Introduction to Kernel Programming

Introduction to Kernel Coding Demystifying Kernel Programming

Outline Context of execution Memory I/O

Mechanism vs Policy Mechanism: Interface to the system resources Policy: How the resource is used Examples: Udev File configuration

Context of execution Possible contexts System Call Interrupt Handling Tasklets Kernel threads User space Kernel space Resource Handler Resource User process Kernel thread System Call Handling Interrupt Handling Tasklet

Why do we care? Blocking: Mutual exclusion / Reentrancy Resource Allocation Mixed context code System responsiveness Crashes – what's at stake

Interface General Pattern Central Data Structure Register entry points Entry point definition Know your subsystem SUBSYSTEM Resource Handler interface { meth1 meth2 ... } Register deregister meth1 (DS) meth2 (DS) Container consumer

Example – Fileops VFS USER KERNEL DRIVER/FS MODULE fleops { myopen myread myclose } Register deregister myopen (FILE) myread myclose M,M:FOPS open(fd) read write

Registration For certain type, e.g. filesystem For specific objects e.g. file ops Detection by the driver – legacy Detection by a bus driver

struct vfsmount * vfs_kern_mount( struct file_system_type *type, int flags, const char *name, void *data) { struct vfsmount *mnt; int error; mnt = alloc_vfsmnt(name); ... error = type->get_sb(type, flags, name, data, mnt); ... mnt->mnt_mountpoint = mnt->mnt_root; ... return mnt; } static struct file_system_type ** find_filesystem (const char *name, unsigned len) { struct file_system_type **p; for (p=&file_systems; *p; p=&(*p)->next) if (strlen((*p)->name) == len && strncmp((*p)->name, name, len) == 0) break; return p; } struct vfsmount * do_kern_mount( const char *fstype, int flags, const char *name, void *data) { struct file_system_type *type = get_fs_type(fstype); struct vfsmount *mnt; ... mnt = vfs_kern_mount(type, flags, name, data); ... return mnt; } int register_filesystem(struct file_system_type * fs) { int res = 0; struct file_system_type ** p; ... INIT_LIST_HEAD(&fs->fs_supers); write_lock(&file_systems_lock); p = find_filesystem(fs->name, strlen(fs->name)); if (*p) res = -EBUSY; else *p = fs; write_unlock(&file_systems_lock); return res; } struct file_system_type *get_fs_type(const char *name) { struct file_system_type *fs; unsigned len = ... strlen(name); read_lock(&file_systems_lock); fs = *(find_filesystem(name, len)); read_unlock(&file_systems_lock); if (!fs && (request_module("%.*s", len, name) == 0)) { read_lock(&file_systems_lock); fs = *(find_filesystem(name, len)); if (fs && !try_module_get(fs->owner)) fs = NULL; read_unlock(&file_systems_lock); } return fs; } VFS EXT3 static int ext3_get_sb (struct file_system_type *fs_type, int flags, const char *dev_name, void *data, struct vfsmount *mnt) { return get_sb_bdev(fs_type, flags, dev_name, data, ext3_fill_super, mnt); } static struct file_system_type ext3_fs_type = { .owner = THIS_MODULE, .name = "ext3", .get_sb = ext3_get_sb, .kill_sb = kill_block_super, .fs_flags = FS_REQUIRES_DEV, }; static int __init init_ext3_fs(void) { ... err = register_filesystem (&ext3_fs_type); ... return 0; }

Device Model (Bovet et al) SUBSYSTEM kset kobject attribute1 attribute2 ... Scan actions Resource Handler PCI pci_register_driver probe driver_if{ ... probe } register_device

Interrupts Registering for interrupts Interrupt Handling – fast and alert Critical regions: Spinlocks and SMP systems Memory allocation System is unresponsive, interrupts masked Tasklets – pretty fast, pretty alert Workqueues – sleep all you want

Interrupt Handling DRIVER WORKQ handler ISR Initialization Tasklet request_irq Device Interrupt KERNEL PROPER schedule_ work tasklet_ schedule

static irqreturn_t ipw_isr(int irq, void *data) { struct ipw_priv *priv = data; u32 inta, inta_mask; ... spin_lock(&priv->irq_lock); ... inta_mask = ipw_read32(priv, IPW_INTA_MASK_R); ... if (!(inta & (IPW_INTA_MASK_ALL & inta_mask))) { ... } __ipw_disable_interrupts(priv); inta &= (IPW_INTA_MASK_ALL & inta_mask); ipw_write32(priv, IPW_INTA_RW, inta); priv->isr_inta = inta; tasklet_schedule(&priv->irq_tasklet); spin_unlock(&priv->irq_lock); return IRQ_HANDLED; } static void ipw_bg_link_down(struct work_struct *work) { struct ipw_priv *priv = container_of(work, struct ipw_priv, link_down); mutex_lock(&priv->mutex); ipw_link_down(priv); mutex_unlock(&priv->mutex); } static void ipw_irq_tasklet(struct ipw_priv *priv) { u32 inta, inta_mask, handled = 0; unsigned long flags; spin_lock_irqsave(&priv->irq_lock, flags); inta = ipw_read32(priv, IPW_INTA_RW); inta_mask = ipw_read32(priv, IPW_INTA_MASK_R); inta &= (IPW_INTA_MASK_ALL & inta_mask); spin_unlock_irqrestore(&priv->irq_lock, flags); spin_lock_irqsave(&priv->lock, flags); ... if (inta & IPW_INTA_BIT_RF_KILL_DONE) { ... cancel_delayed_work(&priv->request_scan); ... schedule_work(&priv->link_down); queue_delayed_work(priv->workqueue, &priv->rf_kill, 2 * HZ); handled |= IPW_INTA_BIT_RF_KILL_DONE; } ... spin_unlock_irqrestore(&priv->lock, flags); /* enable all interrupts */ ipw_enable_interrupts(priv); } static int __devinit ipw_pci_probe(struct pci_dev *pdev, const struct pci_device_id *ent) { ... struct ipw_priv *priv; ... err = ipw_setup_deferred_work(priv); ... err = request_irq(pdev->irq, ipw_isr, IRQF_SHARED, DRV_NAME, priv); ... } static int __devinit ipw_setup_deferred_work(struct ipw_priv *priv) { priv->workqueue = create_workqueue(DRV_NAME); ... INIT_WORK(&priv->link_down, ipw_bg_link_down); ... tasklet_init(&priv->irq_tasklet, (void (*)(unsigned long)) ipw_irq_tasklet, (unsigned long)priv); ... } TASKLET ISR WORKQ PROBE

What Address Space?!!! Flat space Access to pointers Symbols Across the boundary copy_to/copy_from

asmlinkage long sys_sendmsg(int fd, struct msghdr __user *msg, unsigned flags) { struct compat_msghdr __user *msg_compat = (struct compat_msghdr __user *)msg; struct socket *sock; struct sockaddr_storage address; struct iovec *iov = iovstack; struct msghdr msg_sys; int err, iov_size, fput_needed; ... if (copy_from_user(&msg_sys, msg, sizeof(struct msghdr))) return -EFAULT; ... sock = sockfd_lookup_light(fd, &err, &fput_needed); ... iov_size = msg_sys.msg_iovlen * sizeof(struct iovec); ... iov = sock_kmalloc(sock->sk, iov_size, GFP_KERNEL); ... err = verify_iovec(&msg_sys, iov, (struct sockaddr *)&address, VERIFY_READ); ... err = sock_sendmsg(sock, &msg_sys, total_len); ... return err; } static struct socket *sock_from_file(struct file *file, int *err) { if (file->f_op == &socket_file_ops) return file->private_data; ... } static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed) { struct file *file; struct socket *sock; file = fget_light(fd, fput_needed); if (file) { sock = sock_from_file(file, err); if (sock) return sock; fput_light(file, *fput_needed); } return NULL; } #define files_fdtable(files) (rcu_dereference((files)->fdt)) static inline void free_fdtable(struct fdtable *fdt) { call_rcu(&fdt->rcu, free_fdtable_rcu); } struct file *fget_light(unsigned int fd, int *fput_needed) { struct file *file; struct files_struct *files = current->files; *fput_needed = 0; ... rcu_read_lock(); file = fcheck_files(files, fd); ... rcu_read_unlock(); ... return file; } static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd) { struct file * file = NULL; struct fdtable *fdt = files_fdtable(files); ... file = rcu_dereference(fdt->fd[fd]); return file; } SOCKETS FS int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr *kaddr) { if (copy_from_user(kaddr, uaddr, ulen)) return -EFAULT; .,, } struct fdtable { ... struct file ** fd; struct rcu_head rcu; ... };

Allocation and flags Page Frame Memory allocation Atomicity : GFP_ATOMIC from Reserved Pfs – no sleep Contiguity Region: GFP_HIGHMEM, GFP_DMA, GFP_KERNEL Slab allocator

Manipulating User memory Remapping page frames Handling page faults Define vm_operations with a page fault handler Mark page frames to fault (e.g. fork in copy on write)

static int fb_mmap(struct file *file, struct vm_area_struct * vma) { int fbidx = iminor(file->f_path.dentry->d_inode); struct fb_info *info = registered_fb[fbidx]; unsigned long off; unsigned long start; u32 len; ... off = vma->vm_pgoff << PAGE_SHIFT; ... lock_kernel(); ... /* frame buffer memory */ start = info->fix.smem_start; len = PAGE_ALIGN((start & ~PAGE_MASK) + info->fix.smem_len); ... unlock_kernel(); start &= PAGE_MASK; .... off += start; vma->vm_pgoff = off >> PAGE_SHIFT; vma->vm_flags |= VM_IO | VM_RESERVED; ... if (io_remap_pfn_range(vma, vma->vm_start, off >> PAGE_SHIFT, vma->vm_end - vma->vm_start, vma->vm_page_prot)) return -EAGAIN; return 0; } int register_framebuffer(struct fb_info *fb_info) { ... registered_fb[i] = fb_info; ... return 0; } static int __devinit nvidiafb_probe(struct pci_dev *pd, const struct pci_device_id *ent) { struct fb_info *info; info = framebuffer_alloc(sizeof(struct nvidia_par), &pd->dev); ... nvidiafb_fix.smem_start = pci_resource_start(pd, 1); ... if (register_framebuffer(info) < 0) { printk(KERN_ERR PFX "error registering nVidia framebuffer\n"); ... } ... return 0; } NVIDIA FRAME BUFFER

Manipulating VMA static int snd_pcm_mmap_status_fault(struct vm_area_struct *area, struct vm_fault *vmf) { struct snd_pcm_substream *substream = area->vm_private_data; struct snd_pcm_runtime *runtime; runtime = substream->runtime; vmf->page = virt_to_page(runtime->status); get_page(vmf->page); return 0; } static struct vm_operations_struct snd_pcm_vm_ops_status = { .fault = snd_pcm_mmap_status_fault, }; static int snd_pcm_mmap_status(struct snd_pcm_substream *substream, struct file *file, struct vm_area_struct *area) { long size; if (!(area->vm_flags & VM_READ)) return -EINVAL; size = area->vm_end - area->vm_start; if (size != PAGE_ALIGN(sizeof(struct snd_pcm_mmap_status))) return -EINVAL; area->vm_ops = &snd_pcm_vm_ops_status; area->vm_private_data = substream; area->vm_flags |= VM_RESERVED; return 0; }

I/O Control data: I/O memory remapping Data transfer: DMA PCI Scatter Gather

static int qla2x00_iospace_config(scsi_qla_host_t *ha) { resource_size_t pio; if (pci_request_selected_regions(ha->pdev, ha->bars, QLA2XXX_DRIVER_NAME)) { goto iospace_error_exit; } /* Use MMIO operations for all accesses. */ if (!(pci_resource_flags(ha->pdev, 1) & IORESOURCE_MEM)) { goto iospace_error_exit; } if (pci_resource_len(ha->pdev, 1) < MIN_IOBASE_LEN) { goto iospace_error_exit; } ha->iobase = ioremap(pci_resource_start(ha->pdev, 1), MIN_IOBASE_LEN); if (!ha->iobase) { goto iospace_error_exit; } return (0); iospace_error_exit: return (-ENOMEM); } #define WRT_REG_WORD(addr, data) writew(data,addr) #define RD_REG_WORD_RELAXED(addr) readw_relaxed(addr) #define ISP_REQ_Q_IN(ha, reg) \ (IS_QLA2100(ha) || IS_QLA2200(ha) ? \ &(reg)->u.isp2100.mailbox4 : \ &(reg)->u.isp2300.req_q_in) int qla2x00_start_scsi(srb_t *sp) { scsi_qla_host_t *ha; ... if (scsi_sg_count(cmd)) { nseg = dma_map_sg(&ha->pdev->dev, scsi_sglist(cmd), scsi_sg_count(cmd), cmd->sc_data_direction); } else nseg = 0; ... /* Set chip new ring index. */ WRT_REG_WORD(ISP_REQ_Q_IN(ha, reg), ha->req_ring_index); RD_REG_WORD_RELAXED(ISP_REQ_Q_IN(ha, reg)); /* PCI Posting. */ }

Know your Subsystem Specific structures Interface (entry points) The resource objects Specific registration interface Specific objects

References Understanding the Linux Kernel (Daniel Bovet, Marco Cesati) Linux Device Drivers (Alessandro Rubini) Linux Kernel Development (Robert Lowe) Essential Linux Device Drivers (Sreekrishman Venkateswaran) Kernel Documentation Code http://www.gelato.unsw.edu.au/~dsw/public-files/kernel-docs/kernel-api/

Introduction to Kernel Programming

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