Embedded FreeBSD Cookbook phần 3 potx

36 Embedded FreeBSD

Cookbook

The copyin function copies len bytes of data from the user mode address

uaddr to the kernel mode address kaddr. The copyout function copies

len bytes of data from the kernel mode address kaddr to the user mode

address uaddr.

The sysent Structure

Every system call in the FreeBSD kernel has a sysent structure, defined in

/usr/include/sys/sysent.h. The sysent structure takes two elements,

the number of parameters, which is two as defined by the dumpmem_args

structure, and the name of the system call function. Listing 3-7 defines the

copymem sysent.

static struct sysent copymem_sysent =

{

4, /* number of parameters */

copymem /* system call */

};

Listing 3-7

System calls are contained in the sysent structure, defined in

/sys/kern/init_sysent.c. When a KLD system call is made, a new

entry is added to the kernel global sysent structure. Listing 3-8 contains a

partial listing of the sysent structure.

/* The casts are bogus but will do for now. */

struct sysent sysent[] = {

{ 0, (sy_call_t *)nosys }, /* 0 = syscall */

{ AS(rexit_args), (sy_call_t *)exit }, /* 1 = exit */

{ 0, (sy_call_t *)fork }, /* 2 = fork */

{ AS(read_args), (sy_call_t *)read }, /* 3 = read */

{ AS(write_args), (sy_call_t *)write }, /* 4 = write */

{ AS(open_args), (sy_call_t *)open }, /* 5 = open */

{ AS(close_args), (sy_call_t *)close }, /* 6 = close */

Listing 3-8

The System Call Number

A system call number must be declared; since there is no system call num￾ber defined, this value should be set to NO_SYSCALL. The kernel defines

the system call number dynamically.

37 Chapter Three

System Calls

static int32_t syscall_num = NO_SYSCALL;

When the KLD system call is loaded into the kernel, systent entry

copymem_sysent is assigned to the first open index in the kernel global

sysent structure. The index into the sysent array is the system call number.

The specifics of installing a new system call are found in the

syscall_register function listed in /sys/kern/kern_syscalls.c.

Listing 3-9 contains the syscall_register function.

int

syscall_register(int *offset, struct sysent *new_sysent,

struct sysent *old_sysent)

{

if (*offset == NO_SYSCALL) {

int i;

for (i = 1; i < SYS_MAXSYSCALL; ++i)

if (sysent[i].sy_call == (sy_call_t

*)lkmnosys)

break;

if (i == SYS_MAXSYSCALL)

return ENFILE;

*offset = i;

} else if (*offset < 0 || *offset >= SYS_MAXSYSCALL)

return EINVAL;

else if (sysent[*offset].sy_call != (sy_call_t *)lkmnosys)

return EEXIST;

*old_sysent = sysent[*offset];

sysent[*offset] = *new_sysent;

return 0;

}

Listing 3-9

When a new system call is added, the kernel function syscall_register

is called. The offset and sysent structure for the new call are passed. If the

offset is NO_SYSCALL, syscall_register scans the sysent structure

looking for an empty system call location. If one is found, the system call is

inserted and the offset is set to the index of the sysent structure, where the

call has been inserted.