needed to support your new code.
Run make from the top source code directory level to produce the new kernel incorporating your new code.
At this point, you must either add a syscall to your libraries, or use the proper _syscalln macro in your user program in order for your programs to access the new system call. The 386DX Microprocessor Programmer's Reference Manual is a helpful reference, as is James Turley's Advanced 80386 Programming Techniques.
A list of Linux/IA32 kernel system calls can be found, with the listings, in the archive file ftp.linuxjournal.com/pub/lj/listings/issue75/4048.tgz. Note: these are not libc “user-space system calls”, but real kernel system calls provided by the Linux kernel. Information source is GNU libc project, http://www.gnu.org/.
Moshe Bar (firstname.lastname@example.org) is an Israeli system administrator and OS researcher who started learning UNIX on a PDP-11 with AT&T UNIX Release 6 back in 1981. He holds an M.Sc. in computer science. His new book Linux Kernel Internals will be published this year. You may visit Moshe's web site at http://www.moelabs.com/.
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