Function shm_map

freebsd/kern/uipc_shm.c:1680–1732

* Helper routines to allow the backing object of a shared memory file * descriptor to be mapped in the kernel. */

 * descriptor to be mapped in the kernel.
 */
int
shm_map(struct file *fp, size_t size, off_t offset, void **memp)
{
	struct shmfd *shmfd;
	vm_offset_t kva, ofs;
	vm_object_t obj;
	int rv;

	if (fp->f_type != DTYPE_SHM)
		return (EINVAL);
	shmfd = fp->f_data;
	obj = shmfd->shm_object;
	VM_OBJECT_WLOCK(obj);
	/*
	 * XXXRW: This validation is probably insufficient, and subject to
	 * sign errors.  It should be fixed.
	 */
	if (offset >= shmfd->shm_size ||
	    offset + size > round_page(shmfd->shm_size)) {
		VM_OBJECT_WUNLOCK(obj);
		return (EINVAL);
	}

	shmfd->shm_kmappings++;
	vm_object_reference_locked(obj);
	VM_OBJECT_WUNLOCK(obj);

	/* Map the object into the kernel_map and wire it. */
	kva = vm_map_min(kernel_map);
	ofs = offset & PAGE_MASK;
	offset = trunc_page(offset);
	size = round_page(size + ofs);
	rv = vm_map_find(kernel_map, obj, offset, &kva, size, 0,
	    VMFS_OPTIMAL_SPACE, VM_PROT_READ | VM_PROT_WRITE,
	    VM_PROT_READ | VM_PROT_WRITE, 0);
	if (rv == KERN_SUCCESS) {
		rv = vm_map_wire(kernel_map, kva, kva + size,
		    VM_MAP_WIRE_SYSTEM | VM_MAP_WIRE_NOHOLES);
		if (rv == KERN_SUCCESS) {
			*memp = (void *)(kva + ofs);
			return (0);
		}
		vm_map_remove(kernel_map, kva, kva + size);
	} else
		vm_object_deallocate(obj);

	/* On failure, drop our mapping reference. */
	VM_OBJECT_WLOCK(obj);
	shmfd->shm_kmappings--;
	VM_OBJECT_WUNLOCK(obj);

	return (vm_mmap_to_errno(rv));
}

/*
 * We require the caller to unmap the entire entry.  This allows us to