Parameter Passing

Like ordinary functions, system calls often require some input/output parameters, which may consist of actual values (i.e., numbers), addresses of variables in the address space of the User Mode process, or even addresses of data structures including pointers to User Mode functions (see Section 10.4).

Since the system_call( ) function is the common entry point for all system calls in Linux, each of them has at least one parameter: the system call number passed in the eax register. For instance, if an application program invokes the fork( ) wrapper routine, the eax register is set to 2 (i.e., _ _NR_fork) before executing the int $0x8 0 assembly language instruction. Because the register is set by the wrapper routines included in the libc library, programmers do not usually care about the system call number.

The fork( ) system call does not require other parameters. However, many system calls do require additional parameters, which must be explicitly passed by the application program. For instance, the mmap( ) system call may require up to six additional parameters (besides the system call number).

The parameters of ordinary C functions are passed by writing their values in the active program stack (either the User Mode stack or the Kernel Mode stack). Since system calls are a special kind of function that cross over from user to kernel land, neither the User Mode or the Kernel Mode stacks can be used. Rather, system call parameters are written in the CPU registers before invoking the int 0x8 0 assembly language instruction. The kernel then copies the parameters stored in the CPU registers onto the Kernel Mode stack before invoking the system call service routine because the latter is an ordinary C function.

Why doesn't the kernel copy parameters directly from the User Mode stack to the Kernel Mode stack? First of all, working with two stacks at the same time is complex; second, the use of registers makes the structure of the system call handler similar to that of other exception handlers.

However, to pass parameters in registers, two conditions must be satisfied:

• The length of each parameter cannot exceed the length of a register (32 bits). HI

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